@PostConstruct

@PostConstruct 是一种初始化方式

以下述代码为例

• TestController中成员变量testService被@Autowired修饰
• TestService将会在TestController构造方法执行后注入

@Component
public class TestService {
    public TestService() {
        System.out.println("TestService construct");
    }
    public void init() {
        System.out.println("TestService init");
    }
}

@RestController
public class TestController {

    @Autowired
    private TestService testService;

    public TestController() {
        System.out.println("TestController construct");
        testService.init();
    }
}

此时启动Spring应用，则会出现报错，其原因就是testService在执行TestController构造方法时并没有完成初始化和注入；
TestController初始化时需要执行依赖中的方法时，这种写法就会引起问题；
此时就可以使用@PostConstruct来解决问题

Caused by: java.lang.NullPointerException: null
at com.example.demo.my.view.TestController.<init>(TestController.java:20) ~[classes/:na]
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:na]
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:na]
at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:na]
at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490) ~[na:na]

调整代码后

@RestController
public class TestController {

    @Autowired
    private TestService testService;

    public TestController() {
        System.out.println("TestController construct");
    }

    @PostConstruct
    public void initBeforeConstruct() {
        System.out.println("TestController PostConstruct");
        testService.init();
    }
}

此时启动输出结果

从中可见总体的执行顺序

1. 构造方法
2. @Autowird
3. @PostConstruct