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初始化

  spring初始化分为三步：初始化前、初始化和初始化后，每一步作用也不相同，废话不多说，上代码:

/*** Initialize the given bean instance, applying factory callbacks* as well as init methods and bean post processors.* <p>Called from {@link #createBean} for traditionally defined beans,* and from {@link #initializeBean} for existing bean instances.** @param beanName the bean name in the factory (for debugging purposes)* @param bean     the new bean instance we may need to initialize* @param mbd      the bean definition that the bean was created with*                 (can also be {@code null}, if given an existing bean instance)* @return the initialized bean instance (potentially wrapped)* @see BeanNameAware* @see BeanClassLoaderAware* @see BeanFactoryAware* @see #applyBeanPostProcessorsBeforeInitialization* @see #invokeInitMethods* @see #applyBeanPostProcessorsAfterInitialization*/
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 {invokeAwareMethods(beanName, bean);}Object wrappedBean = bean;// 初始化前if (mbd == null || !mbd.isSynthetic()) {wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);}// 初始化try {invokeInitMethods(beanName, wrappedBean, mbd);} catch (Throwable ex) {throw new BeanCreationException((mbd != null ? mbd.getResourceDescription() : null),beanName, "Invocation of init method failed", ex);}// 初始化后 AOPif (mbd == null || !mbd.isSynthetic()) {wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);}return wrappedBean;
}

  通过上述代码，我们可以看到：

1. 初始化方法的第一步会先调用invokeAwareMethods()方法进行Aware回调；
2. 调用applyBeanPostProcessorsBeforeInitialization()方法完成初始化前操作；
3. 调用invokeInitMethods()方法完成初始化操作；
4. 调用applyBeanPostProcessorsAfterInitialization()方法完成初始化后操作；

invokeAwareMethods()方法

invokeAwareMethods()方法详解

private void invokeAwareMethods(String beanName, Object bean) {if (bean instanceof Aware) {// beanName回调if (bean instanceof BeanNameAware) {((BeanNameAware) bean).setBeanName(beanName);}// BeanClassLoader回调if (bean instanceof BeanClassLoaderAware) {ClassLoader bcl = getBeanClassLoader();if (bcl != null) {((BeanClassLoaderAware) bean).setBeanClassLoader(bcl);}}//BeanFactory回调if (bean instanceof BeanFactoryAware) {((BeanFactoryAware) bean).setBeanFactory(AbstractAutowireCapableBeanFactory.this);}}
}

  通过上述代码我们可以看到，这里会对BeanNameAware、BeanClassLoaderAware和BeanFactoryAware三个Aware接口进行回调操作；
  回调initializeBean()方法

applyBeanPostProcessorsBeforeInitialization()方法

applyBeanPostProcessorsBeforeInitialization()方法详解

	@Overridepublic Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)throws BeansException {Object result = existingBean;for (BeanPostProcessor processor : getBeanPostProcessors()) {Object current = processor.postProcessBeforeInitialization(result, beanName);if (current == null) {return result;}result = current;}return result;}

  这里我们可以看到，如果Bean有设置BeanPostProcessor接口的postProcessBeforeInitialization()方法在这里进行调用，Spring在这一步会判断bean对象有没有加@PostConstruct和@PreDestroy，如果有赋值给对应的属性，方便后续直接调用；
  回到initializeBean()方法

invokeInitMethods()方法

invokeInitMethods()方法详解

/*** Give a bean a chance to react now all its properties are set,* and a chance to know about its owning bean factory (this object).* This means checking whether the bean implements InitializingBean or defines* a custom init method, and invoking the necessary callback(s) if it does.** @param beanName the bean name in the factory (for debugging purposes)* @param bean     the new bean instance we may need to initialize* @param mbd      the merged bean definition that the bean was created with*                 (can also be {@code null}, if given an existing bean instance)* @throws Throwable if thrown by init methods or by the invocation process* @see #invokeCustomInitMethod*/
protected void invokeInitMethods(String beanName, Object bean, @Nullable RootBeanDefinition mbd)throws Throwable {boolean isInitializingBean = (bean instanceof InitializingBean);if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {if (logger.isTraceEnabled()) {logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");}if (System.getSecurityManager() != null) {try {AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {((InitializingBean) bean).afterPropertiesSet();return null;}, getAccessControlContext());} catch (PrivilegedActionException pae) {throw pae.getException();}} else {((InitializingBean) bean).afterPropertiesSet();}}if (mbd != null && bean.getClass() != NullBean.class) {String initMethodName = mbd.getInitMethodName();if (StringUtils.hasLength(initMethodName) &&!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&!mbd.isExternallyManagedInitMethod(initMethodName)) {invokeCustomInitMethod(beanName, bean, mbd);}}
}

  这里可以看到，首先会判断当前Bean对象是否实现了InitializingBean接口，如果实现了就调用它的afterPropertiesSet()方法，然后判断BeanDefinition中是否指定初始化方法，如果指定啦初始化方法且不是afterPropertiesSet()方法的话，会调用BeanDefinition指定的初始化方法；
  回到initializeBean()方法

applyBeanPostProcessorsAfterInitialization()方法

applyBeanPostProcessorsAfterInitialization()方法详解

	@Overridepublic Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)throws BeansException {Object result = existingBean;for (BeanPostProcessor processor : getBeanPostProcessors()) {Object current = processor.postProcessAfterInitialization(result, beanName);if (current == null) {return result;}result = current;}return result;}

  如果Bean对象实现了BeanPostProcessor接口的postProcessAfterInitialization()方法进行初始化后设置，Spring中的AOP就是基于初始化后 实现的，初始化后返回的对象才是最终的Bean对象。
  回到initializeBean()方法