Spring

Spring核心IOC容器初体验(上)

2020-10-24  本文已影响0人  沈先生的影子

本文只做记录,会带来不好的阅读体验,请慎重!

IOC(Inversion Of Control) 控制反转:

  所谓的控制反转,就是把代码里需要实现对象创建、依赖的代码,反转给容器来实现。

DI(Dependency Injection)依赖注入:

  对象被动接受依赖类不需要自己实例或者寻找,简单来说就是对象不是从容器中查找它依赖的类,而是在容器实例化对象的时候主动将其依赖的类注入。

IOC设计视角:

1.对象和对象的关系如何表示?

答:xml、properties文件等语义化配置文件表示。

2.描述对象关系的文件存储在什么地方?

答:classpath、filesystem、URL网络资源、servletContext等。

3.不同的配置文件对对象的描述不一样,如标准的,自定义声明式,如何统一?

答:对象定义需要统一,所有外部的描述都必须转化成统一的描述定义。

4.如何对不同的配置文件进行解析?

答:针对不同的文件配置语法,采用不同的解析器。

Spring核心容器图

1.BeanFactory

  Spring中Bean的创建是典型的工厂模式,IOC容器提供了管理对象之间依赖关系的服务,在Spring中有许多IOC容器的实现提供给开发者使用,其相互关系如图:

image.png

  其中BeanFactory作为最顶层的接口类,它定义了IOC容器的基本功能规范,BeanFactory有三个重要的子类:

ListableBeanFactory、HierarchaicalBeanFactory、AutowireCapableBeanFactory

但是从类图中看出最终实现的是 DefaultListableBeanFactory,它实现了所有接口。

为什么要定义这么多层次的接口?

  每个接口都有它的使用的场合,它主要是为了区分在Spring内部在操作过程中区分每个对象传递和转换过程,对对象的数据访问锁做的限制。
例如:ListableBeanFactory接口表示这些Bean是可列表化的,而HierarchicalBeanFactory表示这些Bean是有继承关系的,也就是这些Bean有可能有父Bean。AutowireCapableBeanFactory接口定义了Bean的自动装配规则。这三个接口共同定义了Bean的集合、Bean之间的关系、以及Bean行为。

看一下最基础的BeanFactory源码:

/**
 * The root interface for accessing a Spring bean container.
 * This is the basic client view of a bean container;
 * further interfaces such as {@link ListableBeanFactory} and
 * {@link org.springframework.beans.factory.config.ConfigurableBeanFactory}
 * are available for specific purposes.
 *
 * <p>This interface is implemented by objects that hold a number of bean definitions,
 * each uniquely identified by a String name. Depending on the bean definition,
 * the factory will return either an independent instance of a contained object
 * (the Prototype design pattern), or a single shared instance (a superior
 * alternative to the Singleton design pattern, in which the instance is a
 * singleton in the scope of the factory). Which type of instance will be returned
 * depends on the bean factory configuration: the API is the same. Since Spring
 * 2.0, further scopes are available depending on the concrete application
 * context (e.g. "request" and "session" scopes in a web environment).
 *
 * <p>The point of this approach is that the BeanFactory is a central registry
 * of application components, and centralizes configuration of application
 * components (no more do individual objects need to read properties files,
 * for example). See chapters 4 and 11 of "Expert One-on-One J2EE Design and
 * Development" for a discussion of the benefits of this approach.
 *
 * <p>Note that it is generally better to rely on Dependency Injection
 * ("push" configuration) to configure application objects through setters
 * or constructors, rather than use any form of "pull" configuration like a
 * BeanFactory lookup. Spring's Dependency Injection functionality is
 * implemented using this BeanFactory interface and its subinterfaces.
 *
 * <p>Normally a BeanFactory will load bean definitions stored in a configuration
 * source (such as an XML document), and use the {@code org.springframework.beans}
 * package to configure the beans. However, an implementation could simply return
 * Java objects it creates as necessary directly in Java code. There are no
 * constraints on how the definitions could be stored: LDAP, RDBMS, XML,
 * properties file, etc. Implementations are encouraged to support references
 * amongst beans (Dependency Injection).
 *
 * <p>In contrast to the methods in {@link ListableBeanFactory}, all of the
 * operations in this interface will also check parent factories if this is a
 * {@link HierarchicalBeanFactory}. If a bean is not found in this factory instance,
 * the immediate parent factory will be asked. Beans in this factory instance
 * are supposed to override beans of the same name in any parent factory.
 *
 * <p>Bean factory implementations should support the standard bean lifecycle interfaces
 * as far as possible. The full set of initialization methods and their standard order is:
 * <ol>
 * <li>BeanNameAware's {@code setBeanName}
 * <li>BeanClassLoaderAware's {@code setBeanClassLoader}
 * <li>BeanFactoryAware's {@code setBeanFactory}
 * <li>EnvironmentAware's {@code setEnvironment}
 * <li>EmbeddedValueResolverAware's {@code setEmbeddedValueResolver}
 * <li>ResourceLoaderAware's {@code setResourceLoader}
 * (only applicable when running in an application context)
 * <li>ApplicationEventPublisherAware's {@code setApplicationEventPublisher}
 * (only applicable when running in an application context)
 * <li>MessageSourceAware's {@code setMessageSource}
 * (only applicable when running in an application context)
 * <li>ApplicationContextAware's {@code setApplicationContext}
 * (only applicable when running in an application context)
 * <li>ServletContextAware's {@code setServletContext}
 * (only applicable when running in a web application context)
 * <li>{@code postProcessBeforeInitialization} methods of BeanPostProcessors
 * <li>InitializingBean's {@code afterPropertiesSet}
 * <li>a custom init-method definition
 * <li>{@code postProcessAfterInitialization} methods of BeanPostProcessors
 * </ol>
 *
 * <p>On shutdown of a bean factory, the following lifecycle methods apply:
 * <ol>
 * <li>{@code postProcessBeforeDestruction} methods of DestructionAwareBeanPostProcessors
 * <li>DisposableBean's {@code destroy}
 * <li>a custom destroy-method definition
 * </ol>
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @author Chris Beams
 * @since 13 April 2001
 * @see BeanNameAware#setBeanName
 * @see BeanClassLoaderAware#setBeanClassLoader
 * @see BeanFactoryAware#setBeanFactory
 * @see org.springframework.context.ResourceLoaderAware#setResourceLoader
 * @see org.springframework.context.ApplicationEventPublisherAware#setApplicationEventPublisher
 * @see org.springframework.context.MessageSourceAware#setMessageSource
 * @see org.springframework.context.ApplicationContextAware#setApplicationContext
 * @see org.springframework.web.context.ServletContextAware#setServletContext
 * @see org.springframework.beans.factory.config.BeanPostProcessor#postProcessBeforeInitialization
 * @see InitializingBean#afterPropertiesSet
 * @see org.springframework.beans.factory.support.RootBeanDefinition#getInitMethodName
 * @see org.springframework.beans.factory.config.BeanPostProcessor#postProcessAfterInitialization
 * @see DisposableBean#destroy
 * @see org.springframework.beans.factory.support.RootBeanDefinition#getDestroyMethodName
 */
public interface BeanFactory {

    /**
     * Used to dereference a {@link FactoryBean} instance and distinguish it from
     * beans <i>created</i> by the FactoryBean. For example, if the bean named
     * {@code myJndiObject} is a FactoryBean, getting {@code &myJndiObject}
     * will return the factory, not the instance returned by the factory.
     */
    //对FactoryBean的转义定义,因为如果使用bean的名字检索FactoryBean得到的对象是工厂生成的对象,
    //如果需要得到工厂本身,需要转义
    String FACTORY_BEAN_PREFIX = "&";


    /**
     * Return an instance, which may be shared or independent, of the specified bean.
     * <p>This method allows a Spring BeanFactory to be used as a replacement for the
     * Singleton or Prototype design pattern. Callers may retain references to
     * returned objects in the case of Singleton beans.
     * <p>Translates aliases back to the corresponding canonical bean name.
     * Will ask the parent factory if the bean cannot be found in this factory instance.
     * @param name the name of the bean to retrieve
     * @return an instance of the bean
     * @throws NoSuchBeanDefinitionException if there is no bean definition
     * with the specified name
     * @throws BeansException if the bean could not be obtained
     */
    //根据bean的名字,获取在IOC容器中得到bean实例
    Object getBean(String name) throws BeansException;

    /**
     * Return an instance, which may be shared or independent, of the specified bean.
     * <p>Behaves the same as {@link #getBean(String)}, but provides a measure of type
     * safety by throwing a BeanNotOfRequiredTypeException if the bean is not of the
     * required type. This means that ClassCastException can't be thrown on casting
     * the result correctly, as can happen with {@link #getBean(String)}.
     * <p>Translates aliases back to the corresponding canonical bean name.
     * Will ask the parent factory if the bean cannot be found in this factory instance.
     * @param name the name of the bean to retrieve
     * @param requiredType type the bean must match. Can be an interface or superclass
     * of the actual class, or {@code null} for any match. For example, if the value
     * is {@code Object.class}, this method will succeed whatever the class of the
     * returned instance.
     * @return an instance of the bean
     * @throws NoSuchBeanDefinitionException if there is no such bean definition
     * @throws BeanNotOfRequiredTypeException if the bean is not of the required type
     * @throws BeansException if the bean could not be created
     */
    //根据bean的名字和Class类型来得到bean实例,增加了类型安全验证机制。
    <T> T getBean(String name, @Nullable Class<T> requiredType) throws BeansException;

    /**
     * Return an instance, which may be shared or independent, of the specified bean.
     * <p>Allows for specifying explicit constructor arguments / factory method arguments,
     * overriding the specified default arguments (if any) in the bean definition.
     * @param name the name of the bean to retrieve
     * @param args arguments to use when creating a bean instance using explicit arguments
     * (only applied when creating a new instance as opposed to retrieving an existing one)
     * @return an instance of the bean
     * @throws NoSuchBeanDefinitionException if there is no such bean definition
     * @throws BeanDefinitionStoreException if arguments have been given but
     * the affected bean isn't a prototype
     * @throws BeansException if the bean could not be created
     * @since 2.5
     */
    Object getBean(String name, Object... args) throws BeansException;

