@PostConstruct基本:
@PostConstruct注解好多人以为是Spring提供的。其实是Java自己的注解。
Java中该注解的说明:@PostConstruct该注解被用来修饰一个非静态的void()方法。被@PostConstruct修饰的方法会在服务器加载Servlet的时候运行,并且只会被服务器执行一次。PostConstruct在构造函数之后执行,init()方法之前执行。
通常我们会是在Spring框架中使用到@PostConstruct注解 该注解的方法在整个Bean初始化中的执行顺序:
Constructor(构造方法) -> @Autowired(依赖注入) -> @PostConstruct(注释的方法)
应用:在静态方法中调用依赖注入的Bean中的方法。
package com.example.studySpringBoot.util;
import com.example.studySpringBoot.service.MyMethorClassService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import javax.annotation.PostConstruct;
@Component
public class MyUtils {
private static MyUtils staticInstance = new MyUtils();
@Autowired
private MyMethorClassService myService;
@PostConstruct
public void init(){
staticInstance.myService = myService;
}
public static Integer invokeBean(){
return staticInstance.myService.add(10,20);
}
}
那么Java提供的@PostConstruct注解,Spring是如何实现的呢?
需要先学习下BeanPostProcessor这个接口:
public interface BeanPostProcessor {
/**
* Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean
* initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
* or a custom init-method). The bean will already be populated with property values.
* The returned bean instance may be a wrapper around the original.
*
* 任何Bean实例化,并且Bean已经populated(填充属性) 就会回调这个方法
*
*/
Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;
/**
* Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean
* initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
* or a custom init-method). The bean will already be populated with property values.
* The returned bean instance may be a wrapper around the original.
*
* 任何Bean实例化,并且Bean已经populated(填充属性) 就会回调这个方法
*
*/
Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;
那Spring初始化是那里回调这些方法呢?
AbstractApplicationContext.finishBeanFactoryInitialization(...);
beanFactory.preInstantiateSingletons();
DefaultListableBeanFactory.getBean(beanName);
AbstractBeanFactory.doGetBean();
AbstractAutowireCapableBeanFactory.createBean(....)
populateBean(beanName, mbd, instanceWrapper);
initializeBean(...)
//调用BeanPostProcessor.postProcessBeforeInitialization()方法
applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
//调用BeanPostProcessor.postProcessBeforeInitialization()方法
applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
BeanPostProcessor有个实现类CommonAnnotationBeanPostProcessor,就是专门处理@PostConstruct @PreDestroy注解。
CommonAnnotationBeanPostProcessor的父类InitDestroyAnnotationBeanPostProcessor()
InitDestroyAnnotationBeanPostProcessor.postProcessBeforeInitialization()
InitDestroyAnnotationBeanPostProcessor.findLifecycleMetadata()
// 组装生命周期元数据
InitDestroyAnnotationBeanPostProcessor.buildLifecycleMetadata()
// 查找@PostConstruct注释的方法
InitDestroyAnnotationBeanPostProcessor.initAnnotationType
// 查找@PreDestroy注释方法
InitDestroyAnnotationBeanPostProcessor.destroyAnnotationType
// 反射调用
metadata.invokeInitMethods(bean, beanName);
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