一般场景是一个Bean A依赖Bean B,而Bean B也依赖Bean A ：Bean A → Bean B → Bean A当然我们也可以添加更多的依赖层次，比如：Bean A → Bean B → Bean C → Bean D → Bean E → Bean A

(资料图)

当Spring上下文在加载所有的bean时，他会尝试按照他们他们关联关系的顺序进行创建。比如，如果不存在循环依赖时，例如：Bean A → Bean B → Bean CSpring会先创建Bean C，再创建Bean B（并将Bean C注入到Bean B中），最后再创建Bean A(并将Bean B注入到Bean A中)。但是，如果我们存在循环依赖，Spring上下文不知道应该先创建哪个Bean,因为它们依赖于彼此。在这种情况下，Spring会在加载上下文时，抛出一个BeanCurrentlyInCreationException。当我们使用构造方法进行注入时，就会遇到这种情况。如果您使用其它类型的注入，你应该不会遇到这个问题。因为它是在需要时才会被注入，而不是上下文加载被要求注入。

我们定义两个Bean并且互相依赖（通过构造函数注入）。

package com.test.demo.service;import org.springframework.stereotype.Component;import javax.annotation.Resource;@Componentpublic class A {    private B b;    public A (B b) {        this.b = b;    }}

package com.test.demo.service;import org.springframework.stereotype.Component;import javax.annotation.Resource;@Componentpublic class B {    private A a ;    public B(A a) {        this.a = a;    }}

