Spring/SpringBoot--循环依赖--解决方式/如何解决--实例

红太狼 2022-09-12 05:48 496阅读 0赞

原文网址：[Spring/SpringBoot--循环依赖--解决方式/如何解决--实例\_IT利刃出鞘的博客-CSDN博客][Spring_SpringBoot--_--_--_IT_-CSDN]

# 简介 #

**说明**

本文介绍Spring的循环依赖什么时候会出现以及如何解决。

**循环依赖场景**

1.  单例field的循环依赖 （采用三级缓存+提前暴露对象的方法解决）
2.  构造器的循环依赖 （无法解决循环依赖问题）
3.  多例field的循环依赖 （无法解决循环依赖问题）
    
    1.  对于多例的bean，Spring不缓存"prototype"作用域的bean，不提前暴露一个创建中的bean。

# 问题复现 #

公共代码

package com.example.controller;
    
    import com.example.tmp.A;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.web.bind.annotation.GetMapping;
    import org.springframework.web.bind.annotation.RestController;
    
    @RestController
    public class HelloController {
        @Autowired
        A a;
    
        @GetMapping("/test1")
        public String test1() {
            return a.getTest();
        }
    }

## 1.构造器注入 ##

**代码**

package com.example.tmp;
     
    import org.springframework.stereotype.Component;
     
    @Component
    public class A {
        // @Autowired
        B b;
    
        public A(B b){
            this.b = b;
        }
     
        private String name = "Tony";
     
        public String getName() {
            return name;
        }
     
        public void setName(String name) {
            this.name = name;
        }
     
        public String getTest() {
            return b.getAge().toString() + name;
        }
    }

package com.example.tmp;
     
    import org.springframework.stereotype.Component;
     
    @Component
    public class B {
        // @Autowired
        A a;
    
        public B(A a) {
            this.a = a;
        }
     
        private Integer age = 20;
     
        public Integer getAge() {
            return age;
        }
     
        public void setAge(Integer age) {
            this.age = age;
        }
    }

**测试**

**启动结果**

.   ____          _            __ _ _
     /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
    ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
     \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
      '  |____| .__|_| |_|_| |_\__, | / / / /
     =========|_|==============|___/=/_/_/_/
     :: Spring Boot ::        (v2.3.0.RELEASE)
    
    2021-04-03 10:40:18.732  INFO 5872 --- [           main] com.example.DemoApplication              : Starting DemoApplication on DESKTOP-QI6B9ME with PID 5872 (E:\work\Idea_proj\demo_JAVA\demo_SpringBoot\target\classes started by Liu in E:\work\Idea_proj\demo_JAVA\demo_SpringBoot)
    2021-04-03 10:40:18.748  INFO 5872 --- [           main] com.example.DemoApplication              : No active profile set, falling back to default profiles: default
    2021-04-03 10:40:20.904  INFO 5872 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
    2021-04-03 10:40:20.920  INFO 5872 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
    2021-04-03 10:40:20.920  INFO 5872 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.35]
    2021-04-03 10:40:20.998  INFO 5872 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
    2021-04-03 10:40:20.998  INFO 5872 --- [           main] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 2219 ms
    2021-04-03 10:40:24.436  WARN 5872 --- [           main] ConfigServletWebServerApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'helloController': Unsatisfied dependency expressed through field 'a'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'a' defined in file [E:\work\Idea_proj\demo_JAVA\demo_SpringBoot\target\classes\com\example\tmp\A.class]: Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'b' defined in file [E:\work\Idea_proj\demo_JAVA\demo_SpringBoot\target\classes\com\example\tmp\B.class]: Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
    2021-04-03 10:40:24.451  INFO 5872 --- [           main] o.apache.catalina.core.StandardService   : Stopping service [Tomcat]
    2021-04-03 10:40:24.451  INFO 5872 --- [           main] ConditionEvaluationReportLoggingListener : 
    
    Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
    2021-04-03 10:40:24.467 ERROR 5872 --- [           main] o.s.b.d.LoggingFailureAnalysisReporter   : 
    
    ***************************
    APPLICATION FAILED TO START
    ***************************
    
    Description:
    
