@PostConstruct注解详解

@PostConstruct注解是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。可作为一些数据的常规化加载，比如数据字典之类的。

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);