Spring源码解析之Bean的生命周期

一、Bean的实例化概述

前一篇分析了BeanDefinition的封装过程，最终将beanName与BeanDefinition以一对一映射关系放到beanDefinitionMap容器中，这一篇重点分析如何利用bean的定义信息BeanDefinition实例化bean。

二、流程概览

其实bean的实例化过程比较复杂，中间细节很多，为了抓住重点，先将核心流程梳理出来，主要包含以下几个流程：

step1: 通过反射创建实例； step2：给实例属性赋初始值； step3：如果Bean类实现BeanNameAware接口，则将通过传递Bean的名称来调用setBeanName()方法；如果Bean类实现BeanClassLoaderAware接口，则将通过传递加载此Bean的ClassLoader对象的实例来调用setBeanClassLoader()方法；如果Bean类实现BeanFactoryAware接口，则将通过传递BeanFactory对象的实例来调用setBeanFactory()方法； step4: 如果有类实现BeanPostProcessors接口，则将在初始化之前调用postProcessBeforeInitialization()方法； step5：如果Bean类实现了InitializingBean接口，将调用afterPropertiesSet()方法，如果配置文件中的Bean定义包含init-method属性，则该属性的值将解析为Bean类中的方法名称，并将调用该方法； step6: 如果有类实现BeanPostProcessors接口，则将在初始化之后调用postProcessAfterInitialization()方法； step7：如果Bean类实现DisposableBean接口，则当Application不再需要Bean引用时，将调用destroy()方法；如果配置文件中的Bean定义包含destroy-method属性，那么将调用Bean类中的相应方法定义。三、源码分析

进入AbstractApplicationContext中的fresh()方法，找到finishBeanFactoryInitialization(beanFactory)方法，该类是bean的实例化的入口，具体的实例化由preInstantiateSingletons()方法触发，见如下代码：

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.//xml解析时，把所有beanName都缓存到beanDefinitionNames了List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);// Trigger initialization of all non-lazy singleton beans...for (String beanName : beanNames) {//把父BeanDefinition里面的属性拿到子BeanDefinition中RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);//如果不是抽象的，单例的，非懒加载的就实例化if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {//判断bean是否实现了FactoryBean接口if (isFactoryBean(beanName)) {Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);if (bean instanceof FactoryBean) {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 {// 实例化过程getBean(beanName);}}}

上述代码主要看getBean方法，随后进入doGetBean方法:

protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)throws BeansException {String beanName = transformedBeanName(name);Object bean;// 从缓存中获取bean.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 {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// 创建bean实例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);}...}

由上述代码可知，先从缓存中获取bean，如果没有，则创建bean，最重要的方法就是getSingleton，该方法第二个参数是个函数式接口，进入getSingleton方法，当调用singletonObject = singletonFactory.getObject()时，会触发函数式接口中的createBean方法，随后一路进入doCreateBean，这个方法里面完成了所有实例化所需的步骤:

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {// Instantiate the bean.// 真正开始创建bean的实例.BeanWrapper instanceWrapper = null;if (mbd.isSingleton()) {instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null) {instanceWrapper = createBeanInstance(beanName, mbd, args);}Object bean = instanceWrapper.getWrappedInstance();Class<?> beanType = instanceWrapper.getWrappedClass();if (beanType != NullBean.class) {mbd.resolvedTargetType = beanType;}// Allow post-processors to modify the merged bean definition.synchronized (mbd.postProcessingLock) {if (!mbd.postProcessed) {try {applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);}catch (Throwable ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,'Post-processing of merged bean definition failed', ex);}mbd.postProcessed = true;}}// Eagerly cache singletons to be able to resolve circular references// even when triggered by lifecycle interfaces like BeanFactoryAware.boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&isSingletonCurrentlyInCreation(beanName));if (earlySingletonExposure) {if (logger.isTraceEnabled()) {logger.trace('Eagerly caching bean ’' + beanName +'’ to allow for resolving potential circular references');}addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));}// Initialize the bean instance.Object exposedObject = bean;try {// 属性赋值populateBean(beanName, mbd, instanceWrapper);// 初始化beanexposedObject = initializeBean(beanName, exposedObject, mbd);}catch (Throwable ex) {if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {throw (BeanCreationException) ex;}else {throw new BeanCreationException(mbd.getResourceDescription(), beanName, 'Initialization of bean failed', ex);}}if (earlySingletonExposure) {Object earlySingletonReference = getSingleton(beanName, false);if (earlySingletonReference != null) {if (exposedObject == bean) {exposedObject = earlySingletonReference;}else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {String[] dependentBeans = getDependentBeans(beanName);Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);for (String dependentBean : dependentBeans) {if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {actualDependentBeans.add(dependentBean);}}if (!actualDependentBeans.isEmpty()) {throw new BeanCurrentlyInCreationException(beanName,'Bean with name ’' + beanName + '’ has been injected into other beans [' +StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +'] in its raw version as part of a circular reference, but has eventually been ' +'wrapped. This means that said other beans do not use the final version of the ' +'bean. This is often the result of over-eager type matching - consider using ' +'’getBeanNamesForType’ with the ’allowEagerInit’ flag turned off, for example.');}}}}// Register bean as disposable.// 有必要时，注册bean的销毁try {registerDisposableBeanIfNecessary(beanName, bean, mbd);}catch (BeanDefinitionValidationException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, 'Invalid destruction signature', ex);}return exposedObject;}

