Spring BeanPostProcessor 接口使用

• Spring 中提供了很多 PostProcessor 供开发者进行拓展，例如：BeanPostProcessor、BeanFactoryPostProcessor、BeanValidationPostProcessor 等一系列后处理器。他们的使用方式大多类似，了解其中一个并掌握他的使用方式，其他的可以触类旁通。

• 这里以 BeanPostProcessor 为例展示其使用方式。

• BeanPostProcessor 接口提供了两个供开发者自定义的方法：postProcessBeforeInitialization、postProcessAfterInitialization。

• postProcessBeforeInitialization：该方法主要针对 spring 在 bean 初始化时调用初始化方法前进行自定义处理。

• postProcessAfterInitialization：该方法主要针对 spring 在 bean 初始化时调用初始化方法后进行自定义处理。

• 示例代码：

/**
 * 测试bean
 */
public class Cat {

    private String name;

    private int age;

    public void say() {
        System.out.println("name:" + name);
        System.out.println("age:" + age);
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}
​

/**
 * 自定义后处理器
 */
public class CatBeanPostProcessor implements BeanPostProcessor {

    @Nullable
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if (bean instanceof Cat) {
            //输出原始属性
            Cat cat = (Cat) bean;
            cat.say();
            return bean;
        }
        return bean;
    }

    @Nullable
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        if (bean instanceof Cat) {
            //修改属性值，并返回
            Cat cat = (Cat) bean;
            cat.setName("hello maomi");
            cat.setAge(3);
            return cat;
        }
        return bean;
    }

}
​

/**
 * 运行
 */
public class Run {

    public static void main(String[] args) {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring-bean.xml");
        Cat cat = (Cat) applicationContext.getBean("cat");
        cat.say();
    }

}
​

xml 配置信息

    <!--配置bean并初始化-->
    <bean id="cat" class="com.source.postprocessor.Cat" >
        <property name="name" value="HelloKitty" />
        <property name="age" value="1" />
    </bean>

    <bean id="catBeanPostProcessor" class="com.source.postprocessor.CatBeanPostProcessor" />
​

输出结果：

name:HelloKitty
age:1
name:hello maomi
age:3
​

• 可以看到通过后处理器处理过后的 bean 信息已经改变。最后，看看源码中如何调用自定义实现的。
  • 在初始化 bean 方法中：AbstractAutowireCapableBeanFactory.java

     /**
     * 初始化bean
     */
    protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
        //省略部分无关代码
        Object wrappedBean = bean;
        //初始化前
        if (mbd == null || !mbd.isSynthetic()) {
            wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
        }
  
        try {
            //调用初始化方法初始化bean
            invokeInitMethods(beanName, wrappedBean, mbd);
        }
        catch (Throwable ex) {
            throw new BeanCreationException(
                    (mbd != null ? mbd.getResourceDescription() : null),
                    beanName, "Invocation of init method failed", ex);
        }
        //初始化后
        if (mbd == null || !mbd.isSynthetic()) {
            wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
        }
        return wrappedBean;
    }
  ​

  • postProcessBeforeInitialization 方法调用

    @Override
    public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
            throws BeansException {
  
        Object result = existingBean;
        for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
            //调用自定义postProcessBeforeInitialization方法
            Object current = beanProcessor.postProcessBeforeInitialization(result, beanName);
            if (current == null) {
                return result;
            }
            result = current;
        }
        return result;
    }
  ​

  • postProcessAfterInitialization 方法调用

    @Override
    public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
            throws BeansException {
  
        Object result = existingBean;
        for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
            //自定义postProcessAfterInitialization方法调用
            Object current = beanProcessor.postProcessAfterInitialization(result, beanName);
            if (current == null) {
                return result;
            }
            result = current;
        }
        return result;
    }
  ​

• 以上就是 spring 对自定义方法实现的调用过程。