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spring如何解决单例循环依赖问题?

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1、spring循环依赖场景
 2、循环依赖解决方式: 三级缓存

1、spring循环引用场景

循环依赖的产生可能有很多种情况，例如：

A的构造方法中依赖了B的实例对象，同时B的构造方法中依赖了A的实例对象
A的构造方法中依赖了B的实例对象，同时B的某个field或者setter需要A的实例对象，以及反之
A的某个field或者setter依赖了B的实例对象，同时B的某个field或者setter依赖了A的实例对象，以及反之

Spring对于循环依赖的解决不是无条件的，首先前提条件是针对scope单例并且允许解决循环依赖的对象。以上三种情况: 第一种Spring无法解决, 第二种只能解决一部分情况, 第三种可以解决

以下为对应的示例demo:

 1 public class CirculationA {
 2 
 3     private CirculationB circulationB;
 4 
 5     public CirculationA(CirculationB circulationB) {
 6         this.circulationB = circulationB;
 7     }
 8 
 9     public CirculationA() {
10     }
11 
12     public CirculationB getCirculationB() {
13         return circulationB;
14     }
15 
16     public void setCirculationB(CirculationB circulationB) {
17         this.circulationB = circulationB;
18     }
19 }
20 
21 public class CirculationB {
22     private CirculationA circulationA;
23 
24     public CirculationB(CirculationA circulationA) {
25         this.circulationA = circulationA;
26     }
27     public CirculationB() {
28     }
29 
30     public CirculationA getCirculationA() {
31         return circulationA;
32     }
33 
34     public void setCirculationA(CirculationA circulationA) {
35         this.circulationA = circulationA;
36     }
37 }

pojo.java

ioc-CirculationReference.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" xmlns:p="http://www.springframework.org/schema/p"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
　　 <!--使用构造函数互相引用 -->
    <bean id="circulationb" class="com.nancy.ioc.CirculationReference.CirculationB" >
        <constructor-arg name="circulationA" ref="circulationa"/>
    </bean>
    <bean id="circulationa" class="com.nancy.ioc.CirculationReference.CirculationA" >
        <constructor-arg name="circulationB" ref="circulationb"/>
    </bean>
</beans>

CirculationReferenceTest代码

 1 public class CirculationReferenceTest {
 2     private ApplicationContext applicationContext ;
 3 
 4     @Before
 5     public void beforeApplicationContext(){
 6         applicationContext = new ClassPathXmlApplicationContext("ioc-CirculationReference.xml") ;
 7     }
 8 
 9     @Test
10     public void test(){
11 
12     }
13 
14     @After
15     public void after(){
16         if(applicationContext != null){
17             ((ClassPathXmlApplicationContext)applicationContext).close();
18         }
19     }
20 }

CirculationReferenceTest.java

错误堆栈信息: 验证了spring无法解决第一种循环依赖

 1 org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'circulationb' defined in class path resource [ioc-CirculationReference.xml]: Cannot resolve reference to bean 'circulationa' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'circulationa' defined in class path resource [ioc-CirculationReference.xml]: Cannot resolve reference to bean 'circulationb' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'circulationb': Requested bean is currently in creation: Is there an unresolvable circular reference?
 2 
 3     at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:378)
 4     at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:110)
 5     at org.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:676)
 6     at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:188)
 7     at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1308)
 8     at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1154)
 9     at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:538)
10     at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:498)
11     at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320)
12     at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
13     at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)
14     at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
15     at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:846)
16     at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:863)
17     at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:546)
18     at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:144)
19     at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:85)
20     at com.nancy.ioc.CirculationReferenceTest.beforeApplicationContext(CirculationReferenceTest.java:18)
21     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
22     at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
23     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
24     at java.lang.reflect.Method.invoke(Method.java:498)
25     at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
26     at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
27     at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
28     at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
29     at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
30     at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
31     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
32     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
33     at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
34     at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
35     at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
36     at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
37     at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
38     at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
39     at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
40     at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
41     at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
42     at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
43     at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
44 Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'circulationa' defined in class path resource [ioc-CirculationReference.xml]: Cannot resolve reference to bean 'circulationb' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'circulationb': Requested bean is currently in creation: Is there an unresolvable circular reference?
45     at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:378)
46     at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:110)
47     at org.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:676)
48     at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:188)
49     at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1308)
50     at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1154)
51     at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:538)
52     at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:498)
53     at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320)
54     at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
55     at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)
56     at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
57     at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:367)
58     ... 40 more
59 Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'circulationb': Requested bean is currently in creation: Is there an unresolvable circular reference?
60     at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:339)
61     at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:215)
62     at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)
63     at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
64     at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:367)
65     ... 52 more

修改对应的ioc-CirculationReference.xml如下并再次运行: 验证第二种情况, 此时运行结果正常.