从上述源码中看出bean的实例化主要分为以下三步：

step1:bean的创建； step2:给bean的属性赋值； step3:bean的初始化；

接着得到exposedObject这个已经完全实例化后的bean返回，其中当有必要时，注册bean的销毁，后面再详细看，先抓住主要流程。其中step3也是比较重要的方法，进入该方法:

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {if (System.getSecurityManager() != null) {AccessController.doPrivileged((PrivilegedAction<Object>) () -> {invokeAwareMethods(beanName, bean);return null;}, getAccessControlContext());}else {// 激活aware接口invokeAwareMethods(beanName, bean);}Object wrappedBean = bean;if (mbd == null || !mbd.isSynthetic()) {// 初始化前处理的beanPostProcessorwrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);}try {// 激活 init-method方法invokeInitMethods(beanName, wrappedBean, mbd);}catch (Throwable ex) {throw new BeanCreationException((mbd != null ? mbd.getResourceDescription() : null),beanName, 'Invocation of init method failed', ex);}if (mbd == null || !mbd.isSynthetic()) {// 初始化后处理的beanPostProcessorwrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);}return wrappedBean;}

从上面源码可知，梳理出主要的四个步骤：

step1:激活aware接口，完成aware接口的相关操作； step2:初始化前处理的beanPostProcessor； step3:完成init-method方法； step4:初始化后处理的beanPostProcessor;

BeanPostProcessor作用是对初始化后的bean进行增强处理，在该阶段 BeanPostProcessor 会处理当前容器内所有符合条件的实例化后的 bean 对象。它主要是对 Spring 容器提供的 bean 实例对象进行有效的扩展，允许Spring在初始化 bean 阶段对其进行定制化修改，如处理标记接口或者为其提供代理实现。

四、演示

定义一个MyBeanPostProcessor实现BeanPostProcessor接口

public class MyBeanPostProcessor implements BeanPostProcessor { @Override public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {System.out.println('post Process Before Initialization 被调用...');return bean; } @Override public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {System.out.println('post Process after Initialization 被调用...');return bean; }}

定义一个LifeCycleBean类，实现如下接口：

public class LifeCycleBean implements BeanNameAware, BeanFactoryAware, BeanClassLoaderAware,InitializingBean, DisposableBean { private String property; public String getProperty() {return property; } public void setProperty(String property) {System.out.println('属性注入....');this.property = property; } public LifeCycleBean(){System.out.println('构造函数调用...'); } @Override public void setBeanClassLoader(ClassLoader classLoader) {System.out.println('BeanClassLoaderAware 被调用...'); } @Override public void setBeanFactory(BeanFactory beanFactory) throws BeansException {System.out.println('BeanFactoryAware 被调用...'); } @Override public void setBeanName(String name) {System.out.println('BeanNameAware 被调用...'); } @Override public void destroy() throws Exception {System.out.println('DisposableBean destroy 被调用...'); } @Override public void afterPropertiesSet() throws Exception {System.out.println('InitializingBean afterPropertiesSet 被调用...'); } public void initMethod(){System.out.println('init-method 被调用...'); } public void destroyMethod(){System.out.println('destroy-method 被调用...'); } public void display(){System.out.println('方法调用...'); }}

指定配置文件spring.xml，配置init-method与destroy-method方法

<bean init-method='initMethod' destroy-method='destroyMethod'><property name='property' value='property'/></bean><bean ></bean>

测试类如下：

@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations = {'classpath:spring.xml'})public class TestSpring { @Test public void testLifeCycleBean() { ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext('spring.xml'); }

执行结果：

构造函数调用...属性注入....BeanNameAware 被调用...BeanClassLoaderAware 被调用...BeanFactoryAware 被调用...post Process Before Initialization 被调用...InitializingBean afterPropertiesSet 被调用...init-method 被调用...post Process after Initialization 被调用...DisposableBean destroy 被调用...destroy-method 被调用...

五、总结

本篇从一个初学者的角度概览了bean的整个生命周期，并描述了其中的主要流程，阅读源码的初始阶段，优先抓住主要流程，别陷入细节，并通过跑案例、写注解、画流程图等方式加深理解，后续将继续分析bean实例化中的核心流程、设计思想等。

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