    The dependencies of some of the beans in the application context form a cycle:
    
       helloController (field com.example.tmp.A com.example.controller.HelloController.a)
    ┌─────┐
    |  a defined in file [E:\work\Idea_proj\demo_JAVA\demo_SpringBoot\target\classes\com\example\tmp\A.class]
    ↑     ↓
    |  b defined in file [E:\work\Idea_proj\demo_JAVA\demo_SpringBoot\target\classes\com\example\tmp\B.class]
    └─────┘
    
    
    2021-04-03 10:40:24.467  WARN 5872 --- [           main] o.s.boot.SpringApplication               : Unable to close ApplicationContext
    
    org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'springApplicationAdminRegistrar' defined in class path resource [org/springframework/boot/autoconfigure/admin/SpringApplicationAdminJmxAutoConfiguration.class]: Unsatisfied dependency expressed through method 'springApplicationAdminRegistrar' parameter 1; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.core.env.Environment' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}
    	at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:798) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:539) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1338) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1177) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:557) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:226) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:207) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.event.AbstractApplicationEventMulticaster.retrieveApplicationListeners(AbstractApplicationEventMulticaster.java:245) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.event.AbstractApplicationEventMulticaster.getApplicationListeners(AbstractApplicationEventMulticaster.java:197) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:134) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:403) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:360) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.boot.availability.AvailabilityChangeEvent.publish(AvailabilityChangeEvent.java:81) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.availability.AvailabilityChangeEvent.publish(AvailabilityChangeEvent.java:67) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.doClose(ServletWebServerApplicationContext.java:167) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.context.support.AbstractApplicationContext.close(AbstractApplicationContext.java:978) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.boot.SpringApplication.handleRunFailure(SpringApplication.java:814) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.SpringApplication.run(SpringApplication.java:325) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1237) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at com.example.DemoApplication.main(DemoApplication.java:10) [classes/:na]
    Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.core.env.Environment' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}
    	at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1716) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1272) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1226) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:885) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:789) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	... 23 common frames omitted

## 2.Feild注入多例（@AutoWired） ##

本处将A类设置为多例，将B类设置为单例。（其实，都设为多例或者A类设置为单例，B类设置为多例都是一样的，都会报循环依赖错误）

**代码**

package com.example.tmp;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.beans.factory.config.ConfigurableBeanFactory;
    import org.springframework.context.annotation.Scope;
    import org.springframework.stereotype.Component;
     
    @Component
    @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    public class A {
        @Autowired
        B b;
     
        private String name = "Tony";
     
        public String getName() {
            return name;
        }
     
        public void setName(String name) {
            this.name = name;
        }
     
        public String getTest() {
            return b.getAge().toString() + name;
        }
    }

**测试**

启动结果

.   ____          _            __ _ _
     /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
    ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
     \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
      '  |____| .__|_| |_|_| |_\__, | / / / /
     =========|_|==============|___/=/_/_/_/
     :: Spring Boot ::        (v2.3.0.RELEASE)
    
    2021-04-03 11:09:09.781  INFO 12048 --- [           main] com.example.DemoApplication              : Starting DemoApplication on DESKTOP-QI6B9ME with PID 12048 (E:\work\Idea_proj\demo_JAVA\demo_SpringBoot\target\classes started by Liu in E:\work\Idea_proj\demo_JAVA\demo_SpringBoot)
    2021-04-03 11:09:09.783  INFO 12048 --- [           main] com.example.DemoApplication              : No active profile set, falling back to default profiles: default
    2021-04-03 11:09:10.555  INFO 12048 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
    2021-04-03 11:09:10.562  INFO 12048 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
    2021-04-03 11:09:10.562  INFO 12048 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.35]
    2021-04-03 11:09:10.650  INFO 12048 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
    2021-04-03 11:09:10.650  INFO 12048 --- [           main] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 822 ms
    2021-04-03 11:09:10.698  WARN 12048 --- [           main] ConfigServletWebServerApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'helloController': Unsatisfied dependency expressed through field 'a'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'a': Unsatisfied dependency expressed through field 'b'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'b': Unsatisfied dependency expressed through field 'a'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
    2021-04-03 11:09:10.700  INFO 12048 --- [           main] o.apache.catalina.core.StandardService   : Stopping service [Tomcat]
    2021-04-03 11:09:10.710  INFO 12048 --- [           main] ConditionEvaluationReportLoggingListener : 
    
    Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
    2021-04-03 11:09:10.712 ERROR 12048 --- [           main] o.s.b.d.LoggingFailureAnalysisReporter   : 
    
    ***************************
    APPLICATION FAILED TO START
    ***************************
    
    Description:
    
    The dependencies of some of the beans in the application context form a cycle:
    
       helloController (field com.example.tmp.A com.example.controller.HelloController.a)
    ┌─────┐
    |  a (field com.example.tmp.B com.example.tmp.A.b)
    ↑     ↓
    |  b (field com.example.tmp.A com.example.tmp.B.a)
    └─────┘
    
    
    2021-04-03 11:09:10.717  WARN 12048 --- [           main] o.s.boot.SpringApplication               : Unable to close ApplicationContext
    
    org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'springApplicationAdminRegistrar' defined in class path resource [org/springframework/boot/autoconfigure/admin/SpringApplicationAdminJmxAutoConfiguration.class]: Unsatisfied dependency expressed through method 'springApplicationAdminRegistrar' parameter 1; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.core.env.Environment' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}
    	at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:798) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:539) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1338) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1177) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:557) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:226) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:207) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.event.AbstractApplicationEventMulticaster.retrieveApplicationListeners(AbstractApplicationEventMulticaster.java:245) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.event.AbstractApplicationEventMulticaster.getApplicationListeners(AbstractApplicationEventMulticaster.java:197) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:134) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:403) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:360) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.boot.availability.AvailabilityChangeEvent.publish(AvailabilityChangeEvent.java:81) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.availability.AvailabilityChangeEvent.publish(AvailabilityChangeEvent.java:67) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.doClose(ServletWebServerApplicationContext.java:167) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.context.support.AbstractApplicationContext.close(AbstractApplicationContext.java:978) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.boot.SpringApplication.handleRunFailure(SpringApplication.java:814) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.SpringApplication.run(SpringApplication.java:325) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1237) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at com.example.DemoApplication.main(DemoApplication.java:10) [classes/:na]
    Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.core.env.Environment' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}
    	at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1716) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1272) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1226) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:885) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:789) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	... 23 common frames omitted

## 3.Setter注入多例（@AutoWired） ##

本处将A类设置为多例，将B类设置为单例。（其实，都设为多例或者A类设置为单例，B类设置为多例都是一样的，都会报循环依赖错误）

**代码**

package com.example.tmp;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.beans.factory.config.ConfigurableBeanFactory;
    import org.springframework.context.annotation.Scope;
    import org.springframework.stereotype.Component;
     
    @Component
    @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    public class A {
        B b;
     
        private String name = "Tony";
     
        public String getName() {
            return name;
        }
     
        public void setName(String name) {
            this.name = name;
        }
     
        public String getTest() {
            return b.getAge().toString() + name;
        }
    
        public B getB() {
            return b;
        }
    
        @Autowired
        public void setB(B b) {
            this.b = b;
        }
    }

package com.example.tmp;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
     
    @Component
    public class B {
        A a;
     
        private Integer age = 20;
     
        public Integer getAge() {
            return age;
        }
     
        public void setAge(Integer age) {
            this.age = age;
        }
    
        public A getA() {
            return a;
        }
    
        @Autowired
        public void setA(A a) {
            this.a = a;
        }
    }

**测试**

启动结果

.   ____          _            __ _ _
     /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
    ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
     \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
      '  |____| .__|_| |_|_| |_\__, | / / / /
     =========|_|==============|___/=/_/_/_/
     :: Spring Boot ::        (v2.3.0.RELEASE)
    