    /**
     * Return the bean instance that uniquely matches the given object type, if any.
     * <p>This method goes into {@link ListableBeanFactory} by-type lookup territory
     * but may also be translated into a conventional by-name lookup based on the name
     * of the given type. For more extensive retrieval operations across sets of beans,
     * use {@link ListableBeanFactory} and/or {@link BeanFactoryUtils}.
     * @param requiredType type the bean must match; can be an interface or superclass.
     * {@code null} is disallowed.
     * @return an instance of the single bean matching the required type
     * @throws NoSuchBeanDefinitionException if no bean of the given type was found
     * @throws NoUniqueBeanDefinitionException if more than one bean of the given type was found
     * @throws BeansException if the bean could not be created
     * @since 3.0
     * @see ListableBeanFactory
     */
    <T> T getBean(Class<T> requiredType) throws BeansException;

    /**
     * Return an instance, which may be shared or independent, of the specified bean.
     * <p>Allows for specifying explicit constructor arguments / factory method arguments,
     * overriding the specified default arguments (if any) in the bean definition.
     * <p>This method goes into {@link ListableBeanFactory} by-type lookup territory
     * but may also be translated into a conventional by-name lookup based on the name
     * of the given type. For more extensive retrieval operations across sets of beans,
     * use {@link ListableBeanFactory} and/or {@link BeanFactoryUtils}.
     * @param requiredType type the bean must match; can be an interface or superclass.
     * {@code null} is disallowed.
     * @param args arguments to use when creating a bean instance using explicit arguments
     * (only applied when creating a new instance as opposed to retrieving an existing one)
     * @return an instance of the bean
     * @throws NoSuchBeanDefinitionException if there is no such bean definition
     * @throws BeanDefinitionStoreException if arguments have been given but
     * the affected bean isn't a prototype
     * @throws BeansException if the bean could not be created
     * @since 4.1
     */
    <T> T getBean(Class<T> requiredType, Object... args) throws BeansException;


    /**
     * Does this bean factory contain a bean definition or externally registered singleton
     * instance with the given name?
     * <p>If the given name is an alias, it will be translated back to the corresponding
     * canonical bean name.
     * <p>If this factory is hierarchical, will ask any parent factory if the bean cannot
     * be found in this factory instance.
     * <p>If a bean definition or singleton instance matching the given name is found,
     * this method will return {@code true} whether the named bean definition is concrete
     * or abstract, lazy or eager, in scope or not. Therefore, note that a {@code true}
     * return value from this method does not necessarily indicate that {@link #getBean}
     * will be able to obtain an instance for the same name.
     * @param name the name of the bean to query
     * @return whether a bean with the given name is present
     */
    //提供对bean的检索,看看是否在IOC容器有这个名字的bean
    boolean containsBean(String name);

    /**
     * Is this bean a shared singleton? That is, will {@link #getBean} always
     * return the same instance?
     * <p>Note: This method returning {@code false} does not clearly indicate
     * independent instances. It indicates non-singleton instances, which may correspond
     * to a scoped bean as well. Use the {@link #isPrototype} operation to explicitly
     * check for independent instances.
     * <p>Translates aliases back to the corresponding canonical bean name.
     * Will ask the parent factory if the bean cannot be found in this factory instance.
     * @param name the name of the bean to query
     * @return whether this bean corresponds to a singleton instance
     * @throws NoSuchBeanDefinitionException if there is no bean with the given name
     * @see #getBean
     * @see #isPrototype
     */
    //根据bean名字得到bean实例,并同时判断这个bean是不是单例
    boolean isSingleton(String name) throws NoSuchBeanDefinitionException;

    /**
     * Is this bean a prototype? That is, will {@link #getBean} always return
     * independent instances?
     * <p>Note: This method returning {@code false} does not clearly indicate
     * a singleton object. It indicates non-independent instances, which may correspond
     * to a scoped bean as well. Use the {@link #isSingleton} operation to explicitly
     * check for a shared singleton instance.
     * <p>Translates aliases back to the corresponding canonical bean name.
     * Will ask the parent factory if the bean cannot be found in this factory instance.
     * @param name the name of the bean to query
     * @return whether this bean will always deliver independent instances
     * @throws NoSuchBeanDefinitionException if there is no bean with the given name
     * @since 2.0.3
     * @see #getBean
     * @see #isSingleton
     */
    boolean isPrototype(String name) throws NoSuchBeanDefinitionException;

    /**
     * Check whether the bean with the given name matches the specified type.
     * More specifically, check whether a {@link #getBean} call for the given name
     * would return an object that is assignable to the specified target type.
     * <p>Translates aliases back to the corresponding canonical bean name.
     * Will ask the parent factory if the bean cannot be found in this factory instance.
     * @param name the name of the bean to query
     * @param typeToMatch the type to match against (as a {@code ResolvableType})
     * @return {@code true} if the bean type matches,
     * {@code false} if it doesn't match or cannot be determined yet
     * @throws NoSuchBeanDefinitionException if there is no bean with the given name
     * @since 4.2
     * @see #getBean
     * @see #getType
     */
    boolean isTypeMatch(String name, ResolvableType typeToMatch) throws NoSuchBeanDefinitionException;

    /**
     * Check whether the bean with the given name matches the specified type.
     * More specifically, check whether a {@link #getBean} call for the given name
     * would return an object that is assignable to the specified target type.
     * <p>Translates aliases back to the corresponding canonical bean name.
     * Will ask the parent factory if the bean cannot be found in this factory instance.
     * @param name the name of the bean to query
     * @param typeToMatch the type to match against (as a {@code Class})
     * @return {@code true} if the bean type matches,
     * {@code false} if it doesn't match or cannot be determined yet
     * @throws NoSuchBeanDefinitionException if there is no bean with the given name
     * @since 2.0.1
     * @see #getBean
     * @see #getType
     */
    boolean isTypeMatch(String name, @Nullable Class<?> typeToMatch) throws NoSuchBeanDefinitionException;

    /**
     * Determine the type of the bean with the given name. More specifically,
     * determine the type of object that {@link #getBean} would return for the given name.
     * <p>For a {@link FactoryBean}, return the type of object that the FactoryBean creates,
     * as exposed by {@link FactoryBean#getObjectType()}.
     * <p>Translates aliases back to the corresponding canonical bean name.
     * Will ask the parent factory if the bean cannot be found in this factory instance.
     * @param name the name of the bean to query
     * @return the type of the bean, or {@code null} if not determinable
     * @throws NoSuchBeanDefinitionException if there is no bean with the given name
     * @since 1.1.2
     * @see #getBean
     * @see #isTypeMatch
     */
    //得到bean实例的Class类型
    @Nullable
    Class<?> getType(String name) throws NoSuchBeanDefinitionException;

    /**
     * Return the aliases for the given bean name, if any.
     * All of those aliases point to the same bean when used in a {@link #getBean} call.
     * <p>If the given name is an alias, the corresponding original bean name
     * and other aliases (if any) will be returned, with the original bean name
     * being the first element in the array.
     * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
     * @param name the bean name to check for aliases
     * @return the aliases, or an empty array if none
     * @see #getBean
     */
    //得到bean的别名,如果根据别名检索,那么其原名也会被检索出来
    String[] getAliases(String name);

}

  在BeanFactory里只对IOC容器的基本行为进行定义,根本不关心你的Bean是如何加载的。正如我们只关心工厂能生产什么对象,至于工厂是如何生成对象我们是无须关心。
  如果要知道IOC如如何产生对象的,我们具体要看看IOC容器的实现,Spring提供了许多IOC容器的实现。
比如:GenericApplicationContext、ClassPathXmlApplicationContext等
  ApplicationContext是Spring提供的一个高级IOC容器,它除了能够提供IOC容器的基本功能,还为了用于提供以下附加服务。

1.支持信息源,可实现国际化。(实现MessageSource接口)
2.访问资源。(实现ResourcePatternResolver接口)
3.支持应用事件。(实现ApplicationEventPublisher接口)

2.BeanDefinition

  SpringIOC容器管理了我们定义的各种Bean对象及其相互关系,Bean对象在Spring实现是以BeanDefinition来描述的,其继承体系如下:

image.png

3.BeanDefinitionReader

  Bean的解析过程非常复杂,功能划分很细,因为这里需要被扩展的地方太多了,必须保证灵活性,以应对可能的变化。Bean的解析主要就是对Spring配置文件的解析。这个解析的过程主要通过BeanDefinitionReader来完成,最后看看Spring中BeanDefinitionReader类的结构图:

image.png

现在我们已经对IOC容器有了基本的了解了。

WEB IOC容器初体验

  还是从大家熟悉的DispatcherServlet开始,我们最先想到的还是DispathcherServletinit()方法。在DispatcherServlet中并没有找到init()方法。但是经过探索,往上追索在其父类HttpServletBean中找到了init()方法:

    /**
     * Map config parameters onto bean properties of this servlet, and
     * invoke subclass initialization.
     * @throws ServletException if bean properties are invalid (or required
     * properties are missing), or if subclass initialization fails.
     */
    @Override
    public final void init() throws ServletException {
        if (logger.isDebugEnabled()) {
            logger.debug("Initializing servlet '" + getServletName() + "'");
        }

        // Set bean properties from init parameters.
        PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
        if (!pvs.isEmpty()) {
            try {
                //定位资源
                BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
                //加载配置信息
                ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
                bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
                initBeanWrapper(bw);
                bw.setPropertyValues(pvs, true);
            }
            catch (BeansException ex) {
                if (logger.isErrorEnabled()) {
                    logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
                }
                throw ex;
            }
        }

        // Let subclasses do whatever initialization they like.
        initServletBean();

        if (logger.isDebugEnabled()) {
            logger.debug("Servlet '" + getServletName() + "' configured successfully");
        }
    }

  在init()方法中,真正完成初始化容器动作的逻辑其实在initServletBean(); 继续跟进initServletBean()代码在FrameworkServlet类中:

/**
     * Overridden method of {@link HttpServletBean}, invoked after any bean properties
     * have been set. Creates this servlet's WebApplicationContext.
     */
    @Override
    protected final void initServletBean() throws ServletException {
        getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
        if (this.logger.isInfoEnabled()) {
            this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
        }
        long startTime = System.currentTimeMillis();

        try {

            this.webApplicationContext = initWebApplicationContext();
            initFrameworkServlet();
        }
        catch (ServletException ex) {
            this.logger.error("Context initialization failed", ex);
            throw ex;
        }
        catch (RuntimeException ex) {
            this.logger.error("Context initialization failed", ex);
            throw ex;
        }

        if (this.logger.isInfoEnabled()) {
            long elapsedTime = System.currentTimeMillis() - startTime;
            this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
                    elapsedTime + " ms");
        }
    }

