Spring源码分析（三）（解决循环依赖）

叁歲伎倆 2022-01-27 06:53 263阅读 0赞

**spring解决循环依赖主要通过三级缓存去完成。**

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

**在doGetBean中，它首先会通过getSingleton方法去尝试获取bean。**

Object sharedInstance = getSingleton(beanName);

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
       /** 首先从singletonObjects获取bean实例 */
       Object singletonObject = this.singletonObjects.get(beanName);
       if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
          synchronized (this.singletonObjects) {
             /** 
              * 如果singletonObjects还没有此bean，有两种情况
              * 1.这个bean正在创建状态，先从earlySingletonObjects获取
              * 2.这个bean还没开始创建
              */
             singletonObject = this.earlySingletonObjects.get(beanName);
             if (singletonObject == null && allowEarlyReference) {
                /** 
                 * 如果earlySingletonObjects还没有此bean，有两种情况
                 * 1.说明还未被其他bean注入，正在创建状态，先从singletonFactories获取
                 * 2.该bean还没开始创建
                 */
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                   singletonObject = singletonFactory.getObject();
                   //将此bean放到提前缓存到earlySingletonObjects中
                   this.earlySingletonObjects.put(beanName, singletonObject);
                   //从singletonObject删除bean
                   this.singletonFactories.remove(beanName);
                }
             }
          }
       }
       return singletonObject;
    }

**这个方法做了这么一件事，逐步从三级缓存中获取bean：**

1.  先到singletonObjects中获取，如果有表示实例化已经完成；
2.  否则到earlySingletonObjects获取，如果有表示已经有bean，且存在循环依赖，将此bean作为属性注入了
3.  否则到singletonFactories获取，如果存在循环依赖，且此属性是第一次被其他bean作为属性注入

相应的在doCreateBean中有这么一串代码：

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
    			throws BeanCreationException {
    
    		// Instantiate the bean.
    		//BeanWrapper是用来持有创建出来的Bean对象的
    		BeanWrapper instanceWrapper = null;
    		//如果是Singleton先把缓存中同名的清除
    		if (mbd.isSingleton()) {
    			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    		}
    		//这是创建Bean的地方由createBeanInstance来完成
    		if (instanceWrapper == null) {
    			instanceWrapper = createBeanInstance(beanName, mbd, args);
    		}
    		final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
    		Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
    		mbd.resolvedTargetType = beanType;
    
    		// Allow post-processors to modify the merged bean definition.
    		synchronized (mbd.postProcessingLock) {
    			if (!mbd.postProcessed) {
    				try {
    					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
    				}
    				catch (Throwable ex) {
    					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
    							"Post-processing of merged bean definition failed", ex);
    				}
    				mbd.postProcessed = true;
    			}
    		}
    
    		// 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.isDebugEnabled()) {
    				logger.debug("Eagerly caching bean '" + beanName +
    						"' to allow for resolving potential circular references");
    			}
    			addSingletonFactory(beanName, new ObjectFactory<Object>() {
    				@Override
    				public Object getObject() throws BeansException {
    					return getEarlyBeanReference(beanName, mbd, bean);
    				}
    			});
    		}
    
    		// Initialize the bean instance.
    		//这里是对Bean的初始化，依赖注入往往在这里发生，
    		// 这个exposedObject在初始化处理完后会返回作为依赖注入完成后的bean
    		Object exposedObject = bean;
    		try {
    			populateBean(beanName, mbd, instanceWrapper);
    			if (exposedObject != null) {
    				exposedObject = initializeBean(beanName, exposedObject, mbd);
    			}
    		}
    		catch (Throwable ex) {
    			if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
    				throw (BeanCreationException) ex;
    			}
    			else {
    				throw new BeanCreationException(
    						mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
    			}
    		}
    
    		if (earlySingletonExposure) {
    			Object earlySingletonReference = getSingleton(beanName, false);
    			if (earlySingletonReference != null) {
    				if (exposedObject == bean) {
    					exposedObject = earlySingletonReference;
    				}
    				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
    					String[] dependentBeans = getDependentBeans(beanName);
    					Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
    					for (String dependentBean : dependentBeans) {
    						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
    							actualDependentBeans.add(dependentBean);
    						}
    					}
    					if (!actualDependentBeans.isEmpty()) {
    						throw new BeanCurrentlyInCreationException(beanName,
    								"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.");
    					}
    				}
    			}
    		}
    
    		// Register bean as disposable.
    		try {
    			registerDisposableBeanIfNecessary(beanName, bean, mbd);
    		}
    		catch (BeanDefinitionValidationException ex) {
    			throw new BeanCreationException(
    					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    		}
    
    		return exposedObject;
    	}

核心在这里：

boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
    				isSingletonCurrentlyInCreation(beanName));
    		if (earlySingletonExposure) {
    			if (logger.isDebugEnabled()) {
    				logger.debug("Eagerly caching bean '" + beanName +
    						"' to allow for resolving potential circular references");
    			}
    			addSingletonFactory(beanName, new ObjectFactory<Object>() {
    				@Override
    				public Object getObject() throws BeansException {
    					return getEarlyBeanReference(beanName, mbd, bean);
    				}
    			});
    		}
    
    
    	protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
    		Assert.notNull(singletonFactory, "Singleton factory must not be null");
    		synchronized (this.singletonObjects) {
    			if (!this.singletonObjects.containsKey(beanName)) {
                    //将beanName存放入singletonFactories中
    				this.singletonFactories.put(beanName, singletonFactory);
    				this.earlySingletonObjects.remove(beanName);
    				this.registeredSingletons.add(beanName);
    			}
    		}
    	}

**    这串代码会在创建bean的时候将自身注册进singletonFactories中。所以可以想象当两个bean互相依赖时，A会将自身注册进singletonFactories中，当它创建B时它可以直接从singletonFactories中获取A的引用所以B能成功创建，当B成功创建后那A也可以从singletonObjects中获取B的引用，最后A也成功创建。**

### 总结 ###

**  Spring通过三级缓存解决了循环依赖的问题，它会在创建bean的时候将自身注册进一个singletonFactory中，然后创建它的依赖bean时会从这个缓存中获取它的引用使那个依赖bean先进行创造，最后完成自身的依赖。**

举个列子：

1.  创建A单例时，放入缓存，然后创建它的依赖B单例。
2.  创建B单例时，发现需要依赖注入A单例，然后去加载A单例，此时从缓存中发现A单例正在创建中，于是直接从缓存得到A单例，完成自身的依赖注入。
3.  B单例依赖完成后，返回给A单例。
4.  A单例再把B单例注入到自身域中完成依赖注入。