@lazy注解处理循环注入问题和彻底理解循环依赖底层原理

1.先看如下demo: B和A相互循环依赖

@Component
public class B {
    @Autowired
    private A a;
}

@Component
public class A {
    @Autowired
    private B b;
}

启动项目：结果没有报错：

2. 加入异步逻辑修改

@Component
public class A {
    @Autowired
    private B b;
   @Async
    public void test(){

    }
}

@Component
public class B {
    @Autowired
    private A a;
}
@EnableAsync
public class App {
    public static void main(String[] args) {
        ApplicationContext ctx = new AnnotationConfigApplicationContext(App.class);
    }

}

启动后：

1 Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Bean with name 'a' has been injected into other beans [b] in its raw version as part of a circular reference, but has eventually been wrapped. This means that said other beans do not use the final version of the bean. This is often the result of over-eager type matching - consider using 'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.
2 Exception in thread "main" org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Bean with name 'a' has been injected into other beans [b] in its raw version as part of a circular reference, but has eventually been wrapped. This means that said other beans do not use the final version of the bean. This is often the result of over-eager type matching - consider using 'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.

解决方案：加入lazy注解：

@Component
public class B {
    @Autowired
    @Lazy
    private A a;
}
@Component
public class A {
    @Autowired
    private B b;
   @Async
    public void test(){

    }
}

启动后：没有异常

上面发现使用@Async异步注解，循环依赖就会报错，有可能是因为有了@Async注解修饰的方法，其对应的类被代理了，那代理了就会报错么？我们继续尝试事务注解看看：

@Component
public class A {
    @Autowired
    private B b;
    @Transactional
    public void test(){

    }
}

@Component
public class B {
    @Autowired
    private A a;
}
@EnableTransactionManagement
public class App {
    public static void main(String[] args) {
        ApplicationContext ctx = new AnnotationConfigApplicationContext(App.class);
    }

}