    /**
     * Initialize and publish the WebApplicationContext for this servlet.
     * <p>Delegates to {@link #createWebApplicationContext} for actual creation
     * of the context. Can be overridden in subclasses.
     * @return the WebApplicationContext instance
     * @see #FrameworkServlet(WebApplicationContext)
     * @see #setContextClass
     * @see #setContextConfigLocation
     */
    protected WebApplicationContext initWebApplicationContext() {

        //先从ServletContext中获得父容器WebAppliationContext
        WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        //声明子容器
        WebApplicationContext wac = null;

        //建立父、子容器之间的关联关系
        if (this.webApplicationContext != null) {
            // A context instance was injected at construction time -> use it
            wac = this.webApplicationContext;
            if (wac instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
                if (!cwac.isActive()) {
                    // The context has not yet been refreshed -> provide services such as
                    // setting the parent context, setting the application context id, etc
                    if (cwac.getParent() == null) {
                        // The context instance was injected without an explicit parent -> set
                        // the root application context (if any; may be null) as the parent
                        cwac.setParent(rootContext);
                    }
                    //这个方法里面调用了AbatractApplication的refresh()方法
                    //模板方法,规定IOC初始化基本流程
                    configureAndRefreshWebApplicationContext(cwac);
                }
            }
        }
        //先去ServletContext中查找Web容器的引用是否存在,并创建好默认的空IOC容器
        if (wac == null) {
            // No context instance was injected at construction time -> see if one
            // has been registered in the servlet context. If one exists, it is assumed
            // that the parent context (if any) has already been set and that the
            // user has performed any initialization such as setting the context id
            wac = findWebApplicationContext();
        }
        //给上一步创建好的IOC容器赋值
        if (wac == null) {
            // No context instance is defined for this servlet -> create a local one
            wac = createWebApplicationContext(rootContext);
        }

        //触发onRefresh方法
        if (!this.refreshEventReceived) {
            // Either the context is not a ConfigurableApplicationContext with refresh
            // support or the context injected at construction time had already been
            // refreshed -> trigger initial onRefresh manually here.
            onRefresh(wac);
        }

        if (this.publishContext) {
            // Publish the context as a servlet context attribute.
            String attrName = getServletContextAttributeName();
            getServletContext().setAttribute(attrName, wac);
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
                        "' as ServletContext attribute with name [" + attrName + "]");
            }
        }

        return wac;
    }

在上面代码中我们看到了熟悉的initWebApplicationContext()方法,继续跟进:

/**
     * Initialize and publish the WebApplicationContext for this servlet.
     * <p>Delegates to {@link #createWebApplicationContext} for actual creation
     * of the context. Can be overridden in subclasses.
     * @return the WebApplicationContext instance
     * @see #FrameworkServlet(WebApplicationContext)
     * @see #setContextClass
     * @see #setContextConfigLocation
     */
    protected WebApplicationContext initWebApplicationContext() {

        //先从ServletContext中获得父容器WebAppliationContext
        WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        //声明子容器
        WebApplicationContext wac = null;

        //建立父、子容器之间的关联关系
        if (this.webApplicationContext != null) {
            // A context instance was injected at construction time -> use it
            wac = this.webApplicationContext;
            if (wac instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
                if (!cwac.isActive()) {
                    // The context has not yet been refreshed -> provide services such as
                    // setting the parent context, setting the application context id, etc
                    if (cwac.getParent() == null) {
                        // The context instance was injected without an explicit parent -> set
                        // the root application context (if any; may be null) as the parent
                        cwac.setParent(rootContext);
                    }
                    //这个方法里面调用了AbatractApplication的refresh()方法
                    //模板方法,规定IOC初始化基本流程
                    configureAndRefreshWebApplicationContext(cwac);
                }
            }
        }
        //先去ServletContext中查找Web容器的引用是否存在,并创建好默认的空IOC容器
        if (wac == null) {
            // No context instance was injected at construction time -> see if one
            // has been registered in the servlet context. If one exists, it is assumed
            // that the parent context (if any) has already been set and that the
            // user has performed any initialization such as setting the context id
            wac = findWebApplicationContext();
        }
        //给上一步创建好的IOC容器赋值
        if (wac == null) {
            // No context instance is defined for this servlet -> create a local one
            wac = createWebApplicationContext(rootContext);
        }

        //触发onRefresh方法
        if (!this.refreshEventReceived) {
            // Either the context is not a ConfigurableApplicationContext with refresh
            // support or the context injected at construction time had already been
            // refreshed -> trigger initial onRefresh manually here.
            onRefresh(wac);
        }

        if (this.publishContext) {
            // Publish the context as a servlet context attribute.
            String attrName = getServletContextAttributeName();
            getServletContext().setAttribute(attrName, wac);
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
                        "' as ServletContext attribute with name [" + attrName + "]");
            }
        }

        return wac;
    }

    /**
     * Retrieve a {@code WebApplicationContext} from the {@code ServletContext}
     * attribute with the {@link #setContextAttribute configured name}. The
     * {@code WebApplicationContext} must have already been loaded and stored in the
     * {@code ServletContext} before this servlet gets initialized (or invoked).
     * <p>Subclasses may override this method to provide a different
     * {@code WebApplicationContext} retrieval strategy.
     * @return the WebApplicationContext for this servlet, or {@code null} if not found
     * @see #getContextAttribute()
     */
    @Nullable
    protected WebApplicationContext findWebApplicationContext() {
        String attrName = getContextAttribute();
        if (attrName == null) {
            return null;
        }
        WebApplicationContext wac =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext(), attrName);
        if (wac == null) {
            throw new IllegalStateException("No WebApplicationContext found: initializer not registered?");
        }
        return wac;
    }

    /**
     * Instantiate the WebApplicationContext for this servlet, either a default
     * {@link org.springframework.web.context.support.XmlWebApplicationContext}
     * or a {@link #setContextClass custom context class}, if set.
     * <p>This implementation expects custom contexts to implement the
     * {@link org.springframework.web.context.ConfigurableWebApplicationContext}
     * interface. Can be overridden in subclasses.
     * <p>Do not forget to register this servlet instance as application listener on the
     * created context (for triggering its {@link #onRefresh callback}, and to call
     * {@link org.springframework.context.ConfigurableApplicationContext#refresh()}
     * before returning the context instance.
     * @param parent the parent ApplicationContext to use, or {@code null} if none
     * @return the WebApplicationContext for this servlet
     * @see org.springframework.web.context.support.XmlWebApplicationContext
     */
    protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
        Class<?> contextClass = getContextClass();
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Servlet with name '" + getServletName() +
                    "' will try to create custom WebApplicationContext context of class '" +
                    contextClass.getName() + "'" + ", using parent context [" + parent + "]");
        }
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException(
                    "Fatal initialization error in servlet with name '" + getServletName() +
                    "': custom WebApplicationContext class [" + contextClass.getName() +
                    "] is not of type ConfigurableWebApplicationContext");
        }
        ConfigurableWebApplicationContext wac =
                (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

        wac.setEnvironment(getEnvironment());
        wac.setParent(parent);
        String configLocation = getContextConfigLocation();
        if (configLocation != null) {
            wac.setConfigLocation(configLocation);
        }
        configureAndRefreshWebApplicationContext(wac);

        return wac;
    }

    protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
        if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
            // The application context id is still set to its original default value
            // -> assign a more useful id based on available information
            if (this.contextId != null) {
                wac.setId(this.contextId);
            }
            else {
                // Generate default id...
                wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
                        ObjectUtils.getDisplayString(getServletContext().getContextPath()) + '/' + getServletName());
            }
        }

        wac.setServletContext(getServletContext());
        wac.setServletConfig(getServletConfig());
        wac.setNamespace(getNamespace());
        wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));

        // The wac environment's #initPropertySources will be called in any case when the context
        // is refreshed; do it eagerly here to ensure servlet property sources are in place for
        // use in any post-processing or initialization that occurs below prior to #refresh
        ConfigurableEnvironment env = wac.getEnvironment();
        if (env instanceof ConfigurableWebEnvironment) {
            ((ConfigurableWebEnvironment) env).initPropertySources(getServletContext(), getServletConfig());
        }

        postProcessWebApplicationContext(wac);
        applyInitializers(wac);
        wac.refresh();
    }

  从上面代码中可以看出configureAndRefreshWebApplicationContext方法中真正调用了refresh()方法,这个是启动IOC容器的入口。IOC容器初始化之后,最后调用了DispatcherServletonRefresh()方法,在onRefresh方法中,又是直接调用initStrategies()方法初始化SpringMvc的九大组件:

    /**
     * Initialize the strategy objects that this servlet uses.
     * <p>May be overridden in subclasses in order to initialize further strategy objects.
     */
    //初始化策略
    protected void initStrategies(ApplicationContext context) {
        //多文件上传的组件
        initMultipartResolver(context);
        //初始化本地语言环境
        initLocaleResolver(context);
        //初始化模板处理器
        initThemeResolver(context);
        //handlerMapping
        initHandlerMappings(context);
        //初始化参数适配器
        initHandlerAdapters(context);
        //初始化异常拦截器
        initHandlerExceptionResolvers(context);
        //初始化视图预处理器
        initRequestToViewNameTranslator(context);
        //初始化视图转换器
        initViewResolvers(context);
        //FlashMap管理器
        initFlashMapManager(context);
    }

基于XML的IOC容器初始化

  IOC容器的初始化包括BeanDefinition的定位、加载、注册这三个基本流程。以ApplicationContext为例子,ApplicationContext系列容器也许是我们最熟悉的容器,因为WEB项目中使用的XmlApplicationContext就属于这个继承体系,还有ClassPathXmlApplicationContext等,其继承体系如下:


image.png

  ApplicationContext允许上下文嵌套,通过保持父上下文可以维持一个上下文体系。对于Bean的查找可以在这个上下文体系中发生,首先检查当前上下文,其次是父上下文,逐级向上,这样为不同的Spring 应用提供了一个共享的Bean定义环境。

1.寻找入口

  ClassPathXmlApplicationContext,通过main()方法启动:

ApplicationContext app = new ClassPathXmlApplicationContext("application.xml");

先看其构造函数的调用:


public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
        this(new String[] {configLocation}, true, null);
}

实际调用:

    /**
     * Create a new ClassPathXmlApplicationContext with the given parent,
     * loading the definitions from the given XML files.
     * @param configLocations array of resource locations
     * @param refresh whether to automatically refresh the context,
     * loading all bean definitions and creating all singletons.
     * Alternatively, call refresh manually after further configuring the context.
     * @param parent the parent context
     * @throws BeansException if context creation failed
     * @see #refresh()
     */
    public ClassPathXmlApplicationContext(
            String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
            throws BeansException {

        super(parent);
        setConfigLocations(configLocations);
        if (refresh) {
            //重启、刷新、重置
            refresh();
        }
    }