这个时候我们我们启动项目就会发现项目启动不起来，如下提示信息

/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/bin/java -XX:TieredStopAtLevel=1 -noverify -Dspring.output.ansi.enabled=always -Dcom.sun.management.jmxremote -Dspring.jmx.enabled=true -Dspring.liveBeansView.mbeanDomain -Dspring.application.admin.enabled=true -javaagent:/private/var/folders/03/_9q6fw895913yznz6wq544_00000gn/T/AppTranslocation/332DF0A0-AF1B-49F1-8739-1BF4A9F4BAF7/d/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=50456:/private/var/folders/03/_9q6fw895913yznz6wq544_00000gn/T/AppTranslocation/332DF0A0-AF1B-49F1-8739-1BF4A9F4BAF7/d/IntelliJ IDEA.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/tools.jar:/Users/haolonglong/IdeaProjects/project_boot/target/classes:/Users/haolonglong/.m2/repository/org/springframework/boot/spring-boot-starter-web/2.4.0-SNAPSHOT/spring-boot-starter-web-2.4.0-20200709.121446-178.jar:/Users/haolonglong/.m2/repository/org/springframework/boot/spring-boot-starter-json/2.4.0-SNAPSHOT/spring-boot-starter-json-2.4.0-20200709.121446-178.jar:/Users/haolonglong/.m2/repository/com/fasterxml/jackson/core/jackson-databind/2.11.1/jackson-databind-2.11.1.jar:/Users/haolonglong/.m2/repository/com/fasterxml/jackson/core/jackson-annotations/2.11.1/jackson-annotations-2.11.1.jar:/Users/haolonglong/.m2/repository/com/fasterxml/jackson/core/jackson-core/2.11.1/jackson-core-2.11.1.jar:/Users/haolonglong/.m2/repository/com/fasterxml/jackson/datatype/jackson-datatype-jdk8/2.11.1/jackson-datatype-jdk8-2.11.1.jar:/Users/haolonglong/.m2/repository/com/fasterxml/jackson/datatype/jackson-datatype-jsr310/2.11.1/jackson-datatype-jsr310-2.11.1.jar:/Users/haolonglong/.m2/repository/com/fasterxml/jackson/module/jackson-module-parameter-names/2.11.1/jackson-module-parameter-names-2.11.1.jar:/Users/haolonglong/.m2/repository/org/springframework/boot/spring-boot-starter-tomcat/2.4.0-SNAPSHOT/spring-boot-starter-tomcat-2.4.0-20200709.121446-178.jar:/Users/haolonglong/.m2/repository/org/apache/tomcat/embed/tomcat-embed-core/9.0.36/tomcat-embed-core-9.0.36.jar:/Users/haolonglong/.m2/repository/org/glassfish/jakarta.el/3.0.3/jakarta.el-3.0.3.jar:/Users/haolonglong/.m2/repository/org/apache/tomcat/embed/tomcat-embed-websocket/9.0.36/tomcat-embed-websocket-9.0.36.jar:/Users/haolonglong/.m2/repository/org/springframework/spring-web/5.3.0-M1/spring-web-5.3.0-M1.jar:/Users/haolonglong/.m2/repository/org/springframework/spring-beans/5.3.0-M1/spring-beans-5.3.0-M1.jar:/Users/haolonglong/.m2/repository/org/springframework/spring-webmvc/5.3.0-M1/spring-webmvc-5.3.0-M1.jar:/Users/haolonglong/.m2/repository/org/springframework/spring-aop/5.3.0-M1/spring-aop-5.3.0-M1.jar:/Users/haolonglong/.m2/repository/org/springframework/spring-context/5.3.0-M1/spring-context-5.3.0-M1.jar:/Users/haolonglong/.m2/repository/org/springframework/spring-expression/5.3.0-M1/spring-expression-5.3.0-M1.jar:/Users/haolonglong/.m2/repository/org/springframework/boot/spring-boot-starter/2.4.0-SNAPSHOT/spring-boot-starter-2.4.0-20200709.121446-178.jar:/Users/haolonglong/.m2/repository/org/springframework/boot/spring-boot/2.4.0-SNAPSHOT/spring-boot-2.4.0-20200709.121446-178.jar:/Users/haolonglong/.m2/repository/org/springframework/boot/spring-boot-autoconfigure/2.4.0-SNAPSHOT/spring-boot-autoconfigure-2.4.0-20200709.121446-178.jar:/Users/haolonglong/.m2/repository/org/springframework/boot/spring-boot-starter-logging/2.4.0-SNAPSHOT/spring-boot-starter-logging-2.4.0-20200709.121446-178.jar:/Users/haolonglong/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar:/Users/haolonglong/.m2/repository/ch/qos/logback/logback-core/1.2.3/logback-core-1.2.3.jar:/Users/haolonglong/.m2/repository/org/apache/logging/log4j/log4j-to-slf4j/2.13.3/log4j-to-slf4j-2.13.3.jar:/Users/haolonglong/.m2/repository/org/apache/logging/log4j/log4j-api/2.13.3/log4j-api-2.13.3.jar:/Users/haolonglong/.m2/repository/org/slf4j/jul-to-slf4j/1.7.30/jul-to-slf4j-1.7.30.jar:/Users/haolonglong/.m2/repository/jakarta/annotation/jakarta.annotation-api/1.3.5/jakarta.annotation-api-1.3.5.jar:/Users/haolonglong/.m2/repository/org/springframework/spring-core/5.3.0-M1/spring-core-5.3.0-M1.jar:/Users/haolonglong/.m2/repository/org/springframework/spring-jcl/5.3.0-M1/spring-jcl-5.3.0-M1.jar:/Users/haolonglong/.m2/repository/org/yaml/snakeyaml/1.26/snakeyaml-1.26.jar:/Users/haolonglong/.m2/repository/org/slf4j/slf4j-api/1.7.30/slf4j-api-1.7.30.jar com.test.demo.DemoApplication  .   ____          _            __ _ _ /\\ / ___"_ __ _ _(_)_ __  __ _ \ \ \ \( ( )\___ | "_ | "_| | "_ \/ _` | \ \ \ \ \\/  ___)| |_)| | | | | || (_| |  ) ) ) )  "  |____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot ::       (v2.4.0-SNAPSHOT)2020-07-10 08:01:20.125  INFO 879 --- [           main] com.test.demo.DemoApplication            : Starting DemoApplication using Java 1.8.0_211 on localhost with PID 879 (/Users/haolonglong/IdeaProjects/project_boot/target/classes started by haolonglong in /Users/haolonglong/IdeaProjects/project_boot)2020-07-10 08:01:20.128  INFO 879 --- [           main] com.test.demo.DemoApplication            : No active profile set, falling back to default profiles: default2020-07-10 08:01:21.143  INFO 879 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)2020-07-10 08:01:21.155  INFO 879 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]2020-07-10 08:01:21.155  INFO 879 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.36]2020-07-10 08:01:21.226  INFO 879 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext2020-07-10 08:01:21.226  INFO 879 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 1051 ms2020-07-10 08:01:21.263  WARN 879 --- [           main] ConfigServletWebServerApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name "a" defined in file [/Users/haolonglong/IdeaProjects/project_boot/target/classes/com/test/demo/service/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 [/Users/haolonglong/IdeaProjects/project_boot/target/classes/com/test/demo/service/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?2020-07-10 08:01:21.265  INFO 879 --- [           main] o.apache.catalina.core.StandardService   : Stopping service [Tomcat]2020-07-10 08:01:21.276  INFO 879 --- [           main] ConditionEvaluationReportLoggingListener : Error starting ApplicationContext. To display the conditions report re-run your application with "debug" enabled.2020-07-10 08:01:21.278 ERROR 879 --- [           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:┌─────┐|  a defined in file [/Users/haolonglong/IdeaProjects/project_boot/target/classes/com/test/demo/service/A.class]↑     ↓|  b defined in file [/Users/haolonglong/IdeaProjects/project_boot/target/classes/com/test/demo/service/B.class]└─────┘Process finished with exit code 1

