Spring BeanPostProcessor

 BeanPostProcessor允许在调用初始化方法前后对 Bean 进行额外的处理。

BeanPostProcessor源码

public interface BeanPostProcessor {

    Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;


    Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

}

BeanPostProcessor定义了两个方法postProcessBeforeInitialization(Object bean, String beanName) 与 postProcessAfterInitialization(Object bean, String beanName)。bean为需要操作的bean对象，beanName为该对象的名字。

执行顺序

postProcessorBeforeInitailization() 方法是在bean实例化，依赖注入之后调用，初始化方法之前。

postProcessorAfterInitailization()方法是初始化方法之后调用。

如示例

public class HelloSpring {

    public HelloSpring() {System.out.println("HelloSpring Constructor");}

    private String message;
    public void setMessage(String message) {
        this.message = message;
        System.out.println("Message was injected");
        }
    public String getMessage() {return message;}

    // 初始化方法
    public void init() {System.out.println("Bean is going through inti");}
}

BeanPostProcessor实现

public class InitHelloSpring implements BeanPostProcessor{

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("Before initialization: " + beanName);
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("After initialization: " + beanName);
        return bean;
    }
}

public class MainApp {

    public static void main(String[] args) {
        AbstractApplicationContext context = new ClassPathXmlApplicationContext("context.xml");
    }
}

执行结果如下：

// bean的实例化与依赖注入
HelloSpring Constructor
Message was injected

// 执行postProcessorBeforeInitailization() 方法
Before initialization: helloSpring

// 执行bean的初始化方法
Bean is going through inti

// 执行postProcessorAfterInitailization()方法
After initialization: helloSpring

postProcessBeforeInitialization() 与 postProcessAfterInitialization()方法的返回值

这两个方法返回一个Object对象，该对象会更新Spring容器其中原有的bean。

如上例中，Spring容器中注入的是HelloSpring类型的bean，bean的名字为helloSpring，如果 postProcessBeforeInitialization() 与 postProcessAfterInitialization()返回的不是原有的bean，则原有的会被替换。

如

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("Before initialization: " + beanName);
        return new Object();
    }

postProcessBeforeInitialization() 方法返回的是Object对象，该对象则会取代Spring容器中名为helloSpring的bean。

    public static void main(String[] args) {

        AbstractApplicationContext context = new ClassPathXmlApplicationContext("context.xml");
        Object obj = context.getBean("helloSpring");
        System.out.println(obj instanceof HelloSpring);
    }

执行结果为false。