  AnnotationConfigApplicationContext、FileSystemXmlApplicationContext、XmlWebApplicationContext等都继承自父容器AbstractApplicationContext主要用到了装饰器模式和策略模式,最终都调用refresh()方法:

2.获取配置路径

  通过分析ClassPathXmlApplictionContext的源代码可以知道,在创建ClassPathXmlApplicationContext容器时,构造方法做以下两项重要工作:

第一,调用父类容器的构造方法,super(parent);为容器设置好Bean资源的加载器。

第二,调用父类AbstractRefreshableConfigApplicationContext的setConfigLocations(configLocations);方法,设置Bean配置信息的定位路径。
这里需要追踪一下AbstractApplicationContext

    /**
     * Set the config locations for this application context in init-param style,
     * i.e. with distinct locations separated by commas, semicolons or whitespace.
     * <p>If not set, the implementation may use a default as appropriate.
     */
    //处理单个资源文件路径为一个字符串的情况
    public void setConfigLocation(String location) {
        //String CONFIG_LOCATION_DELIMITERS = ",; /t/n";
        //即多个资源文件路径之间用” ,; \t\n”分隔,解析成数组形式
        setConfigLocations(StringUtils.tokenizeToStringArray(location, CONFIG_LOCATION_DELIMITERS));
    }

    /**
     * Set the config locations for this application context.
     * <p>If not set, the implementation may use a default as appropriate.
     */
    //解析Bean定义资源文件的路径,处理多个资源文件字符串数组
    public void setConfigLocations(@Nullable String... locations) {
        if (locations != null) {
            Assert.noNullElements(locations, "Config locations must not be null");
            this.configLocations = new String[locations.length];
            for (int i = 0; i < locations.length; i++) {
                // resolvePath为同一个类中将字符串解析为路径的方法
                this.configLocations[i] = resolvePath(locations[i]).trim();
            }
        }
        else {
            this.configLocations = null;
        }
    }

通过这两个方法源码我们可以看出,我们既可以使用一个字符串来配置多个SpringBean的配置信息,也可以使用字符串数组。

到这里,SpringIOC容器会将配置Bean配置信息定位为Spring封装的Resource。
具体可以看看PathMatchingResourcePatternResolver这个类。

第三,开始启动。
  SpringIOC容器对Bean配置资源载入是从refresh();方法开始的,refresh()是一个模板方法,规定了IOC容器的启动流程,有些逻辑要交给其子类去实现。它对Bean配置的资源进行载入ClassPathXmlApplicationContext通过调用其父类AbstractApplicationContextrefresh();函数启动整个IOC容器对Bean定义载入过程,现在我们来详细看看refresh();中的逻辑处理:


    @Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            //1、调用容器准备刷新的方法,获取容器的当时时间,同时给容器设置同步标识
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            //2、告诉子类启动refreshBeanFactory()方法,Bean定义资源文件的载入从
            //子类的refreshBeanFactory()方法启动
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            //3、为BeanFactory配置容器特性,例如类加载器、事件处理器等
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                //4、为容器的某些子类指定特殊的BeanPost事件处理器
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                //5、调用所有注册的BeanFactoryPostProcessor的Bean
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                //6、为BeanFactory注册BeanPost事件处理器.
                //BeanPostProcessor是Bean后置处理器,用于监听容器触发的事件
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                //7、初始化信息源,和国际化相关.
                initMessageSource();

                // Initialize event multicaster for this context.
                //8、初始化容器事件传播器.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                //9、调用子类的某些特殊Bean初始化方法
                onRefresh();

                // Check for listener beans and register them.
                //10、为事件传播器注册事件监听器.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                //11、初始化所有剩余的单例Bean
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                //12、初始化容器的生命周期事件处理器,并发布容器的生命周期事件
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                //13、销毁已创建的Bean
                destroyBeans();

                // Reset 'active' flag.
                //14、取消refresh操作,重置容器的同步标识。
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                //15、重设公共缓存
                resetCommonCaches();
            }
        }
    }

  refresh()方法主要为IOC容器Bean的生命周期管理提供条件,SpringIOC容器载入Bean信息,从其子类容器的refreshBeanFactory()方法启动,所以整个refresh()方法从

ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

  这句代码后都是注册容器的信息源和生命周期事件,我们前面说的载入就是从这距代码开始启动。
  refresh();方法主要作用是:在创建IOC容器前,如果已经有容器存在,则需要把已有的容器销毁和关闭,以保证在refresh之后使用的是新建立起来的IOC容器。它类似于对IOC容器的重启,在新建好的容器中,对容器进行初始化,对Bean配置资源进行载入。

4.创建容器

obtainFreshBeanFactory();方法调用子类容器的refreshBeanFactroy()方法,启动容器载入Bean配置信息的过程:

    /**
     * Tell the subclass to refresh the internal bean factory.
     * @return the fresh BeanFactory instance
     * @see #refreshBeanFactory()
     * @see #getBeanFactory()
     */
    protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        //这里使用了委派设计模式,父类定义了抽象的refreshBeanFactory()方法,具体实现调用子类容器的refreshBeanFactory()方法
        refreshBeanFactory();
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        if (logger.isDebugEnabled()) {
            logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
        }
        return beanFactory;
    }

AbstractApplicationContext类中只抽象定义了refreshBeanFactory();方法,容器真正调用的是其子类AbstractRefreshableApplicationContext实现的方法

    /**
     * This implementation performs an actual refresh of this context's underlying
     * bean factory, shutting down the previous bean factory (if any) and
     * initializing a fresh bean factory for the next phase of the context's lifecycle.
     */
    @Override
    protected final void refreshBeanFactory() throws BeansException {
        //如果已经有容器,销毁容器中的bean,关闭容器
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            //创建IOC容器
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            //对IOC容器进行定制化,如设置启动参数,开启注解的自动装配等
            customizeBeanFactory(beanFactory);
            //调用载入Bean定义的方法,主要这里又使用了一个委派模式,在当前类中只定义了抽象的loadBeanDefinitions方法,具体的实现调用子类容器
              
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

  在这个方法中,先判断了BeanFactroy是否存在,如果存在则先销毁beans并关闭beanFactory,接着创建DefaultListableBeanFactory,并调用了loadBeanDefinitions(beanFactory);装在bean定义。

5.载入配置路径

  AbstractRefreshableApplicationContext中只定义了抽象的
loadBeanDefinitions(beanFactory);方法,容器真正调用的是其子类AbstractXmlApplicationContext

    /**
     * Loads the bean definitions via an XmlBeanDefinitionReader.
     * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
     * @see #initBeanDefinitionReader
     * @see #loadBeanDefinitions
     */
    //实现父类抽象的载入Bean定义方法
    @Override
    protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        // Create a new XmlBeanDefinitionReader for the given BeanFactory.
        //创建XmlBeanDefinitionReader,即创建Bean读取器,并通过回调设置到容器中去,容  器使用该读取器读取Bean定义资源
        XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

        // Configure the bean definition reader with this context's
        // resource loading environment.
        //为Bean读取器设置Spring资源加载器,AbstractXmlApplicationContext的
        //祖先父类AbstractApplicationContext继承DefaultResourceLoader,因此,容器本身也是一个资源加载器
        beanDefinitionReader.setEnvironment(this.getEnvironment());
        beanDefinitionReader.setResourceLoader(this);
        //为Bean读取器设置SAX xml解析器
        beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

        // Allow a subclass to provide custom initialization of the reader,
        // then proceed with actually loading the bean definitions.
        //当Bean读取器读取Bean定义的Xml资源文件时,启用Xml的校验机制
        initBeanDefinitionReader(beanDefinitionReader);
        //Bean读取器真正实现加载的方法
        loadBeanDefinitions(beanDefinitionReader);
    }

    /**
     * Load the bean definitions with the given XmlBeanDefinitionReader.
     * <p>The lifecycle of the bean factory is handled by the {@link #refreshBeanFactory}
     * method; hence this method is just supposed to load and/or register bean definitions.
     * @param reader the XmlBeanDefinitionReader to use
     * @throws BeansException in case of bean registration errors
     * @throws IOException if the required XML document isn't found
     * @see #refreshBeanFactory
     * @see #getConfigLocations
     * @see #getResources
     * @see #getResourcePatternResolver
     */
    //Xml Bean读取器加载Bean定义资源
    protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
        //获取Bean定义资源的定位
        Resource[] configResources = getConfigResources();
        if (configResources != null) {
            //Xml Bean读取器调用其父类AbstractBeanDefinitionReader读取定位
            //的Bean定义资源
            reader.loadBeanDefinitions(configResources);
        }
        //如果子类中获取的Bean定义资源定位为空,则获取FileSystemXmlApplicationContext构造方法中setConfigLocations方法设置的资源
        String[] configLocations = getConfigLocations();
        if (configLocations != null) {
            //Xml Bean读取器调用其父类AbstractBeanDefinitionReader读取定位
            //的Bean定义资源
            reader.loadBeanDefinitions(configLocations);
        }
    }


    /**
     * Return an array of Resource objects, referring to the XML bean definition
     * files that this context should be built with.
     * <p>The default implementation returns {@code null}. Subclasses can override
     * this to provide pre-built Resource objects rather than location Strings.
     * @return an array of Resource objects, or {@code null} if none
     * @see #getConfigLocations()
     */
    //这里又使用了一个委托模式,调用子类的获取Bean定义资源定位的方法
    //该方法在ClassPathXmlApplicationContext中进行实现,对于我们
    //举例分析源码的FileSystemXmlApplicationContext没有使用该方法
    @Nullable
    protected Resource[] getConfigResources() {
        return null;
    }

  以XmlBean读取器的其中一种策略XmlBeanDefinitionReader为例子。XmlBeanDefinitionReader调用其父类AbstractBeanDefinitionReader的reader.loadBeanDefinition()方法读取Bean配置资源。由于我们使用ClassPathXmlApplicationContext作为例子分析,因此getConfigResources返回值为null,因此程序执行reader.loadBeanDefinitions(configLocations);分支。

6.分配路径处理策略

  在XmlBeanDefinitionReader的抽象父类AbstractBeanDefinitionReader定义了载入过程。
具体源码如下:

    //重载方法,调用下面的loadBeanDefinitions(String, Set<Resource>);方法
    @Override
    public int loadBeanDefinitions(String location) throws BeanDefinitionStoreException {
        return loadBeanDefinitions(location, null);
    }

