探秘Spring(2)——PostConstruct注解工作原理解析

提示：本文的代码环境为SpringBoot2.3.4

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前言

Spring已成为java开发web应用必会技术栈，本系列文章着重为大家揭秘spring中的技术点，本文重点介绍Bean实例化之后，依靠注解PostConstruct可以做的一些初始化的工作。

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一、PostConstruct是什么？

从注解的全限定名(javax.annotation.PostConstruct)可以看出这个注解是JDK本身自带的注解，JDK中对该注解有如下的说明

/**
 * 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
 */

翻译成中文的意思就是：

PostConstruct注解作用的方法在在完成依赖注入后会被执行，
用来实现任何初始化操作。该方法必须在这个类的实例对象使用
前被执行。所有支持依赖注入的类都必须支持该注解，即使使用
了该注解的类，不要任何资源的注入，该注解标注的方法也必须
被执行，每个类中只有一个方法可以标注这个注解。使用这个注
解的方法必须完全满足以下的要求：
1、方法不能有任何参数除了在拦截器中定义的方法
2、拦截器中定义的方法必须要使用InvocationContext作为参数
3、非拦截器中定义的方法必须为无参，无返回值
4、给注解可以标注在 public, protected,
  无修饰符, private 修饰的方法
5、方法不能被static修饰除非是客户端应用
6、可是final修饰的方法
7、如果方法抛出了一个不受检(运行时)异常，这个类将不能被使
用，除非是一个EJB项目，EJB可以处理异常，或者修复他们。

二、可以做什么

从上面的英文注释中我们可以发现PostConstruct的作用就是在依赖注入实力化完成后，可以做一些初始化的工作。
下面举个简单的例子：
先看一下配置类,本文中所有的例子都是使用该配置类

@ComponentScan("com.it.sun.course")
@Configuration
public class CommonConfiguration {
   
   
    @Bean()
    public HelloService helloService() {
   
   
        return new HelloService();
    }
}

服务类的代码如下：

@Slf4j
public class HelloService{
   
   

    @Autowired
    private WorldService worldService;

    public HelloService() {
   
   
        System.err.println("initialize HelloService");
    }


    @PostConstruct
    public void postConstruct() {
   
   
        System.out.println(worldService);
        System.err.println("PostConstruct 注解");
    }
}

启动类

public class CourseApplication {
   
   

    public static void main(String[] args) {
   
   
        AnnotationConfigApplicationContext applicationContext =
                new AnnotationConfigApplicationContext(CommonConfiguration.