<?xml version="1.0" encoding="utf-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
    <!-- B的filed依赖A  A构造函数依赖B -->
    <bean id="circulationb" class="com.nancy.ioc.CirculationReference.CirculationB" >
        <property name="circulationA" ref="circulationa"/>
    </bean>
    <bean id="circulationa" class="com.nancy.ioc.CirculationReference.CirculationA" >
        <constructor-arg name="circulationB" ref="circulationb"/>
    </bean>
</beans>

如果将 circulationa 和 circulationb 在ioc-CirculationReference.xml文件声明的顺序调换, 使用构造的circulationa先加载. 再次报循环依赖无法解析为什么会出现这样的情况呢?

2、循环依赖解决方式: 三级缓存

第一个demo标红的错误堆栈部分信息清晰的看出bean创建基本流程, 由refresh()为入口切入, 这里只分析单例bean创建流程:

1)、AbstractBeanFactory.getBean为入口并委托 AbstractBeanFactory.doGetBean创建
2)、AbstractBeanFactory.doGetBean 会首先从AbstractBeanFactory.getSingleton中获取缓存的bean对象, 如果不存在则调用抽象方法createBean, 即子类实现的AbstractAutowireCapableBeanFactory.createBean方法
3)、AbstractAutowireCapableBeanFactory.createBean方法触发doCreateBean依次调用以下方法实现bean创建过程

createBeanInstance: 实例化bean, 如果需要依赖其他对象则首先创建其他对象(发生循环依赖的地方)
addSingletonFactory: 将实例化bean加入三级缓存
populateBean: 初始化bean, 如果需要依赖其他对象则首先创建其他对象(发生循环依赖的地方)
initializeBean
registerDisposableBeanIfNecessary

4)、AbstractAutowireCapableBeanFactory.autowireConstructor使用构造函数进行实例化

5)、最终调用 ConstructorResolver.autowireConstructor 和 ConstructorResolver.resolveConstructorArguments 进行实例化已经解析构造参数

6)、调用BeanDefinitionValueResolver.resolveValueIfNecessary 和 BeanDefinitionValueResolver.resolveReference 模版类解析构造参数

这里只分析流程主干代码:

AbstractBeanFactory为bean创建的入口

 1 @Override
 2     public Object getBean(String name) throws BeansException {
 3         return doGetBean(name, null, null, false);
 4     }
 5 
 6 protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
 7             @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
 8 
 9         final String beanName = transformedBeanName(name);
10         Object bean;
11 
12         // Eagerly check singleton cache for manually registered singletons.
13         //  
14         Object sharedInstance = getSingleton(beanName);
15         if (sharedInstance != null && args == null) {
16             if (logger.isTraceEnabled()) {
17                 if (isSingletonCurrentlyInCreation(beanName)) {
18                     logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
19                             "' that is not fully initialized yet - a consequence of a circular reference");
20                 }
21                 else {
22                     logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
23                 }
24             }
25             bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
26         }
27 
28         else {
29             // Fail if we're already creating this bean instance:
30             // We're assumably within a circular reference.
31             if (isPrototypeCurrentlyInCreation(beanName)) {
32                 throw new BeanCurrentlyInCreationException(beanName);
33             }
34 
35             //......省略......
36             //......省略......
37 
38             try {
39             //......省略......
40 
41                 // Create bean instance.
42                 if (mbd.isSingleton()) {
43                     sharedInstance = getSingleton(beanName, () -> {
44                         try {
45                             return createBean(beanName, mbd, args);
46                         }
47                         catch (BeansException ex) {
48                             // Explicitly remove instance from singleton cache: It might have been put there
49                             // eagerly by the creation process, to allow for circular reference resolution.
50                             // Also remove any beans that received a temporary reference to the bean.
51                             destroySingleton(beanName);
52                             throw ex;
53                         }
54                     });
55                     bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
56                 }
57 
58               //........
59     }
60 
61 
62     protected abstract Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
63             throws BeanCreationException;

