【Spring【IOC】】——10、@PostConstruct注解和@PreDestroy注解_the postconstruct annotation is used on a method t-优快云博客

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在之前的文章中，我们介绍了如何使用@Bean注解指定初始化和销毁的方法，也介绍了使用InitializingBean和DisposableBean来处理bean的初始化和销毁。除此之外，在JDK中还提供了两个注解能够在bean创建完成并且属性赋值完成之后执行一些初始化工作和在容器销毁bean之前通知我们进行一些清理工作。今天，我们就一起来看看这两个注解的用法。

1、@PostConstruct

1.1、源码分析

首先我们看一下@PostConstruct注解他是Java原生的，并不是Spring的，让我们看一下其源码，代码如下所示

/*
 * Copyright (c) 2005, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package javax.annotation;

import java.lang.annotation.*;
import static java.lang.annotation.ElementType.*;
import static java.lang.annotation.RetentionPolicy.*;

/**
 * The PostConstruct annotation is used on a method that needs to be executed
 * after dependency injection is done to perform any initialization. This
 * method MUST be invoked before the class is put into service. This
 * annotation MUST be supported on all classes that support dependency
 * injection. The method annotated with PostConstruct MUST be invoked even
 * if the class does not request any resources to be injected. Only one
 * method can be annotated with this annotation. The method on which the
 * PostConstruct annotation is applied MUST fulfill all of the following
 * criteria:
 * <p>
 * <ul>
 * <li>The method MUST NOT have any parameters except in the case of
 * interceptors in which case it takes an InvocationContext object as
 * defined by the Interceptors specification.</li>
 * <li>The method defined on an interceptor class MUST HAVE one of the
 * following signatures:
 * <p>
 * void &#060;METHOD&#062;(InvocationContext)
 * <p>
 * Object &#060;METHOD&#062;(InvocationContext) throws Exception
 * <p>
 * <i>Note: A PostConstruct interceptor method must not throw application
 * exceptions, but it may be declared to throw checked exceptions including
 * the java.lang.Exception if the same interceptor method interposes on
 * business or timeout methods in addition to lifecycle events. If a
 * PostConstruct interceptor method returns a value, it is ignored by
 * the container.</i>
 * </li>
 * <li>The method defined on a non-interceptor class MUST HAVE the
 * following signature:
 * <p>
 * void &#060;METHOD&#062;()
 * </li>
 * <li>The method on which PostConstruct is applied MAY be public, protected,
 * package private or private.</li>
 * <li>The method MUST NOT be static except for the application client.</li>
 * <li>The method MAY be final.</li>
 * <li>If the method throws an unchecked exception the class MUST NOT be put into
 * service except in the case of EJBs where the EJB can handle exceptions and
 * even recover from them.</li></ul>
 * @since Common Annotations 1.0
 * @see javax.annotation.PreDestroy
 * @see javax.annotation.Resource
 */
@Documented
@Retention (RUNTIME)
@Target(METHOD)
public @interface PostConstruct {
}

@PostConstruct是Java中的注解，不是Spring的注解。
@PostConstruct注解被用来修饰一个非静态的void()方法。被@PostConstruct注解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。被@PostConstruct注解修饰的方法通常在构造函数之后，init()方法之前执行。
通常我们是会在Spring框架中使用到@PostConstruct注解的，该注解的方法在整个bean初始化中的执行顺序如下：

Constructor（构造方法）→@Autowired（依赖注入）→@PostConstruct（注释的方法）

2、@PreDestroy

2.1、源码分析

@PreDestroy注解也并不是Spring的注解，也是Java提供的注解，只不过Spring内部会兼容。我们来看一下其源码：

/*
 * Copyright (c) 2005, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package javax.annotation;

import java.lang.annotation.*;
import static java.lang.annotation.ElementType.*;
import static java.lang.annotation.RetentionPolicy.*;

/**
 * The PreDestroy annotation is used on methods as a callback notification to
 * signal that the instance is in the process of being removed by the
 * container. The method annotated with PreDestroy is typically used to
 * release resources that it has been holding. This annotation MUST be
 * supported by all container managed objects that support PostConstruct
 * except the application client container in Java EE 5. The method on which
 * the PreDestroy annotation is applied MUST fulfill all of the following
 * criteria:
 * <p>
 * <ul>
 * <li>The method MUST NOT have any parameters except in the case of
 * interceptors in which case it takes an InvocationContext object as
 * defined by the Interceptors specification.</li>
 * <li>The method defined on an interceptor class MUST HAVE one of the
 * following signatures:
 * <p>
 * void &#060;METHOD&#062;(InvocationContext)
 * <p>
 * Object &#060;METHOD&#062;(InvocationContext) throws Exception
 * <p>
 * <i>Note: A PreDestroy interceptor method must not throw application
 * exceptions, but it may be declared to throw checked exceptions including
 * the java.lang.Exception if the same interceptor method interposes on
 * business or timeout methods in addition to lifecycle events. If a
 * PreDestroy interceptor method returns a value, it is ignored by
 * the container.</i>
 * </li>
 * <li>The method defined on a non-interceptor class MUST HAVE the
 * following signature:
 * <p>
 * void &#060;METHOD&#062;()
 * </li>
 * <li>The method on which PreDestroy is applied MAY be public, protected,
 * package private or private.</li>
 * <li>The method MUST NOT be static.</li>
 * <li>The method MAY be final.</li>
 * <li>If the method throws an unchecked exception it is ignored except in the
 * case of EJBs where the EJB can handle exceptions.</li>
 * </ul>
 *
 * @see javax.annotation.PostConstruct
 * @see javax.annotation.Resource
 * @since Common Annotations 1.0
 */

@Documented
@Retention (RUNTIME)
@Target(METHOD)
public @interface PreDestroy {
}

被@PreDestroy注解修饰的方法会在服务器卸载Servlet的时候运行，并且只会被服务器调用一次，类似于Servlet的destroy()方法。被@PreDestroy注解修饰的方法会在destroy()方法之后，Servlet被彻底卸载之前执行。执行顺序如下所示：

调用destroy()方法→@PreDestroy→destroy()方法→bean销毁

3、使用

3.1、创建对象

package com.zhz.bean;

import lombok.Data;
import org.springframework.beans.factory.DisposableBean;
import org.springframework.beans.factory.InitializingBean;

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;

/**
 * @author zhouhengzhe
 * @date 2022/11/23
 */
public class PostPerson implements InitializingBean, DisposableBean {

    // 在对象创建完成并且属性赋值完成之后调用
    @PostConstruct
    public void init() {
        System.out.println("构造之后");
    }

    // 在容器销毁（移除）对象之前调用
    @PreDestroy
    public void preDestroy() {
        System.out.println("销毁之前");
    }
    
    @Override
    public void afterPropertiesSet() throws Exception {
        System.out.println("属性赋值");
    }

    @Override
    public void destroy() throws Exception {
        System.out.println("销毁");
    }
}

3.2、创建配置类

package com.zhz.config;

import com.zhz.bean.Person;
import com.zhz.bean.PostPerson;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Scope;

/**
 * 生命周期相关配置
 * @author zhouhengzhe
 * @date 2022/11/21
 */
@Configuration
public class MainConfigByLifeCycle {

    @Bean
    public PostPerson personPerson() {
        return new PostPerson();
    }
}

3.3、测试类


    @Test
    public void test4ByPostConstruct(){
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(MainConfigByLifeCycle.class);
        System.out.println("容器创建完成");
        PostPerson postPerson = applicationContext.getBean(PostPerson.class);
        applicationContext.close();
    }