Spring-IOC

循环依赖

• 面试题
  • 解释一下Spring中的三级缓存
  • 三级缓存是什么？三个Map有什么异同
  • 什么是循环依赖？请你谈谈？看过Spring源码吗？一般说的Spring容器是什么？
  • 如何检测是否存在循环依赖？实际开发中见过循环依赖的异常吗？
  • 多例情况下，循环依赖问题为什么无法解决？
• 什么是循环依赖？
  • 多个bean之间互相依赖，形成了一个闭环。比如，A依赖于B，B依赖于C，C依赖于A。（或AB互相依赖）
  • 通常来说，如果问Spring是如何解决循环依赖的问题的，一定是指默认的单例Bean中，属性互相引用的场景。注：定义bean的时候，可以指定scope为singleton/prototype
• 两种注入方式对循环依赖的影响
  • 循环依赖官网说明
  • 结论
    • 只要bean的注入方式是setter且singleton的就不会有循环依赖问题，Spring用三级缓存解决了。DefaultSingletonBeanRegistry
• Spring容器循环依赖报错演示BeanCurrentlyInCreationException

  • 在Spring容器中注入依赖的对象有两种方式

    • constructor

      • code

        @Component
        public class ServiceA {
            private ServiceB serviceB;
            public ServiceA(ServiceB serviceB){
                this.serviceB = serviceB;
            }
        }
        @Component
        public class ServiceB {
            private ServiceA serviceA;
            public ServiceB(ServiceA serviceA){
                this.serviceA = serviceA;
            }
        }
        public class ClientConstructor {
            public static void main(String[] args) {
                //俄罗斯套娃，无穷无尽
                new ServiceA(new ServiceB(new ServiceA(new ServiceB())));
            }
        }
        

      • 结论

        • 构造器循环依赖是无法解决的
    • setter

      • code

        @Component
        public class ServiceA {
            private ServiceB serviceB;
            public void setServiceB(ServiceB serviceB){
                this.serviceB = serviceB;
            }
        }
        @Component
        public class ServiceB {
            private ServiceA serviceA;
            public void setServiceA(ServiceA serviceA){
                this.serviceA = serviceA;
            }
        }
        public class ClientSetter {
            public static void main(String[] args) {
                //成功注入
                ServiceA serviceA = new ServiceA();
                ServiceB serviceB = new ServiceB();
                serviceA.setServiceB(serviceB);
                serviceB.setServiceA(serviceA);
            }
        }
        

  • Spring IOC Case

    • 默认单例（singleton）的场景是支持循环依赖的，不会报错

      • applicationContext.xml

      <?xml version="1.0" encoding="UTF-8"?>
      <beans xmlns="http://www.springframework.org/schema/beans"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://www.springframework.org/schema/beans
            http://www.springframework.org/schema/beans/spring-beans.xsd
            http://www.springframework.org/schema/context
            http://www.springframework.org/schema/context/spring-context.xsd
            http://www.springframework.org/schema/aop
            http://www.springframework.org/schema/aop/spring-aop.xsd
            http://www.springframework.org/schema/tx
            http://www.springframework.org/schema/tx/spring-tx.xsd">
            <bean id="a" class="com.jjc.mypratice.spring.ioc.circulardepend.A">
                <property name="b" ref="b"/>
            </bean>
            <bean id="b" class="com.jjc.mypratice.spring.ioc.circulardepend.B">
                <property name="a" ref="a"/>
            </bean>
      </beans>
      

      • bean

      public class CircularDependBugTest {
          public static void main(String[] args) {
              ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
              A a = applicationContext.getBean("a", A.class);
              B b = applicationContext.getBean("b", B.class);
          }
      }
        
      

      • 执行结果

    • 原型（prototype）的场景是不支持循环依赖的，会报错

      • applicationContext.xml

      <?xml version="1.0" encoding="UTF-8"?>
      <beans xmlns="http://www.springframework.org/schema/beans"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://www.springframework.org/schema/beans
            http://www.springframework.org/schema/beans/spring-beans.xsd
            http://www.springframework.org/schema/context
            http://www.springframework.org/schema/context/spring-context.xsd
            http://www.springframework.org/schema/aop
            http://www.springframework.org/schema/aop/spring-aop.xsd
            http://www.springframework.org/schema/tx
            http://www.springframework.org/schema/tx/spring-tx.xsd">
            <bean id="a" class="com.jjc.mypratice.spring.ioc.circulardepend.A" scope="prototype">
                <property name="b" ref="b"/>
            </bean>
            <bean id="b" class="com.jjc.mypratice.spring.ioc.circulardepend.B" scope="prototype">
                <property name="a" ref="a"/>
            </bean>
      </beans>
      

      • bean

      public class CircularDependBugTest {
          public static void main(String[] args) {
              ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
              A a = applicationContext.getBean("a", A.class);
              B b = applicationContext.getBean("b", B.class);
          }
      }
        
      

      • 执行结果

  • 结论

    • singleton可以解决循环依赖
• 循环依赖debug
  • 实例化/初始化
    • 实例化create
      • 内存中开辟一块空间
      • 类比，租赁/买好房子，还没置办家具/装修
    • 初始化populate
      • 完成属性的赋值
      • 类比，置办家具/装修
  • 3个Map和4个方法
  • A/B对象在三级缓存中的迁移
  • Debug
• 总结----Spring是如何解决循环依赖的
  • 如何解决
  • 流程