当你有一个循环依赖，很可能你有一个设计问题并且各责任没有得到很好的分离。你应该尽量正确地重新设计组件，以便它们的层次是精心设计的，也没有必要循环依赖。如果不能重新设计组件（可能有很多的原因：遗留代码，已经被测试并不能修改代码，没有足够的时间或资源来完全重新设计......），但有一些变通方法来解决这个问题。

解决Spring 循环依赖的一个简单方法就是对一个Bean使用延时加载。也就是说：这个Bean并没有完全的初始化完，实际上他注入的是一个代理，只有当他首次被使用的时候才会被完全的初始化。

package com.test.demo.service;import org.springframework.context.annotation.Lazy;import org.springframework.stereotype.Component;import javax.annotation.Resource;@Componentpublic class A {    private B b;    public A (@Lazy B b) {        this.b = b;    }}

其中最流行的解决方法，就是Spring文档中建议，使用setter注入。简单地说，你对你须要注入的bean是使用setter注入（或字段注入），而不是构造函数注入。通过这种方式创建Bean，实际上它此时的依赖并没有被注入，只有在你须要的时候他才会被注入进来。

package com.test.demo.service;import org.springframework.context.annotation.Lazy;import org.springframework.stereotype.Component;import javax.annotation.Resource;@Componentpublic class A {    private B b;    public void setB(B b) {        this.b = b;    }    public B getB() {        return b;    }}

package com.test.demo.service;import org.springframework.stereotype.Component;import javax.annotation.Resource;@Componentpublic class B {    private A a ;    public void setA(A a) {        this.a = a;    }    public A getA() {        return a;    }}

打破循环的另一种方式是,在要注入的属性（该属性是一个bean）上使用 @Autowired ,并使用@PostConstruct 标注在另一个方法，且该方法里设置对其他的依赖。我们的Bean将修改成下面的代码：

如果一个Bean实现了ApplicationContextAware，该Bean可以访问Spring上下文，并可以从那里获取到其他的bean。实现InitializingBean接口，表明这个bean在所有的属性设置完后做一些后置处理操作（调用的顺序为init-method后调用）;在这种情况下，我们需要手动设置依赖。

@Componentpublic class CircularDependencyA implements ApplicationContextAware, InitializingBean {     private CircularDependencyB circB;     private ApplicationContext context;     public CircularDependencyB getCircB() {        return circB;    }     @Override    public void afterPropertiesSet() throws Exception {        circB = context.getBean(CircularDependencyB.class);    }     @Override    public void setApplicationContext(final ApplicationContext ctx) throws BeansException {        context = ctx;    }}

public class CircularDependencyB {     private CircularDependencyA circA;     private String message = "Hi!";     @Autowired    public void setCircA(CircularDependencyA circA) {        this.circA = circA;    }     public String getMessage() {        return message;    }}

同样，我们可以运行之前的测试，看看有没有异常抛出，程序结果是否是我们所期望的那样。综上，一般采用第三种setter的方式和第二种@Lazy延迟加载的策略

关于Spring bean的创建，其本质上还是一个对象的创建，既然是对象，读者朋友一定要明白一点就是，一个完整的对象包含两部分：当前对象实例化和对象属性的实例化。在Spring中，对象的实例化是通过反射实现的，而对象的属性则是在对象实例化之后通过一定的方式设置的。这个过程可以按照如下方式进行理解：

然后我们看上面解决循环依赖注入的示例3

@Componentpublic class A {    private B b;    public void setB(B b) {        this.b = b;    }    public B getB() {        return b;    }}

