Spring提供了很多扩展接口,BeanPostProcessor接口和InstantiationAwareBeanPostProcessor接口就是其中两个。
BeanPostProcessor
BeanPostProcessor接口作用是:如果我们需要在Spring容器完成Bean的实例化、配置和其他的初始化前后添加一些自己的逻辑处理,我们就可以定义一个或者多个BeanPostProcessor接口的实现,然后注册到容器中。
Spring中Bean的实例化过程图示:
由上图可以看到,Spring中的BeanPostProcessor在实例化过程处于的位置,BeanPostProcessor接口有两个方法需要实现:postProcessBeforeInitialization和postProcessAfterInitialization,
import org.springframework.beans.factory.config.BeanPostProcessor;
public class MyBeanPostProcessor implements BeanPostProcessor {
public MyBeanPostProcessor() {
super();
System.out.println("这是BeanPostProcessor实现类构造器!!");
}
@Override
public Object postProcessAfterInitialization(Object bean, String arg1)
throws BeansException {
System.out.println("bean处理器:bean创建之后..");
return bean;
}
@Override
public Object postProcessBeforeInitialization(Object bean, String arg1)
throws BeansException {
System.out.println("bean处理器:bean创建之前..");
return bean;
}
}
BeanPostProcessor接口定义如下:
public interface BeanPostProcessor {
/**
* Apply this BeanPostProcessor to the given new bean instance before any bean
* initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
* or a custom init-method). The bean will already be populated with property values.
*/
//实例化、依赖注入完毕,在调用显示的初始化之前完成一些定制的初始化任务
Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;
/**
* Apply this BeanPostProcessor to the given new bean instance after any bean
* initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
* or a custom init-method). The bean will already be populated with property values.
*/
//实例化、依赖注入、初始化完毕时执行
Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;
}
由方法名字也可以看出,前者在实例化及依赖注入完成后、在任何初始化代码(比如配置文件中的init-method)调用之前调用;后者在初始化代码调用之后调用。
注意:
1、接口中的两个方法都要将传入的bean返回,而不能返回null,如果返回的是null那么我们通过getBean方法将得不到目标。
2、BeanFactory和ApplicationContext对待bean后置处理器稍有不同。ApplicationContext会自动检测在配置文件中实现了BeanPostProcessor接口的所有bean,并把它们注册为后置处理器,然后在容器创建bean的适当时候调用它,因此部署一个后置处理器同部署其他的bean并没有什么区别。而使用BeanFactory实现的时候,bean 后置处理器必须通过代码显式地去注册,在IoC容器继承体系中的ConfigurableBeanFactory接口中定义了注册方法:
/**
* Add a new BeanPostProcessor that will get applied to beans created
* by this factory. To be invoked during factory configuration.
* Note: Post-processors submitted here will be applied in the order of
* registration; any ordering semantics expressed through implementing the
* {@link org.springframework.core.Ordered} interface will be ignored. Note
* that autodetected post-processors (e.g. as beans in an ApplicationContext)
* will always be applied after programmatically registered ones.
* @param beanPostProcessor the post-processor to register
*/
void addBeanPostProcessor(BeanPostProcessor beanPostProcessor);
另外,不要将BeanPostProcessor标记为延迟初始化。因为如果这样做,Spring容器将不会注册它们,自定义逻辑也就无法得到应用。假如你在
InstantiationAwareBeanPostProcessor
package org.springframework.beans.factory.config;
import java.beans.PropertyDescriptor;
import org.springframework.beans.BeansException;
import org.springframework.beans.PropertyValues;
public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {
Object postProcessBeforeInstantiation(Class beanClass, String beanName) throws BeansException;
boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException;
PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName)
throws BeansException;
}
其使用方法与上面介绍的BeanPostProcessor接口类似,只时回调时机不同。