    2021-04-03 11:11:45.352  INFO 5928 --- [           main] com.example.DemoApplication              : Starting DemoApplication on DESKTOP-QI6B9ME with PID 5928 (E:\work\Idea_proj\demo_JAVA\demo_SpringBoot\target\classes started by Liu in E:\work\Idea_proj\demo_JAVA\demo_SpringBoot)
    2021-04-03 11:11:45.354  INFO 5928 --- [           main] com.example.DemoApplication              : No active profile set, falling back to default profiles: default
    2021-04-03 11:11:46.169  INFO 5928 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
    2021-04-03 11:11:46.176  INFO 5928 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
    2021-04-03 11:11:46.176  INFO 5928 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.35]
    2021-04-03 11:11:46.255  INFO 5928 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
    2021-04-03 11:11:46.255  INFO 5928 --- [           main] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 860 ms
    2021-04-03 11:11:46.298  WARN 5928 --- [           main] ConfigServletWebServerApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'helloController': Unsatisfied dependency expressed through field 'a'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'a': Unsatisfied dependency expressed through method 'setB' parameter 0; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'b': Unsatisfied dependency expressed through method 'setA' parameter 0; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
    2021-04-03 11:11:46.300  INFO 5928 --- [           main] o.apache.catalina.core.StandardService   : Stopping service [Tomcat]
    2021-04-03 11:11:46.316  INFO 5928 --- [           main] ConditionEvaluationReportLoggingListener : 
    
    Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
    2021-04-03 11:11:46.318 ERROR 5928 --- [           main] o.s.b.d.LoggingFailureAnalysisReporter   : 
    
    ***************************
    APPLICATION FAILED TO START
    ***************************
    
    Description:
    
    The dependencies of some of the beans in the application context form a cycle:
    
       helloController (field com.example.tmp.A com.example.controller.HelloController.a)
    ┌─────┐
    |  a
    ↑     ↓
    |  b
    └─────┘
    
    
    2021-04-03 11:11:46.326  WARN 5928 --- [           main] o.s.boot.SpringApplication               : Unable to close ApplicationContext
    
    org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'springApplicationAdminRegistrar' defined in class path resource [org/springframework/boot/autoconfigure/admin/SpringApplicationAdminJmxAutoConfiguration.class]: Unsatisfied dependency expressed through method 'springApplicationAdminRegistrar' parameter 1; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.core.env.Environment' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}
    	at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:798) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:539) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1338) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1177) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:557) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:226) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:207) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.event.AbstractApplicationEventMulticaster.retrieveApplicationListeners(AbstractApplicationEventMulticaster.java:245) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.event.AbstractApplicationEventMulticaster.getApplicationListeners(AbstractApplicationEventMulticaster.java:197) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:134) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:403) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:360) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.boot.availability.AvailabilityChangeEvent.publish(AvailabilityChangeEvent.java:81) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.availability.AvailabilityChangeEvent.publish(AvailabilityChangeEvent.java:67) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.doClose(ServletWebServerApplicationContext.java:167) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.context.support.AbstractApplicationContext.close(AbstractApplicationContext.java:978) ~[spring-context-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.boot.SpringApplication.handleRunFailure(SpringApplication.java:814) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.SpringApplication.run(SpringApplication.java:325) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1237) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) [spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    	at com.example.DemoApplication.main(DemoApplication.java:10) [classes/:na]
    Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.core.env.Environment' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}
    	at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1716) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1272) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1226) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:885) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:789) ~[spring-beans-5.2.6.RELEASE.jar:5.2.6.RELEASE]
    	... 23 common frames omitted

# 解决方案 #

公共代码

## 1.Feild注入单例（@AutoWired） ##

**代码**

package com.example.tmp;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
     
    @Component
    public class A {
        @Autowired
        B b;
     
        private String name = "Tony";
     
        public String getName() {
            return name;
        }
     
        public void setName(String name) {
            this.name = name;
        }
     
        public String getTest() {
            return b.getAge().toString() + name;
        }
    }

**测试**

> 启动不报错。

> postman访问：[http://localhost:8080/test1][http_localhost_8080_test1]
> 
> 后端结果：不报错
> 
> postman结果: 20Tony

## 2.构造器注入+@Lazy ##

延迟加载：在注入依赖时，先注入代理对象，当首次使用时再创建对象完成注入。

**代码**

package com.example.tmp;
    
    import org.springframework.context.annotation.Lazy;
    import org.springframework.stereotype.Component;
     
    @Component
    public class A {
        B b;
    
        public A(@Lazy B b) {
            this.b = b;
        }
     
        private String name = "Tony";
     
        public String getName() {
            return name;
        }
     
        public void setName(String name) {
            this.name = name;
        }
     
        public String getTest() {
            return b.getAge().toString() + name;
        }
    }

package com.example.tmp;
    
    import org.springframework.context.annotation.Lazy;
    import org.springframework.stereotype.Component;
     
