【Spring】-循环依赖

要讲清楚三级缓存解决set循环依赖问题先要讲清除两个问题，一是spring-ioc
会存在哪几种循环依赖的情况以及哪种循环依赖是无解的，哪种是框架会帮我们处理的
，二是getBean()这个方法干了什么，以及里面用到的三级缓存是怎么优雅的解决循环依赖的

Spring三种循环依赖

引言：循环依赖就是N个类中循环嵌套引用，如果在日常开发中我们用new 对象的方式发生这种循环依赖的话程序会在运行时一直循环调用，直至内存溢出报错。下面说一下Spring是如果解决循环依赖的。

构造器参数循环依赖

表示通过构造器注入构成的循环依赖，此依赖是无法解决的，只能抛出BeanCurrentlyIn CreationException异常表示循环依赖。
ServiceA依赖了ServiceB,同时ServiceB依赖了ServiceA,并且都是构造器注入的方式(需要两者都是相互构造器注入)

Demo:

public class ServiceA {

    private ServiceB serviceB;

    public ServiceA(ServiceB serviceB) {
        this.serviceB = serviceB;
    }

}

public class ServiceB {

    private ServiceA serviceA;

    public ServiceB(ServiceA serviceA) {
        this.serviceA = serviceA;
    }
  
}

走一下：

@Import({ServiceA.class, ServiceB.class})
public class MainConfig {
    
}

@SpringBootApplication
public class DemoApplication {
	public static void main(String[] args) {
		AnnotationConfigApplicationContext annotation =
				new AnnotationConfigApplicationContext(MainConfig.class);
	}
}

果不其然:

Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'com.springboot.demo.scan.component.ServiceA': Requested bean is currently in creation: Is there an unresolvable circular reference?

why?:
稍后再说吧。。先把另外两种情况再讲下

set单例循环注入

众所周知，如果两个相互依赖的bean有一方是set注入对方的，那么就会成功注入了
Demo:

public class ServiceA {

    @Resource
    private ServiceB serviceB;

    public ServiceB getServiceB() {
        return serviceB;
    }

    public void setServiceB(ServiceB serviceB) {
        this.serviceB = serviceB;
    }
}

public class ServiceB {

    private ServiceA serviceA;

    public ServiceB(ServiceA serviceA) {
        this.serviceA = serviceA;
    }
}

启动和配置bean的代码就不贴了，同上
走一把，你会发现程序正常运行，完全OJBK

set原型方式注入

原型注入的话，循环依赖是一定会报错的
Demo:

@Scope("prototype")
public class ServiceA {

    @Resource
    private ServiceB serviceB;

    public ServiceB getServiceB() {
        return serviceB;
    }

    public void setServiceB(ServiceB serviceB) {
        this.serviceB = serviceB;
    }
}

public class ServiceB {

    private ServiceA serviceA;

    public ServiceB(ServiceA serviceA) {
        this.serviceA = serviceA;
    }
}

run一下:

Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'com.springboot.demo.scan.component.ServiceB': Requested bean is currently in creation: Is there an unresolvable circular reference?

总结一下

1.相互都是构造器注入，Spring解决不了
2.set单例注入，Spring可以解决
3.set原型注入，Spring解决不了