Spring 面试之 IOC 初始化过程
2019-06-07 本文已影响0人
大菜鸟_
相信大家都知道Spring，我们经常来用他一些特性，比如说他的AOP，IOC，那今天就带大家解析下SpringIOC的加载过程。
我们来看一个例子
AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext(JobService.class);
for (String beanname:context.getBeanDefinitionNames())
{
System.out.println("--------"+beanname);
}
System.out.println("context.getBean(JobService.class) = " + context.getBean(JobService.class));
这点代码很简单 初始化bean，然后再来拿bean，我们点进AnnotationConfigApplicationContext来看
image
进⼊之后 会调用 this()默认无参构造方法
image
调⽤这个⽆参构造⽅法的同时 他会调用⽗类的构造方法，在调用父类构造⽅方法时 他new了一个对象
image
也就是 DefaultListableBeanFactory，当然 这个就是所谓我们平常所说的 bean工厂，其父类就是 BeanFactory，BeanFactory有很多子类，DefaultListableBeanFactory就是其中一个⼦类。
那么 bean的⽣命周期是围绕那个⽅法呢，就是refresh()⽅法。也就是bean的整个生命周期是围绕refresh() 来进行的
在refresh()我们可以看到
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 准备好刷新上下文.
prepareRefresh();
// 返回一个Factory 为什么需要返回一个工厂 因为要对工厂进行初始化
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// 准备bean工厂，以便在此上下文中使用。
prepareBeanFactory(beanFactory);
try {
// 允许在上下文子类中对bean工厂进行后处理。 在spring5 并未对此接口进行实现
postProcessBeanFactory(beanFactory);
// 在spring的环境中去执行已经被注册的 Factory processors
//设置执行自定义的postProcessBeanFactory和spring内部自己定义的
invokeBeanFactoryPostProcessors(beanFactory);
// 注册postProcessor
registerBeanPostProcessors(beanFactory);
// 初始化此上下文的消息源。
initMessageSource();
// 初始化此上下文的事件多播程序。
initApplicationEventMulticaster();
// 在特定上下文子类中初始化其他特殊bean。
onRefresh();
//检查侦听器bean并注册它们。
registerListeners();
// 实例化所有剩余的(非懒加载)单例。
//new 单例对象
finishBeanFactoryInitialization(beanFactory);
// 最后一步:发布相应的事件
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.
destroyBeans();
// Reset 'active' flag.
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...
resetCommonCaches();
}
}
}
那么这里面最重要就是finishBeanFactoryInitialization(beanFactory);这个方法就是描述 spring的一个bean如何初始化
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
}
// Register a default embedded value resolver if no bean post-processor
// (such as a PropertyPlaceholderConfigurer bean) registered any before:
// at this point, primarily for resolution in annotation attribute values.
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
}
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// Stop using the temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(null);
// Allow for caching all bean definition metadata, not expecting further changes.
beanFactory.freezeConfiguration();
// 实例化所有单例对象
beanFactory.preInstantiateSingletons();
}
可以看到前面是一些判断 最重要的就是最后一个方法 beanFactory.preInstantiateSingletons();我们看下preInstantiateSingletons()方法，它是ConfigurableListableBeanFactory这个接口的一个方法 我们直接来看这个接口的实现 是由DefaultListableBeanFactory这个类 来实现
@Override
public void preInstantiateSingletons() throws BeansException {
if (logger.isDebugEnabled()) {
logger.debug("Pre-instantiating singletons in " + this);
}
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
//所有bean的名字
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
getBean(beanName);
}
}
}
我们可以看到用
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
来保存bean的名字，那this.beanDefinitionNames是一个什么东西
/** List of bean definition names, in registration order */
private volatile List<String> beanDefinitionNames = new ArrayList<>(256);
可以看到是beanDefinition的一个名字，那beanDefinition是个什么呢？它是spring当中非常重要的一个概念，在这里简单的提一嘴 我们传统用纯java的方式怎么new对象 是 Student stu=new Student();的方式来实例化对象，但是要交给spring的话，
先通过springScan的方式扫描到类，当他扫描到的时候 他会去new一个beanDefinition对象 他有很多子类 比如说 GenericBeanDefinition generic=new GenericBeanDefinition();然后他会把扫描到的类的各种信息给拿出来，比如说Student的名字是什么：
image
等等很多很多，然后把这个对象放到哪里呢？ 在DefaultListableBeanFactory中有一个Map，叫做
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
就放到这个Map当中 也就是
beanDefinitionMap.put("studentService",generic)这样放进去之后，spring会看你有没有调用拓展接口，拓展接口我们到后面再说 如果没有拓展接口，就会继续调用我们刚刚接着继续的 preInstantiateSingletons()方法，这个方法来完成bean的实例化 总的一句话 BeanDefinition就是用来描述bean的，当然BeanDefinition的知识不仅仅是这些，今天只是简单提一嘴
那么我们现在继续 beanDefinitionNames也就是刚刚那个Map中Key的集合，然后开始循环
//触发所有非延迟加载单例beans的初始化，只要步骤调用getBean
//根据List名字从Map当中把BeanDefinition依次拿出来开始new对象
for (String beanName : beanNames) {
//合并父BeanDefinition
//通过Map的名字拿BeanDefinition
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
//判断当前类是否抽象 是否为单例 是否为懒加载 如果条件都成立 则继续
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//判断该类是否为FactoryBean FactoryBean这里不进行讲解 不懂得朋友可以去了解下
//如果不是FactoryBean
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
//则直接调用getBean
getBean(beanName);
}
}
}