AbstractBeanFactory.java

在DefaultSingletonBeanRegistry使用三级缓存:

// 第一层: 初始化完备的单例bean
/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
// 第二层: 提前暴光的单例对象的Cache 
/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
// 第三层: ObjectFactory工厂bean缓存, 存储实例话后的bean Factory
/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

从缓存中获取单例对象

 1  protected Object getSingleton(String beanName, boolean allowEarlyReference) {
 2         // 首先从第一层缓存获取
 3         Object singletonObject = this.singletonObjects.get(beanName);
 4         // 其次第一层未找到缓存 且 bean处于创建中(例如A定义的构造函数依赖了B对象，得先去创建B对象，或者在populatebean过程中依赖了B对象，得先去创建B对象，此时A处于创建中)
 5         if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
 6             synchronized (this.singletonObjects) {
 7                 singletonObject = this.earlySingletonObjects.get(beanName);
 8                 // 最后第二层未找到缓存 并 允许循环依赖即从工厂类获取对象
 9                 if (singletonObject == null && allowEarlyReference) {
10                     ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
11                     if (singletonFactory != null) {
12                         singletonObject = singletonFactory.getObject();
13                         // 此时会将三级缓存 移入 二级缓存
14                         this.earlySingletonObjects.put(beanName, singletonObject);
15                         this.singletonFactories.remove(beanName);
16                     }
17                 }
18             }
19         }
20         return (singletonObject != NULL_OBJECT ? singletonObject : null);
21     }

getSingleton(String beanName, boolean allowEarlyReference)

创建并缓存单例对象: 创建过程中会暂时先标记bean为创建中, 创建完成之后会清楚该标记并加入第一级缓存

// 创建并注册单例对象 如果存在直接返回
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
        Assert.notNull(beanName, "Bean name must not be null");
        synchronized (this.singletonObjects) {
            Object singletonObject = this.singletonObjects.get(beanName);
            if (singletonObject == null) {
                if (this.singletonsCurrentlyInDestruction) {
                    throw new BeanCreationNotAllowedException(beanName,
                            "Singleton bean creation not allowed while singletons of this factory are in destruction " +
                            "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
                }
                if (logger.isDebugEnabled()) {
                    logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
                }
                // 标记创建开始, 用于标记创建中 多次创建会抛出BeanCurrentlyInCreationException
                beforeSingletonCreation(beanName);
                boolean newSingleton = false;
                boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = new LinkedHashSet<>();
                }
                try {
                    // 使用工厂方法获取单例
                    singletonObject = singletonFactory.getObject();
                    newSingleton = true;
                }
                catch (IllegalStateException ex) {
                    // Has the singleton object implicitly appeared in the meantime ->
                    // if yes, proceed with it since the exception indicates that state.
                    singletonObject = this.singletonObjects.get(beanName);
                    if (singletonObject == null) {
                        throw ex;
                    }
                }
                catch (BeanCreationException ex) {
                    if (recordSuppressedExceptions) {
                        for (Exception suppressedException : this.suppressedExceptions) {
                            ex.addRelatedCause(suppressedException);
                        }
                    }
                    throw ex;
                }
                finally {
                    if (recordSuppressedExceptions) {
                        this.suppressedExceptions = null;
                    }
                    // 标记创建结束
                    afterSingletonCreation(beanName);
                }
                // 保存入一级缓存 并 清空其他缓存
                if (newSingleton) {
                    addSingleton(beanName, singletonObject);
                }
            }
            return singletonObject;
        }
    }

getSingleton(String beanName, ObjectFactory<?> singletonFactory)

标记函数, 重复操作会抛出异常, 循环依赖不能解析抛出异常的触发点

 1 /**
 2      * Callback before singleton creation.
 3      * <p>The default implementation register the singleton as currently in creation.
 4      * @param beanName the name of the singleton about to be created
 5      * @see #isSingletonCurrentlyInCreation
 6      */
 7     protected void beforeSingletonCreation(String beanName) {
 8         if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
 9             throw new BeanCurrentlyInCreationException(beanName);
10         }
11     }
12 
13     /**
14      * Callback after singleton creation.
15      * <p>The default implementation marks the singleton as not in creation anymore.
16      * @param beanName the name of the singleton that has been created
17      * @see #isSingletonCurrentlyInCreation
18      */
19     protected void afterSingletonCreation(String beanName) {
20         if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
21             throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
22         }
23     }

