什么是循环依赖问题

多个bean之间相互依赖，形成了一个闭环。 比如：A依赖于B、B依赖于C、C依赖于A

通常来说，如果问Spring容器内部如何解决循环依赖问题，一定是指默认的单例Bean中，属性相互引用的场景。也就是说，Spring的循环依赖，是Spring容器注入时出现的问题。

示例

项目结构

项目代码

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><groupId>com</groupId><artifactId>spring-circular-dependency</artifactId><version>1.0-SNAPSHOT</version><properties><maven.compiler.source>8</maven.compiler.source><maven.compiler.target>8</maven.compiler.target></properties><dependencies><dependency><groupId>org.springframework</groupId><artifactId>spring-core</artifactId><version>5.2.1.RELEASE</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-beans</artifactId><version>5.2.1.RELEASE</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-context</artifactId><version>5.2.1.RELEASE</version></dependency><dependency><groupId>org.aspectj</groupId><artifactId>aspectjrt</artifactId><version>1.8.9</version></dependency><dependency><groupId>org.apache.geronimo.bundles</groupId><artifactId>aspectjweaver</artifactId><version>1.6.8_2</version></dependency></dependencies></project>

Bean01.java

package com.spring.bean;import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;/*** @author honey* @date 2023-08-23 17:50:53*/
@Component
public class Bean01 {@Autowiredprivate Bean02 bean02;
}

Bean02.java

package com.spring.bean;import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;/*** @author honey* @date 2023-08-23 17:52:55*/
@Component
public class Bean02 {@Autowiredprivate Bean01 bean01;
}

SpringConfig01.java

package com.spring.config;import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;/*** @author honey* @date 2023-08-23 17:59:37*/
@Configuration
@ComponentScan(value = {"com.spring.bean"})
public class SpringConfig01 {
}

SpringTest01.java

package com.spring.test;import com.spring.bean.Bean01;
import com.spring.bean.Bean02;
import com.spring.config.SpringConfig01;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;/*** @author honey* @date 2023-08-23 18:00:24*/
public class SpringTest01 {public static void main(String[] args) {AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(SpringConfig01.class);Bean01 bean01 = applicationContext.getBean("bean01", Bean01.class);Bean02 bean02 = applicationContext.getBean("bean02", Bean02.class);System.out.println(bean01);System.out.println(bean02);}
}

运行结果

Spring默认情况下已经解决了循环依赖问题（单例Bean）

@Async注解导致的问题

在SpringConfig01类上加上@EnableAsync注解

在Bean01类中使用@Async注解修饰的异步方法

Bean01.java

package com.spring.bean;import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;/*** @author honey* @date 2023-08-23 17:50:53*/
@Component
public class Bean01 {@Autowiredprivate Bean02 bean02;@Asyncpublic void test() {System.out.println(Thread.currentThread().getName() + "Bean01测试中...");}
}

运行结果

"C:\Program Files\Java\jdk1.8.0_191\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2021.1.1\lib\idea_rt.jar=57456:C:\Program Files\JetBrains\IntelliJ IDEA 2021.1.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_191\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_191\jre\lib\rt.jar;E:\知识点资料（第四年）\Spring源码解读\代码\spring-circular-dependency\target\classes;D:\maven_jar\org\springframework\spring-core\5.2.1.RELEASE\spring-core-5.2.1.RELEASE.jar;D:\maven_jar\org\springframework\spring-jcl\5.2.1.RELEASE\spring-jcl-5.2.1.RELEASE.jar;D:\maven_jar\org\springframework\spring-beans\5.2.1.RELEASE\spring-beans-5.2.1.RELEASE.jar;D:\maven_jar\org\springframework\spring-context\5.2.1.RELEASE\spring-context-5.2.1.RELEASE.jar;D:\maven_jar\org\springframework\spring-aop\5.2.1.RELEASE\spring-aop-5.2.1.RELEASE.jar;D:\maven_jar\org\springframework\spring-expression\5.2.1.RELEASE\spring-expression-5.2.1.RELEASE.jar;D:\maven_jar\org\aspectj\aspectjrt\1.8.9\aspectjrt-1.8.9.jar;D:\maven_jar\org\apache\geronimo\bundles\aspectjweaver\1.6.8_2\aspectjweaver-1.6.8_2.jar" com.spring.test.SpringTest01
八月 23, 2023 7:07:05 下午 org.springframework.context.support.AbstractApplicationContext refresh
警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'bean01': Bean with name 'bean01' has been injected into other beans [bean02] 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.
Exception in thread "main" org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'bean01': Bean with name 'bean01' has been injected into other beans [bean02] 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.at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:624)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517)at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:879)at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:878)at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550)at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:89)at com.spring.test.SpringTest01.main(SpringTest01.java:15)Process finished with exit code 1