image
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
/**
*通过name获取beanName。这里不使用name直接做beanName
* name可能会以&字符开头，表用调用者想获取FactoryBean本身，而非FactoryBean
*实现类所创建的bean。在BeanFactory中，FactoryBean的实现类和其他的bean存储方式是一致的即
* <beanName, bean> ,beanName中没有&字符的。所以我们需要将name的首字母&移除，这样才能取到
*FactoryBean实例
*/
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 一个注解叫@DependsOn：A类创建 必须B类创建出来再创建A类
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
if (mbd.isSingleton()) {
//真正开始创建对象
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
Object sharedInstance = getSingleton(beanName);重点为这行代码，我们点进去getSingleton(beanName)这个方法来看
image
正在要new一个对象的时候 他会调用getSingleton方法，那么在这个方法中
Object singletonObject = this.singletonObjects.get(beanName);
这行代码是本质，那他是什么东西 我们点进来看
/** Cache of singleton objects: bean name --> bean instance */
//用于存放完全初始化好的bean从该缓存中取出bean可以直接使用
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
很明显 他就是个Map ConcurrentHashMap线程安全的Map，什么意思 这个Map也就是我们常常说说的Spring的容器 如果从微观角度来讲 没错spring容器就是这个Map，如果从宏观上讲，他就不能称之为map 我希望大家能理解什么意思，因为spring环境包含很多组件，各种组件包含在一起以及singletonObjects以及一些后置处理器配合完成一些工作我们称之为Spring容器。
singletonObjects 是什么？没错他就是单例池 他什么要在单例池拿？为什么要调用他？
因为他要解决Spring当中的循环依赖问题，相信大家既然有看源码的能力，相信大家也明白什么是循环依赖这里不再细说
在这个类里也有介绍
/** Cache of early singleton objects: bean name --> bean instance */
//存放原始的bean对象用于解决循环依赖，注意：存到里面的对象还没被填充到属性
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
什么意思 一个bean放到singletonObjects之后，把一个对象new出来之后，如果这个对象要循环引用那spring就会先把他放到earlySingletonObjects这个当中.get是因为怕对象已经放到early当中所以先去get一遍，这里也只是简单题一嘴
那我们继续看
if (sharedInstance != null && args == null) {//如果条件成立的话
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);//直接返回调
//如果条件不成立直接else
if (isPrototypeCurrentlyInCreation(beanName)) //判断这个类是否正在创建，什么意思 因为spring正在创建一类的时候他会进行标识这个类我正在创建中，然后获取bean工厂
BeanFactory parentBeanFactory = getParentBeanFactory();//这里不重要
我们继续来看
sharedInstance = getSingleton(beanName, () -> { try { return createBean(beanName, mbd, args); } 到这一步的时候，才开始真正创建对象，这里又一个getSingleton这个方法跟上面那个不是同一个方法，这个地方用到lambdas表达式，我们点进去看这个getSingleton
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
Object singletonObject = this.singletonObjects.get(beanName);
这里关键：
第一个getSingleton：单例池拿，拿不到到缓存池拿，拿不到返回null 第二个：也是先从单例池拿 如果为null， if (this.singletonsCurrentlyInDestruction)判断对象有没有开始创建，
然后 beforeSingletonCreation(beanName);我们点进去来看
protected void beforeSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
singletonsCurrentlyInCreation是什么意思
/** Names of beans that are currently in creation */
private final Set<String> singletonsCurrentlyInCreation =
Collections.newSetFromMap(new ConcurrentHashMap<>(16));
也是一个Lsit，什么意思 把它放到这个List当中表示正在创建
紧接着调用createBean(beanName, mbd, args);开始创建对象
我们来看createBean
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
try {
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
到这里 不要以为完了，其实还没完最重要的部分
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
这个是第一次调用spring的后置处理器，spring的生命周期一共围绕了9个后置处理器，这个是第一个
我们先不管这些后置处理器是干嘛的 先把他找出来我们点进resolveBeforeInstantiation
@Nullable
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
Object bean = null;
if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
// Make sure bean class is actually resolved at this point.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
Class<?> targetType = determineTargetType(beanName, mbd);
if (targetType != null) {
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);点进来看
@Nullable
protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
if (result != null) {
return result;
}
}
}
return null;
}
他会先拿出来所有后置处理器 然后判断是不是属于他
我们返回刚刚
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
在doCreateBean里面调用第二个后置处理器，我这里都不再一一寻找了，直接列出来吧
第一次：InstantiationAwareBeanPostProcessor --postProcessBeforeInstantiation
第二次：SmartInstantiationAwareBeanPostProcessor—determineCandidateConstructors—由后置处理器决定返回那些构造方法
第三次：MergedBeanDefinitionPostProcessor——postProcessMergedBeanDefinition------缓存的
第四次：SmartInstantiationAwareBeanPostProcessor—getEarlyBeanReference----把对象放到Early当中--处理循环引用
第五次：InstantiationAwareBeanPostProcessor—postProcessAfterInstantiation---判断要不要填充属性
第六次：InstantiationAwareBeanPostProcessor—postProcessPropertyValues—处理属性的值
第七次：BeanPostProcessor—postProcessBeforeInitialization —处理AOP
第八次：BeanPostProcessor----postProcessAfterInitialization
第九次为销毁
那么到这里一个完整的springBean的初始化
本文出自：cnblogs.com/zgwjava/p/10839732.html
【END】
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