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bean
   exposedObject = 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beanPostProcessor
   wrappedBean = 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beanPostProcessor
   wrappedBean = 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 id="lifeCycle" class="com.wzj.bean.LifeCycleBean"
    init-method="initMethod" destroy-method="destroyMethod">
  <property name="property" value="property"/>
 </bean>
 <bean id="myBeanPostProcessor" class="com.wzj.bean.MyBeanPostProcessor" >
 </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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