Spring在扫描bean发现某个类方法被@Async修饰时，会通过后置处理器AsyncAnnotationBeanPostProcessor生成代理对象，而该后置处理器的顺序比处理AOP的后置处理器还靠后，因此会导致Spring处理不了循环依赖。

使用@Lazy注解解决@Async注解导致的问题

在Bean01类中循环依赖的属性上使用@Lazy注解

运行结果

开启Aop使用代理对象示例

项目结构

项目代码

AspectAop.java

package com.spring.aop;import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;
import org.springframework.stereotype.Component;/*** @author honey* @date 2023-08-23 18:26:33*/
@Component
@Aspect
public class AspectAop {@Pointcut("execution (* com.spring.bean.*.*(..))")public void pointcut() {}@Around(value = "pointcut()")public Object doAround(ProceedingJoinPoint point) throws Throwable {System.out.println("doAround advice start");Object result = point.proceed();System.out.println("doAround advice end");return result;}
}

需要在Bean01和Bean02中加上一个普通方法，如：

package com.spring.bean;import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;/*** @author honey* @date 2023-08-23 17:52:55*/
@Component
public class Bean02 {@Autowiredprivate Bean01 bean01;public void add() {}
}

SpringConfig01.java

package com.spring.config;import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.EnableAspectJAutoProxy;/*** @author honey* @date 2023-08-23 17:59:37*/
@Configuration
@ComponentScan(value = {"com.spring.bean", "com.spring.aop"})
@EnableAspectJAutoProxy
public class SpringConfig01 {
}

运行结果

Spring是如何解决循环依赖问题的

Spring底层通过三级缓存解决了循环依赖问题。

一级缓存：singletonObjects，也叫作单例池，存放已经经历了完整生命周期的Bean对象；

/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

二级缓存：earlySingletonObjects，存放早期暴露出来的Bean对象，Bean的生命周期未结束（属性还未填充完整）；

/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

三级缓存：singletonFactories，存放可以生成Bean的工厂；

/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

原理

假设有两个对象分别是A和B，其中A依赖于B，B依赖于A

1. 在创建A对象时需要B对象，于是将A对象存入三级缓存，再去创建B对象；
2. 在创建B对象时发现需要A对象，于是先查一级缓存，没有，再查二级缓存，还是没有，再查三级缓存，找到了A对象，然后把三级缓存里面的A对象存入二级缓存，并删除三级缓存里面的A对象；
3. B对象创建完成，将B对象存入一级缓存（此时B对象依赖的A对象依然是创建中状态），获取到B对象后回来接着创建A对象，直到创建完成，并将A对象存入一级缓存中（如果其它对象依赖于A对象和B对象，可以直接从一级缓存里面获取）；

源码解读

---

在创建A对象时，A对象完成实例化后，会将A对象封装成ObjectFactory存入三级缓存。

AbstractBeanFactory.java

AbstractAutowireCapableBeanFactory.java

ObjectFactory.java

如果该对象是单例对象且开启了循环依赖且该对象正在创建中，则会将该对象封装成ObjectFactory对象存入三级缓存。其中ObjectFactory是一个函数接口，在调用getObject()方法时，才会真正去执行lambda语句调用getEarlyBeanReference()方法。

// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {if (logger.isTraceEnabled()) {logger.trace("Eagerly caching bean '" + beanName +"' to allow for resolving potential circular references");}addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}

DefaultSingletonBeanRegistry.java

---

在为A对象设置属性时，发现A对象依赖于B对象，则会去尝试获取B对象，由于此时B对象还未被创建，所以B对象也会走和A对象一样的创建逻辑，不同的是在为B对象设置属性时，发现B对象依赖于A对象，可以从三级缓存中获取得到A对象。

AbstractBeanFactory.java

DefaultSingletonBeanRegistry.java

此时存入二级缓存的是不完整对象，调用singletonFactory.getObject()方法实际上是在调用getEarlyBeanReference(beanName, mbd, bean)方法。

如果是正常情况下，返回原始对象

InstantiationAwareBeanPostProcessorAdapter.java

如果是开启了Aop的情况下，返回Aop代理对象

AbstractAutoProxyCreator.java

---

将获取到的A对象设置为B对象的属性并执行完B对象的完整生命周期后，将B对象存入一级缓存中并从二级缓存、三级缓存中移除。此处也使用了ObjectFactory这个函数接口。

AbstractBeanFactory.java

DefaultSingletonBeanRegistry.java

---

获取得到B对象后，接着执行A对象的生命周期，执行完成后和B对象一样存入一级缓存中。

---

什么情况下Spring无法解决循环依赖问题

1. 构造器注入的循环依赖问题；
2. 非单例对象的循环依赖问题；