标记和清除bean处于创建中方法

AbstractAutowireCapableBeanFactory中doCreateBean: 核心createBeanInstance、addSingletonFactory、populateBean

 1 protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
 2             throws BeanCreationException {
 3 
 4         // Instantiate the bean.
 5         BeanWrapper instanceWrapper = null;
 6         if (mbd.isSingleton()) {
 7             instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
 8         }
 9         // 创建bean实例化, 此时bean并未进行
10         if (instanceWrapper == null) {
11             instanceWrapper = createBeanInstance(beanName, mbd, args);
12         }
13         //.........
14 
15         // bean为单例并 允许循环依赖 且 处于创建中 加入3级缓存
16         // Eagerly cache singletons to be able to resolve circular references
17         // even when triggered by lifecycle interfaces like BeanFactoryAware.
18         boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
19                 isSingletonCurrentlyInCreation(beanName));
20         if (earlySingletonExposure) {
21             if (logger.isTraceEnabled()) {
22                 logger.trace("Eagerly caching bean '" + beanName +
23                         "' to allow for resolving potential circular references");
24             }
25             addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
26         }
27 
28         // Initialize the bean instance.
29         Object exposedObject = bean;
30         try {
31             // 对bean属性进行复制
32             populateBean(beanName, mbd, instanceWrapper);
33             // 调用初始化方法
34             exposedObject = initializeBean(beanName, exposedObject, mbd);
35         }
36         catch (Throwable ex) {
37             if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
38                 throw (BeanCreationException) ex;
39             }
40             else {
41                 throw new BeanCreationException(
42                         mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
43             }
44         }
45 
46         if (earlySingletonExposure) {
47             Object earlySingletonReference = getSingleton(beanName, false);
48             if (earlySingletonReference != null) {
49                 if (exposedObject == bean) {
50                     exposedObject = earlySingletonReference;
51                 }
52                 else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
53                     String[] dependentBeans = getDependentBeans(beanName);
54                     Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
55                     for (String dependentBean : dependentBeans) {
56                         if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
57                             actualDependentBeans.add(dependentBean);
58                         }
59                     }
60                     if (!actualDependentBeans.isEmpty()) {
61                         throw new BeanCurrentlyInCreationException(beanName,
62                                 "Bean with name '" + beanName + "' has been injected into other beans [" +
63                                 StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
64                                 "] in its raw version as part of a circular reference, but has eventually been " +
65                                 "wrapped. This means that said other beans do not use the final version of the " +
66                                 "bean. This is often the result of over-eager type matching - consider using " +
67                                 "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
68                     }
69                 }
70             }
71         }
72 
73         // Register bean as disposable.
74         try {
75             registerDisposableBeanIfNecessary(beanName, bean, mbd);
76         }
77         catch (BeanDefinitionValidationException ex) {
78             throw new BeanCreationException(
79                     mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
80         }
81 
82         return exposedObject;
83     }   
84 
85 protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
86     Assert.notNull(singletonFactory, "Singleton factory must not be null");
87     synchronized (this.singletonObjects) {
88         if (!this.singletonObjects.containsKey(beanName)) {
89             this.singletonFactories.put(beanName, singletonFactory);
90             this.earlySingletonObjects.remove(beanName);
91             this.registeredSingletons.add(beanName);
92         }
93     }
94 }

View Code

ConstructorResolver

public BeanWrapper autowireConstructor(String beanName, RootBeanDefinition mbd,
            @Nullable Constructor<?>[] chosenCtors, @Nullable Object[] explicitArgs) {

         // ........

            int minNrOfArgs;
            if (explicitArgs != null) {
                minNrOfArgs = explicitArgs.length;
            }
            else {
                ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
                resolvedValues = new ConstructorArgumentValues();
                minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
            }

           // ........
}

    private int resolveConstructorArguments(String beanName, RootBeanDefinition mbd, BeanWrapper bw,
            ConstructorArgumentValues cargs, ConstructorArgumentValues resolvedValues) {

        //.......

