在使用spring的场景中，有时会碰到如下的一种情况，即bean之间的循环引用。即两个bean之间互相进行引用的情况。这时，在spring xml配置文件中，就会出现如下的配置：

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<bean id="beanA" class="BeanA" p:beanB-ref="beaB"/>
<bean id="beanB" class="BeanB" p:beanA-ref="beaA"/>

并且，在一般情况下，这个配置在现有的spring3.0中是可以正常工作的，前提是没有对beanA和beanB进行增强。但是，如果任意一方进行了增强，比如通过spring的代理对beanA进行了增强，即实际返回的对象和原始对象不一致的情况，在这种情况下，就会报如下一个错误：

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"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] 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."

这个错误即对于一个bean，其所引用的对象并不是由spring容器最终生成的对象，而只是一个原始对象，而spring不允许这种情况出现，即持有过程中间对象。那么，这个错误是如何产生的，以及在spring内部，是如何来检测这种情况的呢。这就得从spring如何创建一个对象，以及如何处理bean间引用，以及spring使用何种策略处理循环引用问题说起。

这里会涉及到在spring内部所使用的两个内部属性，singletonFactories和earlySingletonObjects，这两个属性在类DefaultSingletonBeanRegistry中被定义，定义如下:

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/** Cache of singleton factories: bean name --> ObjectFactory */
    private final Map<String, ObjectFactory> singletonFactories = new HashMap<String, ObjectFactory>();
 
    /** Cache of early singleton objects: bean name --> bean instance */
    private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>();

官方对此的属性定义不是很明确，这里我们可以这样来理解。

  • singletonFactories，用于存储在spring内部所使用的beanName->对象工厂的引用，一旦最终对象被创建(通过objectFactory.getObject())，此引用信息将删除
  • earlySingletonObjects，用于存储在创建Bean早期对创建的原始bean的一个引用，注意这里是原始bean，即使用工厂方法或构造方法创建出来的对象，一旦对象最终创建好，此引用信息将删除

从上面的解释，可以看出，这两个对象都是一个临时工。在所有的对象创建完毕之后，此两个对象的size都为0。

那么再来看下这两个对象如何进行协作：

方法1：

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/** 
* Add the given singleton factory for building the specified singleton
     * if necessary.
     * <p>To be called for eager registration of singletons, e.g. to be able to
     * resolve circular references.
     * @param beanName the name of the bean
     * @param singletonFactory the factory for the singleton object
     */
    protected void addSingletonFactory(String beanName, ObjectFactory singletonFactory) {
        Assert.notNull(singletonFactory, "Singleton factory must not be null");
        synchronized (this.singletonObjects) {
            if (!this.singletonObjects.containsKey(beanName)) {
                this.singletonFactories.put(beanName, singletonFactory);
                this.earlySingletonObjects.remove(beanName);
                this.registeredSingletons.add(beanName);
            }
        }
    }

方法2：

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 /**
     * Return the (raw) singleton object registered under the given name.
     * <p>Checks already instantiated singletons and also allows for an early
     * reference to a currently created singleton (resolving a circular reference).
     * @param beanName the name of the bean to look for
     * @param allowEarlyReference whether early references should be created or not
     * @return the registered singleton object, or <code>null</code> if none found
     */
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
            synchronized (this.singletonObjects) {
                singletonObject = this.earlySingletonObjects.get(beanName);
                if (singletonObject == null && allowEarlyReference) {
                    ObjectFactory singletonFactory = this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        singletonObject = singletonFactory.getObject();
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }
        return (singletonObject != NULL_OBJECT ? singletonObject : null);
    }

方法3：

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/**
     * Add the given singleton object to the singleton cache of this factory.
     * <p>To be called for eager registration of singletons.
     * @param beanName the name of the bean
     * @param singletonObject the singleton object
     */
    protected void addSingleton(String beanName, Object singletonObject) {
        synchronized (this.singletonObjects) {
            this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
            this.singletonFactories.remove(beanName);
            this.earlySingletonObjects.remove(beanName);
            this.registeredSingletons.add(beanName);
        }
    }

方法1和方法2中的官方注释都很明显地显示了，针对于循环引用的处理，即能够处理循环引用问题。

详细见后两篇
Spring中循环引用的处理-2
Spring中循环引用的处理-3

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