spring中BeanPostProcessor之一：InstantiationAwareBeanPostProcessor（02）
在上篇博客中写道了bean后置处理器InstantiationAwareBeanPostProcessor，只介绍了其中一个方法的作用及用法，现在来看postProcessBeforeInstantiation方法。
一、概述
postProcessBeforeInstantiation方法定义在InstantiationAwareBeanPostProcessor接口中，方法的定义如下，
@Nullable default Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException { return null; }
从上面的代码中可以看到该方法默认返回null。
二、详述
postProcessBeforeInstantiation方法是用来做什么的，在看源码的过程中，在createBean方法中找到了该方法的调用，下面只贴出相关代码，
try { // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance. //1、调用beanPostProcessor即bean的后置处理器，这里会调用2次后置处理器 Object bean = resolveBeforeInstantiation(beanName, mbdToUse); if (bean != null) { return bean; } } catch (Throwable ex) { throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "BeanPostProcessor before instantiation of bean failed", ex); }
在resolveBeforeInstantiation方法中进行了调用，resolveBeforeInstantiation方法返回值如果不为null，则该方法直接返回bean，也就是说resolveBeforeInstantiation方法至关重要，下面是resolveBeforeInstantiation方法，
@Nullable protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) { Object bean = null; if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) { // Make sure bean class is actually resolved at this point. if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) { Class<?> targetType = determineTargetType(beanName, mbd); if (targetType != null) { //如果是InstantiationAwareBeanPostProcessor的实例，则执行其postProcessBeforeInstantiation方法 bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName); //如果上面的postProcessBeforeInstantiation方法返回值不为null，则执行所有beanPostProcessor的postProcessAfterInitialization方法 //bean不为null，则说明postProcesBeforeInstantiation方法中的返回值是一个不为null的对象 if (bean != null) { bean = applyBeanPostProcessorsAfterInitialization(bean, beanName); } } } mbd.beforeInstantiationResolved = (bean != null); } return bean; }
在resolveBeforeInstantiation方法中首先调用了applyBeanPostProcessorsBeforeInstantiation方法，该方法中便会调用InstantiationAwareBeanPostProcessor接口中的postProcessBeforeInstantiation方法，且如果applyBeanPostPorcessorsBeforeInstantiation方法返回值不为null，才会调用applyBeanPostProcessAfterIntialization方法，下面先看applyBeanPostProcessorsBeforeInstantiation方法
@Nullable protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) { for (BeanPostProcessor bp : getBeanPostProcessors()) { if (bp instanceof InstantiationAwareBeanPostProcessor) { InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp; Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName); if (result != null) { return result; } } } return null; }
从上面方法的定义看到，该方法会遍历benaFactory中的beanPostProcessor，并且判断是否为InstantiationAwareBeanPostPrecessor的类型，如果是执行其postProcessBeforeInstantiation方法，这里默认注册的beanPostProcessor该方法的返回值均为null。稍后自定义一个BeanPostProcessor实现InstantiationAwareBeanPostProcessor接口。
下面看applyBeanPostProcessAfterIntializtion方法，
@Override public 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; }
改方法的逻辑是遍历beanFactory中的所有的beanPostProcessor，执行其postProcessAfterInitialization方法，该方法定义在BeanPostProcessor接口中，默认返回bean，如下，
@Nullable default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { return bean; }
从上面可以看出默认返回的是bean参数的值，也就是如果该方法未实现则默认返回参数中的bean。
再次回到resolveBeforeInstantiation方法，再看其逻辑有以下几种方法返回值的组合，
1、applyBeanPostProcessBeforeInstantiation返回值为null，则resolveBeforeInstantiation方法返回null；
2、applyBeanPostProcessBeforeInstantiation返回值bean不为null，applyBeanPostProcessAfterInitialization方法返回值为null，则resolveBeforeInstantiationf方法返回值为bean；
3、applyBeanPostProcessBeforeInstantiation返回值bean不为null，applyBeanPostProcessAfterInitialization方法返回值bean1不为为null，则resolveBeforeInstantiationf方法返回值为bean1；
从resolveBeforeInstantiation方法分析，该方法的返回值，直接决定了createBean方法的返回值，也就是说applyBeanPostProcessBeforeInstantiation方法返回的bean不为null，下面的方法不会执行。
再来看调用resolveBeforeInstantiation方法时的注释
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance. Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
大体意思时给BeanPostProcessor一个机会返回代理对象而不是目标对象的实例，所以这里resolveBeforeInstantiation方法返回的必然时一个代理对象（JDK和CGLib）。看下面的例子
自定义的BeanPostProcessor实现了InstantiationAwareBeanPostProcessor
package cn.com.my.test; import org.springframework.beans.BeansException; import org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessor; import org.springframework.stereotype.Component; import net.sf.cglib.proxy.Enhancer; @Component public class MyInstantiationAwareBeanPostProcessor implements InstantiationAwareBeanPostProcessor { @Override public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { // TODO Auto-generated method stub if("userService".equals(beanName)) { UserService us=(UserService)bean; Enhancer enhancer = new Enhancer(); //设置目标类的字节码文件 enhancer.setSuperclass(UserService.class); //设置回调函数 enhancer.setCallback(new MyMethodInterceptor()); //这里的creat方法就是正式创建代理类 UserService proxyUs = (UserService)enhancer.create(); return proxyUs; } return InstantiationAwareBeanPostProcessor.super.postProcessBeforeInitialization(bean, beanName); } }
当beanName等于userUservice时返回的是经过cglib代理后的对象。在MyInstantiationAwareBeanPostProcessor类中仅实现了postProcessBeforeInitialization方法，未实现postProcessAfterInitialization方法，所以resolveBeforeInstantiation方法的返回值即未postProcessBeforeInitialization方法的返回值，在上面的类中就是使用cglib代理后的UserService实例。
代理类MyMethodInterceptor，实现cglib的MethodInterceptor接口
package cn.com.my.test; import java.lang.reflect.Method; import org.springframework.cglib.proxy.MethodProxy; import net.sf.cglib.proxy.MethodInterceptor; public class MyMethodInterceptor implements MethodInterceptor{ @Override public Object intercept(Object arg0, Method arg1, Object[] arg2, net.sf.cglib.proxy.MethodProxy arg3) throws Throwable { // TODO Auto-generated method stub Object object = arg3.invokeSuper(arg0, arg2); return object; } }
下面是测试类
package cn.com.my.test; import org.springframework.context.annotation.AnnotationConfigApplicationContext; public class Test { public static void main(String[] args) { // TODO Auto-generated method stub AnnotationConfigApplicationContext ac=new AnnotationConfigApplicationContext(Config.class); UserService us=ac.getBean(UserService.class); System.out.println("us:"+us); } }
看下面的结果，
us:cn.com.my.test.UserService$$EnhancerByCGLIB$$ffa582b4@5fe94a96
返回的是UserService的一个经过cglib代理后的对象。到这里发现真好强大，返回的一个代理对象。
三、适用场合
实现InstantiationAwareBeanPostProcessor接口的postProcessBeforeInitialization方法，通过返回一个代理对象的方式，达到改变目标类类型的目的。在不想改变现有类的逻辑而又想借助现有类实现其他功能，就可以使用这种方式。像AOP就是这种实现，AnnotationAwareAspectJAutoProxyCreator类便是InstantiationAwareBeanPostProcessor的一个实现。
原创不易，有不当之处，欢迎指正，谢谢！
