[spring源码学习]五-BeanPostProcessor的使用

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

posted @ 2016-04-21 16:14  孤子  阅读(10094)  评论(1编辑  收藏  举报