    @Component
    public class B {
        A a;
    
        public B(@Lazy A a) {
            this.a = a;
        }
     
        private Integer age = 20;
     
        public Integer getAge() {
            return age;
        }
     
        public void setAge(Integer age) {
            this.age = age;
        }
    
    }

**测试**

> 启动不报错。

> postman访问：[http://localhost:8080/test1][http_localhost_8080_test1]
> 
> 后端结果：不报错
> 
> postman结果: 20Tony

## 3.Setter/Field注入单例 ##

**代码**

package com.example.tmp;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
     
    @Component
    public class A {
        B b;
     
        private String name = "Tony";
     
        public String getName() {
            return name;
        }
     
        public void setName(String name) {
            this.name = name;
        }
     
        public String getTest() {
            return b.getAge().toString() + name;
        }
    
        public B getB() {
            return b;
        }
    
        @Autowired
        public void setB(B b) {
            this.b = b;
        }
    }

package com.example.tmp;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
     
    @Component
    public class B {
        A a;
    
        private Integer age = 20;
     
        public Integer getAge() {
            return age;
        }
     
        public void setAge(Integer age) {
            this.age = age;
        }
    
        public A getA() {
            return a;
        }
    
        @Autowired
        public void setA(A a) {
            this.a = a;
        }
    }

**测试**

> 启动不报错。

> postman访问：[http://localhost:8080/test1][http_localhost_8080_test1]
> 
> 后端结果：不报错
> 
> postman结果: 20Tony

## 4.@PostConstruct ##

**代码**

package com.example.tmp;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
    
    import javax.annotation.PostConstruct;
    
    @Component
    public class A {
        @Autowired
        B b;
    
        @PostConstruct
        public void init() {
            b.setA(this);
        }
     
        private String name = "Tony";
     
        public String getName() {
            return name;
        }
     
        public void setName(String name) {
            this.name = name;
        }
     
        public String getTest() {
            return b.getAge().toString() + name;
        }
    }

package com.example.tmp;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
     
    @Component
    public class B {
        @Autowired
        A a;
    
        private Integer age = 20;
     
        public Integer getAge() {
            return age;
        }
     
        public void setAge(Integer age) {
            this.age = age;
        }
    
        public A getA() {
            return a;
        }
    
        public void setA(A a) {
            this.a = a;
        }
    }

**测试**

> 启动不报错。

> postman访问：[http://localhost:8080/test1][http_localhost_8080_test1]
> 
> 后端结果：不报错
> 
> postman结果: 20Tony

## 5.实现ApplicationContextAware与InitializingBean ##

**代码**

package com.example.tmp;
    
    import org.springframework.beans.BeansException;
    import org.springframework.beans.factory.InitializingBean;
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.ApplicationContextAware;
    import org.springframework.stereotype.Component;
    
    @Component
    public class A implements ApplicationContextAware, InitializingBean {
        B b;
    
        private ApplicationContext context;
    
        private String name = "Tony";
     
        public String getName() {
            return name;
        }
     
        public void setName(String name) {
            this.name = name;
        }
     
        public String getTest() {
            return b.getAge().toString() + name;
        }
    
        @Override
        public void afterPropertiesSet() throws Exception {
            this.b = context.getBean(B.class);
        }
    
        @Override
        public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
            this.context = applicationContext;
        }
    }

**测试**

> 启动不报错。

> postman访问：[http://localhost:8080/test1][http_localhost_8080_test1]
> 
> 后端结果：不报错
> 
> postman结果: 20Tony

# 其他网址 #

[Spring--原理--循环依赖--@Autowired\_IT利刃出鞘的博客-CSDN博客][Spring--_--_--_Autowired_IT_-CSDN]

《Spring源码深度解析》=> 5.6 循环依赖

[Spring_SpringBoot--_--_--_IT_-CSDN]: https://knife.blog.csdn.net/article/details/120407487
[http_localhost_8080_test1]: http://localhost:8080/test1
[Spring--_--_--_Autowired_IT_-CSDN]: https://knife.blog.csdn.net/article/details/108711870