了解Spring Boot @Autowired

Question

我不明白Spring boots注释是如何@Autowired正确工作的.这是一个简单的例子:

@SpringBootApplication
public class App {
    @Autowired
    public Starter starter;

    public static void main(String[] args) {
        SpringApplication.run(App.class, args);
    }
    public App() {
        System.out.println("init App");
        //starter.init();
    }
}

-

@Repository
public class Starter {
    public Starter() {System.out.println("init Starter");}
    public void init() { System.out.println("call init"); }
}

当我执行这个代码,我得到的日志init App和init Starter,所以春天创建该对象.但是当我从中调用init方法Starter时App,我得到了一个NullPointerException.还有更多我需要使用注释@Autowired来初始化我的对象吗？

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'app': Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [{package}.App$$EnhancerBySpringCGLIB$$87a7ccf]: Constructor threw exception; nested exception is java.lang.NullPointerException

Answer 1

当您init从类的构造函数调用该方法时App,Spring尚未将依赖项自动装入该App对象.如果要在Spring完成创建并自动装配App对象后调用此方法,则添加带@PostConstruct注释的方法来执行此操作,例如:

@SpringBootApplication
public class App {
    @Autowired
    public Starter starter;

    public static void main(String[] args) {
        SpringApplication.run(App.class, args);
    }

    public App() {
        System.out.println("constructor of App");
    }

    @PostConstruct
    public void init() {
        System.out.println("Calling starter.init");
        starter.init();
    }
}