Spring@PostConstruct注解和构造方法的调用顺序

先看下@PostConstruct的注解

 * 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 -

自己翻译一下，意思是：

PostConstruct注解用于方法上，该方法在初始化的依赖注入操作之后被执行。这个方法必须在class被放到service之后被执行，这个注解所在的类必须支持依赖注入。

父类class，被@component注解修饰，说明会被spring扫描并创建。在默认构造方法里加上输出打印，init方法被@PostConstruct修饰

@Component
public class ParentBean implements InitializingBean{
    
    public ParentBean() {
        System.out.println("ParentBean construct");
    }
    
    @PostConstruct
    public void init(){
        System.out.println("ParentBean init");
    }

    public void afterPropertiesSet() throws Exception {
        System.out.println("ParentBean afterPropertiesSet");
    }

}

子类class，也被 @component注解修饰，其余配置和父类class一样

@Component
public class SonBean extends ParentBean{
    public SonBean() {
        System.out.println("SonBean construct");
    }
    
    @PostConstruct
    public void init(){
        System.out.println("SonBean init");
    }
    @Override
    public void afterPropertiesSet() throws Exception {
        System.out.println("SonBean afterPropertiesSet");
    }
    
}

然后我们使用maven命令 jetty:run，控制台输出如下：

[INFO] Initializing Spring root WebApplicationContext
ParentBean construct
ParentBean init
ParentBean afterPropertiesSet
ParentBean construct
SonBean construct
SonBean init
SonBean afterPropertiesSet
[INFO] Set web app root system property: 'webapp.root' = [J:\androidworkspace\com\src\main\webapp\]
[INFO] Initializing log4j from [classpath:log4j.properties]
[INFO] Started SelectChannelConnector@0.0.0.0:8088
[INFO] Started Jetty Server
[INFO] Starting scanner at interval of 3 seconds.

可以看出优先执行依然是构造方法，这个是java的语言决定的，毕竟spring只是建立在java之上的框架。然后才是被PostConstruct修饰的方法，要注意的是这个方法在对象的初始化和依赖都完成之后才会执行，所以不必担心执行这个方法的时候有个别成员属性没有被初始化为null的情况发生。在init方法之后执行的才是afterPropertiesSet方法，这个方法必须实现InitializingBean接口，这个接口很多spring的bean都实现了他，从他的方法名就能看出在属性都被设置了之后执行，也属于springbean初始化方法。

其实spring提供了很多接口以供开发者在bean初始化后调用，可以在官网上查阅相关文档。

参考：https://my.oschina.net/wwwd/blog/810530