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DispathcherServlet的init()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DispatcherServlet的onRefresh()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AbstractApplicationContext的refresh();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ResourceLoader与ApplicationContext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DefaultResourceLoader的getResource();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XmlBeanDefinitionReader的loadBeanDefinitions(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BeanDefinition和BeanDefinitionHold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。