    /**
     * Load bean definitions from the specified resource location.
     * <p>The location can also be a location pattern, provided that the
     * ResourceLoader of this bean definition reader is a ResourcePatternResolver.
     * @param location the resource location, to be loaded with the ResourceLoader
     * (or ResourcePatternResolver) of this bean definition reader
     * @param actualResources a Set to be filled with the actual Resource objects
     * that have been resolved during the loading process. May be {@code null}
     * to indicate that the caller is not interested in those Resource objects.
     * @return the number of bean definitions found
     * @throws BeanDefinitionStoreException in case of loading or parsing errors
     * @see #getResourceLoader()
     * @see #loadBeanDefinitions(org.springframework.core.io.Resource)
     * @see #loadBeanDefinitions(org.springframework.core.io.Resource[])
     */
    public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
        //获取在IoC容器初始化过程中设置的资源加载器
        ResourceLoader resourceLoader = getResourceLoader();
        if (resourceLoader == null) {
            throw new BeanDefinitionStoreException(
                    "Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
        }

        if (resourceLoader instanceof ResourcePatternResolver) {
            // Resource pattern matching available.
            try {
                //将指定位置的Bean定义资源文件解析为Spring IOC容器封装的资源
                //加载多个指定位置的Bean定义资源文件
                Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
                //委派调用其子类XmlBeanDefinitionReader的方法,实现加载功能
                int loadCount = loadBeanDefinitions(resources);
                if (actualResources != null) {
                    for (Resource resource : resources) {
                        actualResources.add(resource);
                    }
                }
                if (logger.isDebugEnabled()) {
                    logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
                }
                return loadCount;
            }
            catch (IOException ex) {
                throw new BeanDefinitionStoreException(
                        "Could not resolve bean definition resource pattern [" + location + "]", ex);
            }
        }
        else {
            // Can only load single resources by absolute URL.
            //将指定位置的Bean定义资源文件解析为Spring IOC容器封装的资源
            //加载单个指定位置的Bean定义资源文件
            Resource resource = resourceLoader.getResource(location);
            //委派调用其子类XmlBeanDefinitionReader的方法,实现加载功能
            int loadCount = loadBeanDefinitions(resource);
            if (actualResources != null) {
                actualResources.add(resource);
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
            }
            return loadCount;
        }
    }

    //重载方法,调用loadBeanDefinitions(String);
    @Override
    public int loadBeanDefinitions(String... locations) throws BeanDefinitionStoreException {
        Assert.notNull(locations, "Location array must not be null");
        int counter = 0;
        for (String location : locations) {
            counter += loadBeanDefinitions(location);
        }
        return counter;
    }

  AbstractRefreshableConfigApplictionContext的loadBeanDefintion(Resource .. resources)方法实际上调用了AbstractBeanDefinitionReader的loadBeanDefintion()方法。从对AbstractBeanDefinitionRead的loadBeanDefintion()方法源码分析得出结论:
 调用资源加载器的获取资源方法esourceLoader.getResource(location);,获取到要加载的资源。
真正执行加载功能是其子类XmlBeanDefinitionReader的loadBeanDefintion()方法。跟进去发现getResources()方法其实定义在ResourcePatternResolver中。
ResourcePatternResolver类图:

;

 &emsp从上面可以看到ResourceLoaderApplicationContext的继承关系,可以看出其实际调用是DefaultResourceLoader中的getSource()方法定位Resource,因为ClassPathXmlApplicaitonContext本身就是DefaultResourceLoader的实现类,所以此时又回到ClassPathXmlApplicationContext中来了。

7.配置解析文件路径

  XmlBeanDefinitionReader通过调用ClassPathXmlApplicationContext的父类DefaultResourceLoadergetResource();方法获取资源,其源码如下:

    //获取Resource的具体实现方法
    @Override
    public Resource getResource(String location) {
        Assert.notNull(location, "Location must not be null");

        for (ProtocolResolver protocolResolver : this.protocolResolvers) {
            Resource resource = protocolResolver.resolve(location, this);
            if (resource != null) {
                return resource;
            }
        }
        //如果是类路径的方式,那需要使用ClassPathResource 来得到bean 文件的资源对象
        if (location.startsWith("/")) {
            return getResourceByPath(location);
        }
        else if (location.startsWith(CLASSPATH_URL_PREFIX)) {
            return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());
        }
        else {
            try {
                // Try to parse the location as a URL...
                // 如果是URL 方式,使用UrlResource 作为bean 文件的资源对象
                URL url = new URL(location);
                return (ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : new UrlResource(url));
            }
            catch (MalformedURLException ex) {
                // No URL -> resolve as resource path.
                //如果既不是classpath标识,又不是URL标识的Resource定位,则调用
                //容器本身的getResourceByPath方法获取Resource
                return getResourceByPath(location);
            }
        }
    }

  ** DefaultResourceLoader**提供了getResourceByPath()方法实现,就是为了处理既不是classpath标识,又不是URL的Resource定位的这种情况。

protected Resource getResourceByPath(String path) {
        return new ClassPathContextResource(path, getClassLoader());
}

  在ClassPathResource中完成了对整个路径的解析。这样,我们就可以从类路径上对IOC配置文件进行加载,当然我们可以按照这个逻辑从任何地方加载,在Spring中我们看到它提供的各种资源抽象,比如ClassPathResource、URLResource、FileStystemResource等来供我们使用。上面我们看到的是定位Resource的过程,这只是加载过程的一部分。例如FileSystemXmlApplication容器就重写了getResourceByPath();方法:

@Override
protected Resource getResourceByPath(String path) {
        if (path.startsWith("/")) {
            path = path.substring(1);
        }
        //这里使用文件系统资源对象来定义bean 文件
        return new FileSystemResource(path);
}

通过子类覆盖,巧妙完成了讲类路径变为文件路径。

8.开始读取配置内容

  继续回到XmlBeanDefinitionReaderloadBeanDefinitions(Resource ... resources)方法看到代表bean文件的资源定义以后的加载过程。

//XmlBeanDefinitionReader加载资源的入口方法
    @Override
    public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
        //将读入的XML资源进行特殊编码处理
        return loadBeanDefinitions(new EncodedResource(resource));
    }
//这里是载入XML形式Bean定义资源文件方法
    public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
        Assert.notNull(encodedResource, "EncodedResource must not be null");
        if (logger.isInfoEnabled()) {
            logger.info("Loading XML bean definitions from " + encodedResource.getResource());
        }

        Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
        if (currentResources == null) {
            currentResources = new HashSet<>(4);
            this.resourcesCurrentlyBeingLoaded.set(currentResources);
        }
        if (!currentResources.add(encodedResource)) {
            throw new BeanDefinitionStoreException(
                    "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
        }
        try {
            //将资源文件转为InputStream的IO流
            InputStream inputStream = encodedResource.getResource().getInputStream();
            try {
                //从InputStream中得到XML的解析源
                InputSource inputSource = new InputSource(inputStream);
                if (encodedResource.getEncoding() != null) {
                    inputSource.setEncoding(encodedResource.getEncoding());
                }
                //这里是具体的读取过程
                return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
            }
            finally {
                //关闭从Resource中得到的IO流
                inputStream.close();
            }
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(
                    "IOException parsing XML document from " + encodedResource.getResource(), ex);
        }
        finally {
            currentResources.remove(encodedResource);
            if (currentResources.isEmpty()) {
                this.resourcesCurrentlyBeingLoaded.remove();
            }
        }
    }

//从特定XML文件中实际载入Bean定义资源的方法
    protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
            throws BeanDefinitionStoreException {
        try {
            //将XML文件转换为DOM对象,解析过程由documentLoader实现
            Document doc = doLoadDocument(inputSource, resource);
            //这里是启动对Bean定义解析的详细过程,该解析过程会用到Spring的Bean配置规则
            return registerBeanDefinitions(doc, resource);
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
        catch (SAXParseException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
        }
        catch (SAXException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "XML document from " + resource + " is invalid", ex);
        }
        catch (ParserConfigurationException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Parser configuration exception parsing XML from " + resource, ex);
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "IOException parsing XML document from " + resource, ex);
        }
        catch (Throwable ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Unexpected exception parsing XML document from " + resource, ex);
        }
    }

  通过源码分析,载入Bean配置信息的最后一步是讲Bean配置信息转换为Document对象,该过程由doLoadDocument(inputSource, resource);方法实现。

9.准备文档对象

  DocumentLoader将Bean配置的资源转化为Document对象源码如下:


//使用标准的JAXP将载入的Bean定义资源转换成document对象
@Override
public Document loadDocument(InputSource inputSource, EntityResolver entityResolver,
            ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception {

        //创建文件解析器工厂
        DocumentBuilderFactory factory = createDocumentBuilderFactory(validationMode, namespaceAware);
        if (logger.isDebugEnabled()) {
            logger.debug("Using JAXP provider [" + factory.getClass().getName() + "]");
        }
        //创建文档解析器
        DocumentBuilder builder = createDocumentBuilder(factory, entityResolver, errorHandler);
        //解析Spring的Bean定义资源
        return builder.parse(inputSource);
}

protected DocumentBuilderFactory createDocumentBuilderFactory(int validationMode, boolean namespaceAware)
            throws ParserConfigurationException {

        //创建文档解析工厂
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        factory.setNamespaceAware(namespaceAware);

        //设置解析XML的校验
        if (validationMode != XmlValidationModeDetector.VALIDATION_NONE) {
            factory.setValidating(true);
            if (validationMode == XmlValidationModeDetector.VALIDATION_XSD) {
                // Enforce namespace aware for XSD...
                factory.setNamespaceAware(true);
                try {
                    factory.setAttribute(SCHEMA_LANGUAGE_ATTRIBUTE, XSD_SCHEMA_LANGUAGE);
                }
                catch (IllegalArgumentException ex) {
                    ParserConfigurationException pcex = new ParserConfigurationException(
                            "Unable to validate using XSD: Your JAXP provider [" + factory +
                            "] does not support XML Schema. Are you running on Java 1.4 with Apache Crimson? " +
                            "Upgrade to Apache Xerces (or Java 1.5) for full XSD support.");
                    pcex.initCause(ex);
                    throw pcex;
                }
            }
        }

        return factory;
}
protected DocumentBuilder createDocumentBuilder(DocumentBuilderFactory factory,
            @Nullable EntityResolver entityResolver, @Nullable ErrorHandler errorHandler)
            throws ParserConfigurationException {

        DocumentBuilder docBuilder = factory.newDocumentBuilder();
        if (entityResolver != null) {
            docBuilder.setEntityResolver(entityResolver);
        }
        if (errorHandler != null) {
            docBuilder.setErrorHandler(errorHandler);
        }
        return docBuilder;
}