可以看到，这里A和B中各自都以对方为自己的全局属性。这里首先需要说明的一点是，Spring实例化bean是通过ApplicationContext.getBean()方法来进行的。如果要获取的对象依赖了另一个对象，那么其首先会创建当前对象，然后通过递归的调用ApplicationContext.getBean()方法来获取所依赖的对象，最后将获取到的对象注入到当前对象中。这里我们以上面的首先初始化A对象实例为例进行讲解。

图中getBean()表示调用Spring的ApplicationContext.getBean()方法，而该方法中的参数，则表示我们要尝试获取的目标对象。图中的黑色箭头表示一开始的方法调用走向，走到最后，返回了Spring中缓存的A对象之后，表示递归调用返回了，此时使用绿色的箭头表示。从图中我们可以很清楚的看到，B对象的a属性是在第三步中注入的半成品A对象，而A对象的b属性是在第二步中注入的成品B对象，此时半成品的A对象也就变成了成品的A对象，因为其属性已经设置完成了。

重点方法流程如下

对于Spring处理循环依赖问题的方式，我们这里通过上面的流程图其实很容易就可以理解，需要注意的一个点就是，Spring是如何标记开始生成的A对象是一个半成品，并且是如何保存A对象的。

protected  T doGetBean(final String name, @Nullable final Class requiredType,  @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException { // 尝试通过bean名称获取目标bean对象，比如这里的A对象 Object sharedInstance = getSingleton(beanName);  // 我们这里的目标对象都是单例的 if (mbd.isSingleton()) {  // 这里就尝试创建目标对象，第二个参数传的就是一个ObjectFactory类型的对象，这里是使用Java8的lamada  // 表达式书写的，只要上面的getSingleton()方法返回值为空，则会调用这里的getSingleton()方法来创建  // 目标对象  sharedInstance = getSingleton(beanName, () -> {   try {    // 尝试创建目标对象    return createBean(beanName, mbd, args);   } catch (BeansException ex) {    throw ex;   }  }); } return (T) bean;}

@Nullableprotected Object getSingleton(String beanName, boolean allowEarlyReference) { // 尝试从缓存中获取成品的目标对象，如果存在，则直接返回 Object singletonObject = this.singletonObjects.get(beanName); // 如果缓存中不存在目标对象，则判断当前对象是否已经处于创建过程中，    //在前面的讲解中，第一次尝试获取A对象的实例之后，就会将A对象标记为正在创建中，    //因而最后再尝试获取A对象的时候，这里的if判断就会为true if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {  synchronized (this.singletonObjects) {   singletonObject = this.earlySingletonObjects.get(beanName);   if (singletonObject == null && allowEarlyReference) {    // 这里的singletonFactories是一个Map，其key是bean的名称，    //而值是一个ObjectFactory类型的对象，    //这里对于A和B而言，调用图其getObject()方法返回的就是A和B对象的实例，无论是否是半成品    ObjectFactory singletonFactory = this.singletonFactories.get(beanName);    if (singletonFactory != null) {     // 获取目标对象的实例     singletonObject = singletonFactory.getObject();     this.earlySingletonObjects.put(beanName, singletonObject);     this.singletonFactories.remove(beanName);    }   }  } } return singletonObject;}

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException { // 实例化当前尝试获取的bean对象，比如A对象和B对象都是在这里实例化的 BeanWrapper instanceWrapper = null; if (mbd.isSingleton()) {  instanceWrapper = this.factoryBeanInstanceCache.remove(beanName); } if (instanceWrapper == null) {  instanceWrapper = createBeanInstance(beanName, mbd, args); }  // 判断Spring是否配置了支持提前暴露目标bean，也就是是否支持提前暴露半成品的bean boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences   && isSingletonCurrentlyInCreation(beanName)); if (earlySingletonExposure) {  // 如果支持，为了解决循环依赖，再bean初始化完成前将创建实例的objectFactory加入工厂  addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean)); } try {  // 在初始化实例之后，这里就是判断当前bean是否依赖了其他的bean，如果依赖了，  // 就会递归的调用getBean()方法尝试获取目标bean  populateBean(beanName, mbd, instanceWrapper); } catch (Throwable ex) {  // 省略... }  return exposedObject;}

/** Cache of singleton objects: bean name --> bean instance */private final Map singletonObjects = new ConcurrentHashMap(256);/** Cache of singleton factories: bean name --> ObjectFactory */private final Map> singletonFactories = new HashMap>(16);/** Cache of early singleton objects: bean name --> bean instance */private final Map earlySingletonObjects = new HashMap(16);