@PostConstruct的顺序

其实从依赖注入的字面意思就可以知道，要将对象p注入到对象a，那么首先就必须得生成对象p与对象a，才能执行注入。所以，如果一个类A中有个成员变量p被@Autowired注解，那么@Autowired注入是发生在A的构造方法执行完之后的。
如果想在生成对象时候完成某些初始化操作，而偏偏这些初始化操作又依赖于依赖注入，那么就无法在构造函数中实现。为此，可以使用@PostConstruct注解一个方法来完成初始化，@PostConstruct注解的方法将会在依赖注入完成后被自动调用。
Constructor >> @Autowired >> @PostConstruct

public Class AAA {  
    @Autowired  
    private BBB b;  

    public AAA() {  
        System.out.println("此时b还未被注入: b = " + b);  
    }  

    @PostConstruct  
    private void init() {  
        System.out.println("@PostConstruct将在依赖注入完成后被自动调用: b = " + b);  
    }  
}

• 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