  上面的解析过程是调用JavaEE标准的JAXP标准进行处理。SpringIOC容器根据定位的Bean配置信息,将其加载读入并转换为Document对象完成。

10.分配解析策略

  XmlBeanDefinitionReader类中的doLoadBeanDefinition()方法是从特定XML文件中实际载入Bean配置资源的方法,该方法在载入Bean配置资源之后将其转换为Document对象,接下来调用registerBeanDefinitions(Document doc, Resource resource);启动SpringIOC容器对Bean定义的解析过程,

    //按照Spring的Bean语义要求将Bean定义资源解析并转换为容器内部数据结构
    public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        //得到BeanDefinitionDocumentReader来对xml格式的BeanDefinition解析
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        //获得容器中注册的Bean数量
        int countBefore = getRegistry().getBeanDefinitionCount();
        //解析过程入口,这里使用了委派模式,BeanDefinitionDocumentReader只是个接口,
        //具体的解析实现过程有实现类DefaultBeanDefinitionDocumentReader完成
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        //统计解析的Bean数量
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }

  Bean配置资源的载入解析分为以下两个过程:

1.通过调用XML解析器将Bean配置信息转换得到Document对象。这一步没有按照Spring的Bean规则进行解析。这一步是载入过程。

2.完成XML通用解析后,按照Sping定义规则对Document对象进行解析,其解析过程是在接口BeanDefinitionDocumentReader的实现类中实现的。

11.配置载入内存

  BeanDefinitionDocumentReader接口通过registerBeanDefinition()方法调用其实现类DefaultBeanDefinitionDocumentReader对Document对象进行解析:

    //根据Spring DTD对Bean的定义规则解析Bean定义Document对象
    @Override
    public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        //获得XML描述符
        this.readerContext = readerContext;
        logger.debug("Loading bean definitions");
        //获得Document的根元素
        Element root = doc.getDocumentElement();
        doRegisterBeanDefinitions(root);
    }
    protected void doRegisterBeanDefinitions(Element root) {
        // Any nested <beans> elements will cause recursion in this method. In
        // order to propagate and preserve <beans> default-* attributes correctly,
        // keep track of the current (parent) delegate, which may be null. Create
        // the new (child) delegate with a reference to the parent for fallback purposes,
        // then ultimately reset this.delegate back to its original (parent) reference.
        // this behavior emulates a stack of delegates without actually necessitating one.

        //具体的解析过程由BeanDefinitionParserDelegate实现,
        //BeanDefinitionParserDelegate中定义了Spring Bean定义XML文件的各种元素
        BeanDefinitionParserDelegate parent = this.delegate;
        this.delegate = createDelegate(getReaderContext(), root, parent);

        if (this.delegate.isDefaultNamespace(root)) {
            String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
            if (StringUtils.hasText(profileSpec)) {
                String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
                        profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
                if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
                    if (logger.isInfoEnabled()) {
                        logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
                                "] not matching: " + getReaderContext().getResource());
                    }
                    return;
                }
            }
        }

        //在解析Bean定义之前,进行自定义的解析,增强解析过程的可扩展性
        preProcessXml(root);
        //从Document的根元素开始进行Bean定义的Document对象
        parseBeanDefinitions(root, this.delegate);
        //在解析Bean定义之后,进行自定义的解析,增加解析过程的可扩展性
        postProcessXml(root);

        this.delegate = parent;
    }
    //创建BeanDefinitionParserDelegate,用于完成真正的解析过程
    protected BeanDefinitionParserDelegate createDelegate(
            XmlReaderContext readerContext, Element root, @Nullable BeanDefinitionParserDelegate parentDelegate) {

        BeanDefinitionParserDelegate delegate = new BeanDefinitionParserDelegate(readerContext);
        //BeanDefinitionParserDelegate初始化Document根元素
        delegate.initDefaults(root, parentDelegate);
        return delegate;
    }
    //使用Spring的Bean规则从Document的根元素开始进行Bean定义的Document对象
    protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
        //Bean定义的Document对象使用了Spring默认的XML命名空间
        if (delegate.isDefaultNamespace(root)) {
            //获取Bean定义的Document对象根元素的所有子节点
            NodeList nl = root.getChildNodes();
            for (int i = 0; i < nl.getLength(); i++) {
                Node node = nl.item(i);
                //获得Document节点是XML元素节点
                if (node instanceof Element) {
                    Element ele = (Element) node;
                    //Bean定义的Document的元素节点使用的是Spring默认的XML命名空间
                    if (delegate.isDefaultNamespace(ele)) {
                        //使用Spring的Bean规则解析元素节点
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                        //没有使用Spring默认的XML命名空间,则使用用户自定义的解//析规则解析元素节点
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
        else {
            //Document的根节点没有使用Spring默认的命名空间,则使用用户自定义的
            //解析规则解析Document根节点
            delegate.parseCustomElement(root);
        }
    }
    //使用Spring的Bean规则解析Document元素节点
    private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        //如果元素节点是<Import>导入元素,进行导入解析
        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
            importBeanDefinitionResource(ele);
        }
        //如果元素节点是<Alias>别名元素,进行别名解析
        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
            processAliasRegistration(ele);
        }
        //元素节点既不是导入元素,也不是别名元素,即普通的<Bean>元素,
        //按照Spring的Bean规则解析元素
        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
            processBeanDefinition(ele, delegate);
        }
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
            // recurse
            doRegisterBeanDefinitions(ele);
        }
    }

    //解析<Import>导入元素,从给定的导入路径加载Bean定义资源到Spring IoC容器中
    protected void importBeanDefinitionResource(Element ele) {
        //获取给定的导入元素的location属性
        String location = ele.getAttribute(RESOURCE_ATTRIBUTE);
        //如果导入元素的location属性值为空,则没有导入任何资源,直接返回
        if (!StringUtils.hasText(location)) {
            getReaderContext().error("Resource location must not be empty", ele);
            return;
        }

        // Resolve system properties: e.g. "${user.dir}"
        //使用系统变量值解析location属性值
        location = getReaderContext().getEnvironment().resolveRequiredPlaceholders(location);

        Set<Resource> actualResources = new LinkedHashSet<>(4);

        // Discover whether the location is an absolute or relative URI
        //标识给定的导入元素的location是否是绝对路径
        boolean absoluteLocation = false;
        try {
            absoluteLocation = ResourcePatternUtils.isUrl(location) || ResourceUtils.toURI(location).isAbsolute();
        }
        catch (URISyntaxException ex) {
            // cannot convert to an URI, considering the location relative
            // unless it is the well-known Spring prefix "classpath*:"
            //给定的导入元素的location不是绝对路径
        }

        // Absolute or relative?
        //给定的导入元素的location是绝对路径
        if (absoluteLocation) {
            try {
                //使用资源读入器加载给定路径的Bean定义资源
                int importCount = getReaderContext().getReader().loadBeanDefinitions(location, actualResources);
                if (logger.isDebugEnabled()) {
                    logger.debug("Imported " + importCount + " bean definitions from URL location [" + location + "]");
                }
            }
            catch (BeanDefinitionStoreException ex) {
                getReaderContext().error(
                        "Failed to import bean definitions from URL location [" + location + "]", ele, ex);
            }
        }
        else {
            // No URL -> considering resource location as relative to the current file.
            //给定的导入元素的location是相对路径
            try {
                int importCount;
                //将给定导入元素的location封装为相对路径资源
                Resource relativeResource = getReaderContext().getResource().createRelative(location);
                //封装的相对路径资源存在
                if (relativeResource.exists()) {
                    //使用资源读入器加载Bean定义资源
                    importCount = getReaderContext().getReader().loadBeanDefinitions(relativeResource);
                    actualResources.add(relativeResource);
                }
                //封装的相对路径资源不存在
                else {
                    //获取Spring IOC容器资源读入器的基本路径
                    String baseLocation = getReaderContext().getResource().getURL().toString();
                    //根据Spring IOC容器资源读入器的基本路径加载给定导入路径的资源
                    importCount = getReaderContext().getReader().loadBeanDefinitions(
                            StringUtils.applyRelativePath(baseLocation, location), actualResources);
                }
                if (logger.isDebugEnabled()) {
                    logger.debug("Imported " + importCount + " bean definitions from relative location [" + location + "]");
                }
            }
            catch (IOException ex) {
                getReaderContext().error("Failed to resolve current resource location", ele, ex);
            }
            catch (BeanDefinitionStoreException ex) {
                getReaderContext().error("Failed to import bean definitions from relative location [" + location + "]",
                        ele, ex);
            }
        }
        Resource[] actResArray = actualResources.toArray(new Resource[actualResources.size()]);
        //在解析完<Import>元素之后,发送容器导入其他资源处理完成事件
        getReaderContext().fireImportProcessed(location, actResArray, extractSource(ele));
    }

  通过上述Spring IOC容器对载入的Bean定义Document解析可以看出,Spring配置文件可以使用<import>元素来导入IOC容器所需要的其他资源,Spring IOC容器在解析时会将指定导入的资源加载到容器中。使用<aliase>别名时,Spring IOC容器首先将别名元素所定义的别名注册到容器去。

  对于既不是<import>元素,又不是<aliase>元素的元素,即Spring配置文件中普通的<bean>元素的解析由BeanDefinitionParserDelegate类的
parseBeanDefinitionElement()方法来实现。这个解析过程非常复杂。

12.载入<bean>元素

  Bean配置信息中的<import>和<alias>元素解析在DefaultBeanDefinitionDocumentReader中已经完成,对Bean配置信息中使用最多的<bean>元素交由BeanDefinitionParserDelegate来解析:

    //解析<Bean>元素的入口
    @Nullable
    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele) {
        return parseBeanDefinitionElement(ele, null);
    }
    //解析Bean定义资源文件中的<Bean>元素,这个方法中主要处理<Bean>元素的id,name和别名属性
    @Nullable
    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, @Nullable BeanDefinition containingBean) {
        //获取<Bean>元素中的id属性值
        String id = ele.getAttribute(ID_ATTRIBUTE);
        //获取<Bean>元素中的name属性值
        String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

        //获取<Bean>元素中的alias属性值
        List<String> aliases = new ArrayList<>();

        //将<Bean>元素中的所有name属性值存放到别名中
        if (StringUtils.hasLength(nameAttr)) {
            String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
            aliases.addAll(Arrays.asList(nameArr));
        }

