Spring中bean的生命周期之getSingleton方法

Spring中bean的生命周期

要想讲清楚spring中bean的生命周期，真的是不容易，以AnnotationConfigApplicationContext上下文为基础来讲解bean的生命周期，AnnotationConfigApplicationContext是基于注解的上下文，使用XML的方式现在很少见，所以以此上下文为基础，使用XML的上下文ClassPathXmlApplicationContext的步骤和AnnotationConfigApplicationContext类似。

了解过spring源码的都知道，在AnnotationConfigApplicationContext中调用了父类AbstractApplicationContext的refresh()方法，

refresh方法中调用了10个左右的方法，具体方法不一一细看，看今天分析的重点方法finishBeanFactoryInitialization方法，

@Override public void refresh () throws BeansException , IllegalStateException { synchronized ( this . startupShutdownMonitor ) { // Prepare this context for refreshing. prepareRefresh ();   // Tell the subclass to refresh the internal bean factory. ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory ();   // Prepare the bean factory for use in this context. prepareBeanFactory ( beanFactory );   try { // Allows post-processing of the bean factory in context subclasses. postProcessBeanFactory ( beanFactory );   // Invoke factory processors registered as beans in the context. invokeBeanFactoryPostProcessors ( beanFactory );   // Register bean processors that intercept bean creation. registerBeanPostProcessors ( beanFactory );   // Initialize message source for this context. initMessageSource ();   // Initialize event multicaster for this context. initApplicationEventMulticaster ();   // Initialize other special beans in specific context subclasses. onRefresh ();   // Check for listener beans and register them. registerListeners ();   // Instantiate all remaining (non-lazy-init) singletons. finishBeanFactoryInitialization ( beanFactory );   // Last step: publish corresponding event. 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 (); } } }

在该方法中又调用了preInstantiateSingletons()方法，这个方法才是今天的主角，在该方法中会生成所有非懒加载的单例bean。方法定义如下

@Override public void preInstantiateSingletons () throws BeansException { if ( logger . isTraceEnabled ()) { logger . trace ( "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. 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 ); } } }   // Trigger post-initialization callback for all applicable beans... for ( String beanName : beanNames ) { Object singletonInstance = getSingleton ( beanName ); if ( singletonInstance instanceof SmartInitializingSingleton ) { final SmartInitializingSingleton smartSingleton = ( SmartInitializingSingleton ) singletonInstance ; if ( System . getSecurityManager () != null ) { AccessController . doPrivileged (( PrivilegedAction < Object >) () -> { smartSingleton . afterSingletonsInstantiated (); return null ; }, getAccessControlContext ()); } else { smartSingleton . afterSingletonsInstantiated (); } } } }

该方法完成的主要逻辑是遍历所有的beanName，调用getBean(beanName)方法生成单例bean，在遍历过程中对FactoryBean做了特殊的判断，大家都知道FactoryBean是一种特殊的bean，在《spring中FactoryBean是什么bean》重点分析该bean。重点看getBean(beanName)方法，getBean(beanName)方法调用了doGetBean(beanName)方法，

@Override public Object getBean ( String name ) throws BeansException { return doGetBean ( name , null , null , false ); }

doGetBean方法定义如下，

protected < T > T doGetBean ( final String name , @Nullable final Class < T > requiredType , @Nullable final Object [] args , boolean typeCheckOnly ) throws BeansException {   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 . isTraceEnabled ()) { if ( isSingletonCurrentlyInCreation ( beanName )) { logger . trace ( "Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference" ); } else { logger . trace ( "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 if ( requiredType != null ) { // No args -> delegate to standard getBean method. return parentBeanFactory . getBean ( nameToLookup , requiredType ); } else { return ( T ) parentBeanFactory . getBean ( nameToLookup ); } }   if (! typeCheckOnly ) { markBeanAsCreated ( beanName ); }   try { final RootBeanDefinition mbd = getMergedLocalBeanDefinition ( beanName ); checkMergedBeanDefinition ( mbd , beanName , args );   // Guarantee initialization of beans that the current bean depends on. 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 ()) { 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 . isTraceEnabled ()) { logger . trace ( "Failed to convert bean '" + name + "' to required type '" + ClassUtils . getQualifiedName ( requiredType ) + "'" , ex ); } throw new BeanNotOfRequiredTypeException ( name , requiredType , bean . getClass ()); } } return ( T ) bean ; }

doGetBean方法中有getSingleton、getObjectForBeanInstance、createBean等方法是很重要的。在doGetBean方法中首先调用getSingleton方法检查单例缓存中是否有该bean，如果没有则判断当前bean的作用域是单例（singleton）还是原型（prototype），如果是单例的，再次调用getSingleton方法，不过这次的该方法是重载的一个；如果是原型的则调用createBean方法生成bean，上面几个步骤生成的beanInstance均要调用getObjectForBeanInstance方法获得bean对象。先看getSingleton方法

@Nullable protected Object getSingleton ( String beanName , boolean allowEarlyReference ) { Object singletonObject = this . singletonObjects . get ( beanName ); if ( singletonObject == null && isSingletonCurrentlyInCreation ( beanName )) { synchronized ( this . singletonObjects ) { singletonObject = this . earlySingletonObjects . get ( beanName ); if ( singletonObject == null && allowEarlyReference ) { ObjectFactory <?> singletonFactory = this . singletonFactories . get ( beanName ); if ( singletonFactory != null ) { singletonObject = singletonFactory . getObject (); this . earlySingletonObjects . put ( beanName , singletonObject ); this . singletonFactories . remove ( beanName ); } } } } return singletonObject ; }

在该方法中涉及到三个对象singletonObjects、earlySingletonObjects、singletonFactories，使用这三个对象可以很好的解决循环依赖的问题，这里暂时不讲这个，先看其中的逻辑，

执行的逻辑大概是上面的过程。对singletonObject、earlySingletonObjects、singletonFactories逐级判断，其中singletonFactories中存储的是提前暴露的实例工厂，用来生成一个实例或者实例的代理类。

本文重点对spring生成bean的入门进行了分析，重点分析了代码的调用过程及getSingleton方法，该方法还有一个重载的方法，放到后面分析。

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原文链接：https://HdhCmsTestcnblogs测试数据/teach/p/14951175.html

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