通过Spring提供的接口,改变Spring对Bean的初始化行为

Spring对开闭原则的体现,提供了大量的回调接口,例如BeanPostProcess，该接口的作用是我们可以添加自己的逻辑来控制Bean的初始化构建行为，如我们不想返回Bean本身而是需要返回他的代理对象，修改所依赖的属性对象等等。
下图是BeanPostProcess的关系结构图：
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InstantiationAwareBeanPostProcessor 该接口非常重要，在Spring的Aop自动代理的实现中就需要使用该接口，原因很简单，我们的织入我们需要的我们的一些列的逻辑到目标中，所以我们返回的应该目标对象的代理对象。

 public abstract class AbstractAutoProxyCreator extends ProxyConfig
		implements SmartInstantiationAwareBeanPostProcessor, BeanClassLoaderAware, BeanFactoryAware,
		Ordered, AopInfrastructureBean {

.............................................................
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
		Object cacheKey = getCacheKey(beanClass, beanName);

		if (!this.targetSourcedBeans.contains(cacheKey)) {
			if (this.advisedBeans.contains(cacheKey) || this.nonAdvisedBeans.contains(cacheKey)) {
				return null;
			}
			if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) {
				this.nonAdvisedBeans.add(cacheKey);
				return null;
			}
		}

		// Create proxy here if we have a custom TargetSource.
		// Suppresses unnecessary default instantiation of the target bean:
		// The TargetSource will handle target instances in a custom fashion.
                //获取目标源
		TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
		if (targetSource != null) {
			this.targetSourcedBeans.add(beanName);
			Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
                        //重点看这句,创建代理对象，通过ProxyFactory（aop的代码编程方式）
			Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
			this.proxyTypes.put(cacheKey, proxy.getClass());
                        //然后返回代理对象
			return proxy;
		}

		return null;
	}



}

以InstantiationAwareBeanPostProcessor为例（其他的都一样类似)

public class InstantiationAwareBeanPostProcessorImp implements
		InstantiationAwareBeanPostProcessor {

	@Override
	public Object postProcessBeforeInitialization(Object bean, String beanName)
			throws BeansException {
		System.out.println("postProcessBeforeInitialization" + bean
				+ " beanName:" + beanName);

		return bean;
	}

	@Override
	public Object postProcessAfterInitialization(Object bean, String beanName)
			throws BeansException {
		// TODO Auto-generated method stub
		return bean;
	}

	/**
	 *
	 */
	@Override
	public Object postProcessBeforeInstantiation(Class<?> beanClass,
			String beanName) throws BeansException {

		System.out.println("postProcessBeforeInstantiation(class)" + beanClass
				+ " beanName:" + beanName);
		
		//此处修改了Bean本身所需要的实例对象,阻断了Spring的创建Bean执行流                              程

		return new InstantiationAwareBeanPostProcessorImp();
	}

	@Override
	public boolean postProcessAfterInstantiation(Object bean, String beanName)
			throws BeansException {
		// TODO Auto-generated method stub
		return false;
	}

	@Override
	public PropertyValues postProcessPropertyValues(PropertyValues pvs,
			PropertyDescriptor[] pds, Object bean, String beanName)
			throws BeansException {
		System.out.println("postProcessPropertyValues");
		return null;
	}

}

当Spring在执行创建Bean的时候，会先去执行InstantiationAwareBeanPostProcessor 该接口里面的postProcessBeforeInstantiation如果返回的不是null则调用BeanPostProcessor.postProcessAfterInitialization方法，如上例我们修改了bean本省应该返回的实例，只要这2个方法不返回NULL，Spring就会停止后面的Bean的创建，直接返回该实例（return new InstantiationAwareBeanPostProcessorImp())

@Override
	protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
			throws BeanCreationException {

		if (logger.isDebugEnabled()) {
			logger.debug("Creating instance of bean '" + beanName + "'");
		}
		// Make sure bean class is actually resolved at this point.
		resolveBeanClass(mbd, beanName);

		// Prepare method overrides.
		try {
			mbd.prepareMethodOverrides();
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
					beanName, "Validation of method overrides failed", ex);
		}

		try {
			// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
                       //在这里执行InstantiationAwareBeanPostProcessor里面的方法,
                      // 如果返回值不为NULL直接退出
			Object bean = resolveBeforeInstantiation(beanName, mbd);
			if (bean != null) {
				return bean;
			}
		}
		catch (Throwable ex) {
			throw new BeanCreationException(mbd.getResourceDescription(), beanName,
					"BeanPostProcessor before instantiation of bean failed", ex);
		}
                //否则去执行后续的创建过程
		Object beanInstance = doCreateBean(beanName, mbd, args);
		if (logger.isDebugEnabled()) {
			logger.debug("Finished creating instance of bean '" + beanName + "'");
		}
		return beanInstance;
	}