        String beanName = id;
        //如果<Bean>元素中没有配置id属性时,将别名中的第一个值赋值给beanName
        if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
            beanName = aliases.remove(0);
            if (logger.isDebugEnabled()) {
                logger.debug("No XML 'id' specified - using '" + beanName +
                        "' as bean name and " + aliases + " as aliases");
            }
        }

        //检查<Bean>元素所配置的id或者name的唯一性,containingBean标识<Bean>
        //元素中是否包含子<Bean>元素
        if (containingBean == null) {
            //检查<Bean>元素所配置的id、name或者别名是否重复
            checkNameUniqueness(beanName, aliases, ele);
        }

        //详细对<Bean>元素中配置的Bean定义进行解析的地方
        AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
        if (beanDefinition != null) {
            if (!StringUtils.hasText(beanName)) {
                try {
                    if (containingBean != null) {
                        //如果<Bean>元素中没有配置id、别名或者name,且没有包含子元素
                        //<Bean>元素,为解析的Bean生成一个唯一beanName并注册
                        beanName = BeanDefinitionReaderUtils.generateBeanName(
                                beanDefinition, this.readerContext.getRegistry(), true);
                    }
                    else {
                        //如果<Bean>元素中没有配置id、别名或者name,且包含了子元素
                        //<Bean>元素,为解析的Bean使用别名向IOC容器注册
                        beanName = this.readerContext.generateBeanName(beanDefinition);
                        // Register an alias for the plain bean class name, if still possible,
                        // if the generator returned the class name plus a suffix.
                        // This is expected for Spring 1.2/2.0 backwards compatibility.
                        //为解析的Bean使用别名注册时,为了向后兼容
                        //Spring1.2/2.0,给别名添加类名后缀
                        String beanClassName = beanDefinition.getBeanClassName();
                        if (beanClassName != null &&
                                beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
                                !this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
                            aliases.add(beanClassName);
                        }
                    }
                    if (logger.isDebugEnabled()) {
                        logger.debug("Neither XML 'id' nor 'name' specified - " +
                                "using generated bean name [" + beanName + "]");
                    }
                }
                catch (Exception ex) {
                    error(ex.getMessage(), ele);
                    return null;
                }
            }
            String[] aliasesArray = StringUtils.toStringArray(aliases);
            return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
        }
        //当解析出错时,返回null
        return null;
    }
    //详细对<Bean>元素中配置的Bean定义其他属性进行解析
    //由于上面的方法中已经对Bean的id、name和别名等属性进行了处理
    //该方法中主要处理除这三个以外的其他属性数据
    @Nullable
    public AbstractBeanDefinition parseBeanDefinitionElement(
            Element ele, String beanName, @Nullable BeanDefinition containingBean) {
        //记录解析的<Bean>
        this.parseState.push(new BeanEntry(beanName));

        //这里只读取<Bean>元素中配置的class名字,然后载入到BeanDefinition中去
        //只是记录配置的class名字,不做实例化,对象的实例化在依赖注入时完成
        String className = null;

        //如果<Bean>元素中配置了parent属性,则获取parent属性的值
        if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
            className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
        }
        String parent = null;
        if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
            parent = ele.getAttribute(PARENT_ATTRIBUTE);
        }

        try {
            //根据<Bean>元素配置的class名称和parent属性值创建BeanDefinition
            //为载入Bean定义信息做准备
            AbstractBeanDefinition bd = createBeanDefinition(className, parent);

            //对当前的<Bean>元素中配置的一些属性进行解析和设置,如配置的单态(singleton)属性等
            parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
            //为<Bean>元素解析的Bean设置description信息
            bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));

            //对<Bean>元素的meta(元信息)属性解析
            parseMetaElements(ele, bd);
            //对<Bean>元素的lookup-method属性解析
            parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
            //对<Bean>元素的replaced-method属性解析
            parseReplacedMethodSubElements(ele, bd.getMethodOverrides());

            //解析<Bean>元素的构造方法设置
            parseConstructorArgElements(ele, bd);
            //解析<Bean>元素的<property>设置
            parsePropertyElements(ele, bd);
            //解析<Bean>元素的qualifier属性
            parseQualifierElements(ele, bd);

            //为当前解析的Bean设置所需的资源和依赖对象
            bd.setResource(this.readerContext.getResource());
            bd.setSource(extractSource(ele));

            return bd;
        }
        catch (ClassNotFoundException ex) {
            error("Bean class [" + className + "] not found", ele, ex);
        }
        catch (NoClassDefFoundError err) {
            error("Class that bean class [" + className + "] depends on not found", ele, err);
        }
        catch (Throwable ex) {
            error("Unexpected failure during bean definition parsing", ele, ex);
        }
        finally {
            this.parseState.pop();
        }

        //解析<Bean>元素出错时,返回null
        return null;
    }

  对Spring配置文件比较熟悉的人,通过上述代码的分析,就会明白我们在Spring配置文件中<Bean>元素的中配置的属性就是通过该方法解析和设置到Bean中。

注:在解析<Bean>元素过程中,并没有创建和实例化对象。只是创建了Bean对象的定义类BeanDefinition,将<Bean>元素中的配置信息设置到BeanDefinition中作为记录,当依赖注入时候才使用这些记录创建和实例化具体Bean对象。

  上面方法中配置元数据(meta)、qualifier等的解析,在Spring中配置时使用不多,我们在使用Spring的<Bean>元素时,配置最多的就是<property>。

13.载入<property>元素

  BeanDefinitionParserDelegate在解析<Bean>调用parsePropertyElement()方法解析<Bean>元素中<property>属性子元素,源码如下:

    //解析<property>元素
    public void parsePropertyElement(Element ele, BeanDefinition bd) {
        //获取<property>元素的名字
        String propertyName = ele.getAttribute(NAME_ATTRIBUTE);
        if (!StringUtils.hasLength(propertyName)) {
            error("Tag 'property' must have a 'name' attribute", ele);
            return;
        }
        this.parseState.push(new PropertyEntry(propertyName));
        try {
            //如果一个Bean中已经有同名的property存在,则不进行解析,直接返回。
            //即如果在同一个Bean中配置同名的property,则只有第一个起作用
            if (bd.getPropertyValues().contains(propertyName)) {
                error("Multiple 'property' definitions for property '" + propertyName + "'", ele);
                return;
            }
            //解析获取property的值
            Object val = parsePropertyValue(ele, bd, propertyName);
            //根据property的名字和值创建property实例
            PropertyValue pv = new PropertyValue(propertyName, val);
            //解析<property>元素中的属性
            parseMetaElements(ele, pv);
            pv.setSource(extractSource(ele));
            bd.getPropertyValues().addPropertyValue(pv);
        }
        finally {
            this.parseState.pop();
        }
    }
    //解析获取property值
    @Nullable
    public Object parsePropertyValue(Element ele, BeanDefinition bd, @Nullable String propertyName) {
        String elementName = (propertyName != null) ?
                        "<property> element for property '" + propertyName + "'" :
                        "<constructor-arg> element";

        // Should only have one child element: ref, value, list, etc.
        //获取<property>的所有子元素,只能是其中一种类型:ref,value,list,etc等
        NodeList nl = ele.getChildNodes();
        Element subElement = null;
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            //子元素不是description和meta属性
            if (node instanceof Element && !nodeNameEquals(node, DESCRIPTION_ELEMENT) &&
                    !nodeNameEquals(node, META_ELEMENT)) {
                // Child element is what we're looking for.
                if (subElement != null) {
                    error(elementName + " must not contain more than one sub-element", ele);
                }
                else {
                    //当前<property>元素包含有子元素
                    subElement = (Element) node;
                }
            }
        }

        //判断property的属性值是ref还是value,不允许既是ref又是value
        boolean hasRefAttribute = ele.hasAttribute(REF_ATTRIBUTE);
        boolean hasValueAttribute = ele.hasAttribute(VALUE_ATTRIBUTE);
        if ((hasRefAttribute && hasValueAttribute) ||
                ((hasRefAttribute || hasValueAttribute) && subElement != null)) {
            error(elementName +
                    " is only allowed to contain either 'ref' attribute OR 'value' attribute OR sub-element", ele);
        }

        //如果属性是ref,创建一个ref的数据对象RuntimeBeanReference
        //这个对象封装了ref信息
        if (hasRefAttribute) {
            String refName = ele.getAttribute(REF_ATTRIBUTE);
            if (!StringUtils.hasText(refName)) {
                error(elementName + " contains empty 'ref' attribute", ele);
            }
            //一个指向运行时所依赖对象的引用
            RuntimeBeanReference ref = new RuntimeBeanReference(refName);
            //设置这个ref的数据对象是被当前的property对象所引用
            ref.setSource(extractSource(ele));
            return ref;
        }
        //如果属性是value,创建一个value的数据对象TypedStringValue
        //这个对象封装了value信息
        else if (hasValueAttribute) {
            //一个持有String类型值的对象
            TypedStringValue valueHolder = new TypedStringValue(ele.getAttribute(VALUE_ATTRIBUTE));
            //设置这个value数据对象是被当前的property对象所引用
            valueHolder.setSource(extractSource(ele));
            return valueHolder;
        }
        //如果当前<property>元素还有子元素
        else if (subElement != null) {
            //解析<property>的子元素
            return parsePropertySubElement(subElement, bd);
        }
        else {
            // Neither child element nor "ref" or "value" attribute found.
            //propery属性中既不是ref,也不是value属性,解析出错返回null
            error(elementName + " must specify a ref or value", ele);
            return null;
        }
    }

  通过上面的代码,我们可以了解在Spring配置文件中,<Bean>元素中<property>元素的相关配置是如何处理的:
1.ref被封装为指向依赖对象一个引用。
2.value配置都会被封装为一个字符串对象。
3.ref和value都通过"解析的数据类型属性值.setSource(extractSource(ele));方法将属性值/与引用的属性关联起来。

在方法的最后对于<property>元素的子元素通过parsePropertySubElement(subElement, bd);方法解析

14.载入<property>子元素

  BeanDefinitionParserDelegate类中parsePropertySubElement(subElement, bd);方法对<property>中的子元素解析,源码如下:

