Spring中BeanPostProcessor
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:
1 public interface BeanPostProcessor { 2 3 /** 4 * Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean 5 * initialization callbacks (like InitializingBean's {@code afterPropertiesSet} 6 * or a custom init-method). The bean will already be populated with property values. 7 */ 8 //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 9 Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException; 10 11 12 /** 13 * Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean 14 * initialization callbacks (like InitializingBean's {@code afterPropertiesSet} 15 * or a custom init-method). The bean will already be populated with property values. 16 */ 17 //实例化、依赖注入、初始化完毕时执行 18 Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException; 19 20 }
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. * <p>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<beans />元素的定义中使用了'default-lazy-init'属性,请确信你的各个BeanPostProcessor标记为'lazy-init="false"'。
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; }