spring中BeanPostProcessor之一:InstantiationAwareBeanPostPro
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 != ) { 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, protected Object resolveBeforeInstantiation(String beanName,RootBeanDefinition mbd) { Object bean = ; 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 != ) { 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 ) { bean = applyBeanPostProcessorsAfterInitialization(bean,beanName); } } } mbd.beforeInstantiationResolved = (bean != ); } 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 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 != result; } } } ; } 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) BeansException { Object result = existingBean; (BeanPostProcessor processor : getBeanPostProcessors()) { Object current = processor.postProcessAfterInitialization(result,beanName); if (current == result; } result = current; } 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, default Object postProcessAfterInitialization(Object bean,1)"> 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; org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessor; org.springframework.stereotype.Component; net.sf.cglib.proxy.Enhancer; @Component public class MyInstantiationAwareBeanPostProcessor implements InstantiationAwareBeanPostProcessor { @Override public Object postProcessBeforeInitialization(Object bean,1)"> TODO Auto-generated method stub if("userService".equals(beanName)) { UserService us=(UserService)bean; Enhancer enhancer = Enhancer(); 设置目标类的字节码文件 enhancer.setSuperclass(UserService.class); 设置回调函数 enhancer.setCallback( MyMethodInterceptor()); 这里的creat方法就是正式创建代理类 UserService proxyUs = (UserService)enhancer.create(); 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 java.lang.reflect.Method; org.springframework.cglib.proxy.MethodProxy; net.sf.cglib.proxy.MethodInterceptor; class MyMethodInterceptor MethodInterceptor{ @Override Object intercept(Object arg0,Method arg1,Object[] arg2,net.sf.cglib.proxy.MethodProxy arg3) Throwable { TODO Auto-generated method stub Object object = arg3.invokeSuper(arg0,arg2); object; } } 下面是测试类 org.springframework.context.annotation.AnnotationConfigApplicationContext; Test { static void main(String[] args) { TODO Auto-generated method stub AnnotationConfigApplicationContext ac=new AnnotationConfigApplicationContext(Config.); UserService us=ac.getBean(UserService.); 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! (编辑:李大同) 【声明】本站内容均来自网络,其相关言论仅代表作者个人观点,不代表本站立场。若无意侵犯到您的权利,请及时与联系站长删除相关内容! |