    //解析<property>元素中ref,value或者集合等子元素
    @Nullable
    public Object parsePropertySubElement(Element ele, @Nullable BeanDefinition bd, @Nullable String defaultValueType) {
        //如果<property>没有使用Spring默认的命名空间,则使用用户自定义的规则解析内嵌元素
        if (!isDefaultNamespace(ele)) {
            return parseNestedCustomElement(ele, bd);
        }
        //如果子元素是bean,则使用解析<Bean>元素的方法解析
        else if (nodeNameEquals(ele, BEAN_ELEMENT)) {
            BeanDefinitionHolder nestedBd = parseBeanDefinitionElement(ele, bd);
            if (nestedBd != null) {
                nestedBd = decorateBeanDefinitionIfRequired(ele, nestedBd, bd);
            }
            return nestedBd;
        }
        //如果子元素是ref,ref中只能有以下3个属性:bean、local、parent
        else if (nodeNameEquals(ele, REF_ELEMENT)) {
            // A generic reference to any name of any bean.
            //可以不再同一个Spring配置文件中,具体请参考Spring对ref的配置规则
            String refName = ele.getAttribute(BEAN_REF_ATTRIBUTE);
            boolean toParent = false;
            if (!StringUtils.hasLength(refName)) {
                // A reference to the id of another bean in a parent context.
                //获取<property>元素中parent属性值,引用父级容器中的Bean
                refName = ele.getAttribute(PARENT_REF_ATTRIBUTE);
                toParent = true;
                if (!StringUtils.hasLength(refName)) {
                    error("'bean' or 'parent' is required for <ref> element", ele);
                    return null;
                }
            }
            if (!StringUtils.hasText(refName)) {
                error("<ref> element contains empty target attribute", ele);
                return null;
            }
            //创建ref类型数据,指向被引用的对象
            RuntimeBeanReference ref = new RuntimeBeanReference(refName, toParent);
            //设置引用类型值是被当前子元素所引用
            ref.setSource(extractSource(ele));
            return ref;
        }
        //如果子元素是<idref>,使用解析ref元素的方法解析
        else if (nodeNameEquals(ele, IDREF_ELEMENT)) {
            return parseIdRefElement(ele);
        }
        //如果子元素是<value>,使用解析value元素的方法解析
        else if (nodeNameEquals(ele, VALUE_ELEMENT)) {
            return parseValueElement(ele, defaultValueType);
        }
        //如果子元素是null,为<property>设置一个封装null值的字符串数据
        else if (nodeNameEquals(ele, NULL_ELEMENT)) {
            // It's a distinguished null value. Let's wrap it in a TypedStringValue
            // object in order to preserve the source location.
            TypedStringValue nullHolder = new TypedStringValue(null);
            nullHolder.setSource(extractSource(ele));
            return nullHolder;
        }
        //如果子元素是<array>,使用解析array集合子元素的方法解析
        else if (nodeNameEquals(ele, ARRAY_ELEMENT)) {
            return parseArrayElement(ele, bd);
        }
        //如果子元素是<list>,使用解析list集合子元素的方法解析
        else if (nodeNameEquals(ele, LIST_ELEMENT)) {
            return parseListElement(ele, bd);
        }
        //如果子元素是<set>,使用解析set集合子元素的方法解析
        else if (nodeNameEquals(ele, SET_ELEMENT)) {
            return parseSetElement(ele, bd);
        }
        //如果子元素是<map>,使用解析map集合子元素的方法解析
        else if (nodeNameEquals(ele, MAP_ELEMENT)) {
            return parseMapElement(ele, bd);
        }
        //如果子元素是<props>,使用解析props集合子元素的方法解析
        else if (nodeNameEquals(ele, PROPS_ELEMENT)) {
            return parsePropsElement(ele);
        }
        //既不是ref,又不是value,也不是集合,则子元素配置错误,返回null
        else {
            error("Unknown property sub-element: [" + ele.getNodeName() + "]", ele);
            return null;
        }
    }

  通过上面的代码,可以看出Spring配置文件中,对<property>元素中配置的array、list、set、map、prop等各种集合子元素的都通过上面的方法解析,生成对应的数据对象,比如:ManagedList、ManagedArray、ManagedSet等,这些Managed类是Spring对象BeanDefinition的数据封装,对集合数据类型的具体解析有各自的解析方法实现。

15.载入<list>的子元素

  BeanDefinitionParserDelegate类中有parseListElement()方法就是具体解析<property>元素的<list>集合子元素,源码如下:

    //解析<list>集合子元素
    public List<Object> parseListElement(Element collectionEle, @Nullable BeanDefinition bd) {
        //获取<list>元素中的value-type属性,即获取集合元素的数据类型
        String defaultElementType = collectionEle.getAttribute(VALUE_TYPE_ATTRIBUTE);
        //获取<list>集合元素中的所有子节点
        NodeList nl = collectionEle.getChildNodes();
        //Spring中将List封装为ManagedList
        ManagedList<Object> target = new ManagedList<>(nl.getLength());
        target.setSource(extractSource(collectionEle));
        //设置集合目标数据类型
        target.setElementTypeName(defaultElementType);
        target.setMergeEnabled(parseMergeAttribute(collectionEle));
        //具体的<list>元素解析
        parseCollectionElements(nl, target, bd, defaultElementType);
        return target;
    }

    //具体解析<list>集合元素,<array>、<list>和<set>都使用该方法解析
    protected void parseCollectionElements(
            NodeList elementNodes, Collection<Object> target, @Nullable BeanDefinition bd, String defaultElementType) {
        //遍历集合所有节点
        for (int i = 0; i < elementNodes.getLength(); i++) {
            Node node = elementNodes.item(i);
            //节点不是description节点
            if (node instanceof Element && !nodeNameEquals(node, DESCRIPTION_ELEMENT)) {
                //将解析的元素加入集合中,递归调用下一个子元素
                target.add(parsePropertySubElement((Element) node, bd, defaultElementType));
            }
        }
    }

  SpringBean配置信息转换的Document对象中的元素层层解析,SpringIOC现在已经将XML形式定义的Bean配置信息转换为SpringIOC所识别的数据结构--->BeanDefinition,它是Bean配置信息中配置的POJO对象在SpringIOC容器中的映射。我们可以通过AbstractBeanDefinition为入口,看到lOC容器进行索引、查询和操作。

  SpringIOC容器对Bean配置资源的解析后,IOC容器大致完成了管理Bean对象的准备工作,即初始化过程,但是最为重要的依赖注入还没有发生,现在在IOC容器中BeanDefinition存储的只是一些静态信息,接下来需要向容器注册Bean定义信息才能全部完成IOC容器的初始化过程。

16.分配注册策略

  DefaultBeanDefinitionDocumentReader对Bean定义转换的Document对象解析的流程中,在其parseDefaultElement()方法中完成对Document对象的解析得到BeanDefinitionBeanDefinitionHold对象。
然后调用BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());方法向IOC容器注册解析Bean,BeanDefinitionReaderUtils注册的源码是:

    //将解析的BeanDefinitionHold注册到容器中
    public static void registerBeanDefinition(
            BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
            throws BeanDefinitionStoreException {

        // Register bean definition under primary name.
        //获取解析的BeanDefinition的名称
        String beanName = definitionHolder.getBeanName();
        //向IOC容器注册BeanDefinition
        registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

        // Register aliases for bean name, if any.
        //如果解析的BeanDefinition有别名,向容器为其注册别名
        String[] aliases = definitionHolder.getAliases();
        if (aliases != null) {
            for (String alias : aliases) {
                registry.registerAlias(beanName, alias);
            }
        }
    }

  DefaultListableBeanFactory中使用一个HashMap的集合对象存放IOC容器中注册解析BeanDefinition,向IOC容器注册的主要源码如下:

image.png

    //向IOC容器注册解析的BeanDefiniton
    @Override
    public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
            throws BeanDefinitionStoreException {

        Assert.hasText(beanName, "Bean name must not be empty");
        Assert.notNull(beanDefinition, "BeanDefinition must not be null");

        //校验解析的BeanDefiniton
        if (beanDefinition instanceof AbstractBeanDefinition) {
            try {
                ((AbstractBeanDefinition) beanDefinition).validate();
            }
            catch (BeanDefinitionValidationException ex) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Validation of bean definition failed", ex);
            }
        }

        BeanDefinition oldBeanDefinition;

        oldBeanDefinition = this.beanDefinitionMap.get(beanName);

        if (oldBeanDefinition != null) {
            if (!isAllowBeanDefinitionOverriding()) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                        "': There is already [" + oldBeanDefinition + "] bound.");
            }
            else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
                // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
                if (this.logger.isWarnEnabled()) {
                    this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
                            "' with a framework-generated bean definition: replacing [" +
                            oldBeanDefinition + "] with [" + beanDefinition + "]");
                }
            }
            else if (!beanDefinition.equals(oldBeanDefinition)) {
                if (this.logger.isInfoEnabled()) {
                    this.logger.info("Overriding bean definition for bean '" + beanName +
                            "' with a different definition: replacing [" + oldBeanDefinition +
                            "] with [" + beanDefinition + "]");
                }
            }
            else {
                if (this.logger.isDebugEnabled()) {
                    this.logger.debug("Overriding bean definition for bean '" + beanName +
                            "' with an equivalent definition: replacing [" + oldBeanDefinition +
                            "] with [" + beanDefinition + "]");
                }
            }
            this.beanDefinitionMap.put(beanName, beanDefinition);
        }
        else {
            if (hasBeanCreationStarted()) {
                // Cannot modify startup-time collection elements anymore (for stable iteration)
                //注册的过程中需要线程同步,以保证数据的一致性
                synchronized (this.beanDefinitionMap) {
                    this.beanDefinitionMap.put(beanName, beanDefinition);
                    List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
                    updatedDefinitions.addAll(this.beanDefinitionNames);
                    updatedDefinitions.add(beanName);
                    this.beanDefinitionNames = updatedDefinitions;
                    if (this.manualSingletonNames.contains(beanName)) {
                        Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
                        updatedSingletons.remove(beanName);
                        this.manualSingletonNames = updatedSingletons;
                    }
                }
            }
            else {
                // Still in startup registration phase
                this.beanDefinitionMap.put(beanName, beanDefinition);
                this.beanDefinitionNames.add(beanName);
                this.manualSingletonNames.remove(beanName);
            }
            this.frozenBeanDefinitionNames = null;
        }

        //检查是否有同名的BeanDefinition已经在IOC容器中注册
        if (oldBeanDefinition != null || containsSingleton(beanName)) {
            //重置所有已经注册过的BeanDefinition的缓存
            resetBeanDefinition(beanName);
        }
    }

  Bean配置信息中的Bean被解析后,已经注册到IOC容器中了,被容器管理起来了,真正完成了IOC容器初始化工作。现在IOC容器已经建立了整个Bean的配置信息,这些BeanDefinition信息已经可以使用,并且可以检索,IOC容器的作用就是对这些注册的Bean定义信息进行处理和维护。注册的Bean定义信息是IOC容器控制反转的基础,正是有了这些注册的数据,容器才可以进行依赖注入。

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