@PostConstruct注解詳解

@PostConstruct注解是Java自己的注解。

Java中該注解的說明：@PostConstruct該注解被用來修飾一個非靜態的void（）方法。被@PostConstruct修飾的方法會在服務器加載Servlet的時候運行，並且只會被服務器執行一次。PostConstruct在構造函數之后執行，init（）方法之前執行。

通常我們會是在Spring框架中使用到@PostConstruct注解 該注解的方法在整個Bean初始化中的執行順序：

Constructor(構造方法) -> @Autowired(依賴注入) -> @PostConstruct(注釋的方法)

應用：在靜態方法中調用依賴注入的Bean中的方法。可作為一些數據的常規化加載，比如數據字典之類的。

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是如何實現的呢？

需要先學習下BeanPostProcessor這個接口：

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實例化，並且Bean已經populated(填充屬性) 就會回調這個方法
     *
     */
	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實例化，並且Bean已經populated(填充屬性) 就會回調這個方法
     *
     */
	Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

那Spring初始化是那里回調這些方法呢？

AbstractApplicationContext.finishBeanFactoryInitialization(...);
    beanFactory.preInstantiateSingletons();
       DefaultListableBeanFactory.getBean(beanName);
          AbstractBeanFactory.doGetBean();
            AbstractAutowireCapableBeanFactory.createBean(....)
                populateBean(beanName, mbd, instanceWrapper);
                initializeBean(...)
                 //調用BeanPostProcessor.postProcessBeforeInitialization()方法
                  applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
                 //調用BeanPostProcessor.postProcessBeforeInitialization()方法
                  applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);

BeanPostProcessor有個實現類CommonAnnotationBeanPostProcessor，就是專門處理@PostConstruct @PreDestroy注解。

CommonAnnotationBeanPostProcessor的父類InitDestroyAnnotationBeanPostProcessor()
 InitDestroyAnnotationBeanPostProcessor.postProcessBeforeInitialization()
    InitDestroyAnnotationBeanPostProcessor.findLifecycleMetadata()
        // 組裝生命周期元數據
        InitDestroyAnnotationBeanPostProcessor.buildLifecycleMetadata()
            // 查找@PostConstruct注釋的方法
            InitDestroyAnnotationBeanPostProcessor.initAnnotationType
            // 查找@PreDestroy注釋方法
            InitDestroyAnnotationBeanPostProcessor.destroyAnnotationType
 // 反射調用          
 metadata.invokeInitMethods(bean, beanName);    

站在巨人肩膀上摘蘋果

https://blog.csdn.net/qq360694660/article/details/82877222