Spring中循环引用的处理-1

2012/08/28 09:49:49 1 Comment

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:

<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:

"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:

/** 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:

/**	 
* 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:

 /**
	 * 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:

/**
	 * 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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已有一个评论

  1. Honwhy Says:

    1)一般的流程是用ObjectFactory被添加到生成singletonFactories中,
    2)然后通过ObjectFacotry生成Raw Bean,将Raw Bean加入earlySingletonObjects,从singletonFactories删除ObjectFacotry
    3)Bean构造完成后加入到singletonObjects,从earlySingletonObjects移除,然后注册是到registeredSingletons

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