        for (ConstructorArgumentValues.ValueHolder valueHolder : cargs.getGenericArgumentValues()) {
            if (valueHolder.isConverted()) {
                resolvedValues.addGenericArgumentValue(valueHolder);
            }
            else {
                Object resolvedValue =
                        valueResolver.resolveValueIfNecessary("constructor argument", valueHolder.getValue());
                ConstructorArgumentValues.ValueHolder resolvedValueHolder = new ConstructorArgumentValues.ValueHolder(
                        resolvedValue, valueHolder.getType(), valueHolder.getName());
                resolvedValueHolder.setSource(valueHolder);
                resolvedValues.addGenericArgumentValue(resolvedValueHolder);
            }
        }

        return minNrOfArgs;
    }

ConstructorResolver.java

BeanDefinitionValueResolver

 1 @Nullable
 2     public Object resolveValueIfNecessary(Object argName, @Nullable Object value) {
 3         // We must check each value to see whether it requires a runtime reference
 4         // to another bean to be resolved.
 5         if (value instanceof RuntimeBeanReference) {
 6             RuntimeBeanReference ref = (RuntimeBeanReference) value;
 7             return resolveReference(argName, ref);
 8         }
 9         //.......
10     }
11 
12     @Nullable
13     private Object resolveReference(Object argName, RuntimeBeanReference ref) {
14         try {
15             Object bean;
16             String refName = ref.getBeanName();
17             refName = String.valueOf(doEvaluate(refName));
18             if (ref.isToParent()) {
19                 if (this.beanFactory.getParentBeanFactory() == null) {
20                     throw new BeanCreationException(
21                             this.beanDefinition.getResourceDescription(), this.beanName,
22                             "Can't resolve reference to bean '" + refName +
23                             "' in parent factory: no parent factory available");
24                 }
25                 bean = this.beanFactory.getParentBeanFactory().getBean(refName);
26             }
27             else {
28                 bean = this.beanFactory.getBean(refName);
29                 this.beanFactory.registerDependentBean(refName, this.beanName);
30             }
31             if (bean instanceof NullBean) {
32                 bean = null;
33             }
34             return bean;
35         }
36         catch (BeansException ex) {
37             throw new BeanCreationException(
38                     this.beanDefinition.getResourceDescription(), this.beanName,
39                     "Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
40         }
41     }

BeanDefinitionValueResolver.java

首先需要明确spring对象如果拿不到构造参数将无法使用构造函数实例化, 所以如果构造函数依赖于其他对象必然先去创建其他对象. 如果是使用默认空参构造则可以实例化, 如果初始化阶段依赖于其他对象必然会去创建其他对象.

第一个demo情况, 当createBeanInstance使用构造函数创建circulationA需要依赖circulationB, 则会暂时停下来并会创建circulationB 并由于对应未创建并不会加入三级缓存. 当使用构造函数创建circulationB需要依赖circulationA, 则也会暂时停下来并会创建circulationA 并由于对应未创建并不会加入三级缓存. 当创建circulationA会再次调用beforeSingletonCreation进行标记, 因为会抛出BeanCurrentlyInCreationException异常终止ioc容器初始化. circulationA和circulationB 由于各自拿不到对应的构造函数参数而无法实例化
第二个demo情况, circulationB使用setter依赖circulationA, 因此createBeanInstance使用默认的空参数构造实例化, 完成之后加入三级缓存并在populateBean中属性进行初始化, 此时需要实例化circulationA. 当使用构造函数创建circulationA需要依赖circulationB, 则也会暂时停下来并去创建circulationB, 由于在缓存中拿到circulationB即完成circulationA实例化. 再次返回circulationB的populateBean方法. 此时circulationA 和 circulationB 加载完成. 由此可以类推, 如果只是通过属性或者 setter方法进行循环依赖 spring可以完美解决.
在第二个demo情况中将 circulationB 和 circulationA 声明顺序进行交换依然导致了加载错误. 根源问题在于与第一种情况一样, circulationA拿不到对应的构造函数参数而无法实例化未进入三级缓存, 故而导致了circulationB再次创建circulationA的时候, 由beforeSingletonCreation抛出BeanCurrentlyInCreationException异常终止ioc容器初始化.

因此得出结果spring解决的循环依赖只是部分, 而无法解决的情况是在使用构造函数互相引用的场景. spring bean声明中应避免第一种情况和第一种情况的变种情况.

参考链接:

Spring源码初探-IOC(4)-Bean的初始化-循环依赖的解决
 Spring-bean的循环依赖以及解决方式

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原文地址：https://www.cnblogs.com/xiaoxing/p/10762686.html