一、接口描述
spring提供了一个接口类-BeanPostProcessor,我们叫他:bean的加工器,应该是在bean的实例化过程中对bean做一些包装处理,里边提供两个方法
public interface BeanPostProcessor { public abstract Object postProcessBeforeInitialization(Object obj, String s) throws BeansException; public abstract Object postProcessAfterInitialization(Object obj, String s) throws BeansException; }
根据类的名称,我们可以猜测两个接口方法的定义分别为:
1、在bean初始化之前执行
2、在bean的初始化之后执行
我们需要到找到spring源码中执行两个方法的代码进行验证,在AbstractAutowireCapableBeanFactory类的方法中,我们找到了执行方法:
二、源码探查
protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) { if (System.getSecurityManager() != null) { AccessController.doPrivileged(new PrivilegedAction<Object>() { @Override public Object run() { invokeAwareMethods(beanName, bean); return null; } }, getAccessControlContext()); } else { invokeAwareMethods(beanName, bean); } Object wrappedBean = bean; if (mbd == null || !mbd.isSynthetic()) { wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName); } try { 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()) { wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName); } return wrappedBean; }
applyBeanPostProcessorsBeforeInitialization和applyBeanPostProcessorsAfterInitialization的具体代码分别为:
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName) throws BeansException { Object result = existingBean; for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) { result = beanProcessor.postProcessBeforeInitialization(result, beanName); if (result == null) { return result; } } return result; }
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName) throws BeansException { Object result = existingBean; for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) { result = beanProcessor.postProcessAfterInitialization(result, beanName); if (result == null) { return result; } } return result; }
根据以上代码,我们得知,在invokeInitMethods的执行前后,spring会分别调用所有的BeanPostProcessor,执行其中的方法,那么invokeInitMethods的具体内容我们仍需要看下,发现此方法主要作用有两个:1、判断bean是否继承了InitializingBean,如果继承接口,执行afterPropertiesSet()方法,2、获得是否设置了init-method属性,如果设置了,就执行设置的方法
protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd) throws Throwable { boolean isInitializingBean = (bean instanceof InitializingBean); if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) { if (logger.isDebugEnabled()) { logger.debug("Invoking afterPropertiesSet() on bean with name ‘" + beanName + "‘"); } if (System.getSecurityManager() != null) { try { AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() { @Override public Object run() throws Exception { ((InitializingBean) bean).afterPropertiesSet(); return null; } }, getAccessControlContext()); } catch (PrivilegedActionException pae) { throw pae.getException(); } } else { ((InitializingBean) bean).afterPropertiesSet(); } } if (mbd != null) { String initMethodName = mbd.getInitMethodName(); if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) && !mbd.isExternallyManagedInitMethod(initMethodName)) { invokeCustomInitMethod(beanName, bean, mbd); } } }
根据以上描述,我们可以看到原有推断有一些问题,两个方法的执行主要是在bean完成初始化之后,准备执行默认方法时候对bean进行包装。
三、应用场景
几个典型的应用如:
1、解析bean的注解,将注解中的字段转化为属性
2、统一将属性在执行前,注入bean中,如数据库访问的sqlMap,如严重服务,这样不需要每个bean都配置属性
3、打印日志,记录时间等。
四、实践
1、定义接口和实例
package com.zjl.beanpostprocessor; public interface DemoService { public void sayHello(); }
public class DemoServiceImpl implements DemoService,NameInit { String name; @Override public void sayHello() { System.out.println("hello "+name); } @Override public void setName(String name) { this.name=name; } }
2、定义bean的配置
<bean id="demoService" class="com.zjl.beanpostprocessor.DemoServiceImpl"> </bean>
此处实例中需要name的值进行打印,但是我们bean中并没有提供此属性
3、定义注入接口
public interface NameInit { public void setName(String name); }
4、定义一个BeanPostProcessor 实例,凡是继承了NameInit的接口,均实例化,注入name值。此处定义接口一方面是要使用接口中提供的setName方法,另一方面减轻系统压力,防止每个bean都进行注入。
/** * 针对继承了接口的bean,注入name * @author lenovo * @time 2016年4月21日 * */ public class NameBeanPostProcessor implements BeanPostProcessor { String name; public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { if(bean instanceof NameInit){ ((NameInit)bean).setName(name); } return bean; } @Override public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { return bean; } }
5、定义bean,注入name的值
<bean id="nameBeanPostProcessor" class="com.zjl.beanpostprocessor.NameBeanPostProcessor"> <property name="name" value="zhangsan"></property> </bean>
6、定义另一个BeanPostProcessor ,仅打印日志
public class LogBeanPostProcessor implements BeanPostProcessor { @Override public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { System.out.println("正在处理"+beanName); return bean; } @Override public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { System.out.println("已经处理完成"+beanName); return bean; } }
7、定义bean
<bean id="logBeanPostProcessor" class="com.zjl.beanpostprocessor.LogBeanPostProcessor"> </bean>
8、测试类
public class BeanPostProcessorTest { public static void main(String[] args) { ApplicationContext context=new FileSystemXmlApplicationContext("beanpostprocessor.xml"); DemoService demoService=(DemoService) context.getBean("demoService"); demoService.sayHello(); } }
9、测试结果为
正在处理demoService 已经处理完成demoService hello zhangsan
总结:根据执行结果,再次验证
1、两个方法均在bean实例化期间已经完成,
2、name属性是根据NameInit接口自动注入
3、由于两个方法执行的时间特殊性,所以打印日志和记录时间意义不大,主要还是用于注入属性和完善配置