bean的加载（七）创建bean

常规bean的创建时在doCreateBean中完成的，我们看看这个方法

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {// Instantiate the bean.BeanWrapper instanceWrapper = null;if (mbd.isSingleton()) {//如果是单例，首先要清除缓存instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null) {//根据指定bean使用对应的策略创建新的实例，如：工厂方法，构造函数自动注入，简单初始化//实例化bean，将BeanDefinition转化为BeanWrapper.instanceWrapper = createBeanInstance(beanName, mbd, args);}final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);// Allow post-processors to modify the merged bean definition.synchronized (mbd.postProcessingLock) {if (!mbd.postProcessed) {//应用MergedBeanDefinitionPostProcessors//bean合并后的处理，Autowired注解正是通过此方法实现诸如类型的预解析。applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);mbd.postProcessed = true;}}//是否需要提早曝光：单例&允许循环依赖&当前bean正在创建中，检测循环依赖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");}//为避免后期循环依赖，可以在bean初始化完成前将创建实例的ObjectFactory加入工厂//依赖处理：在Spring中会有循环依赖的情况，例如，当A中含有B的属性，而B中又含有A的属性时就会//构成一个循环依赖，此时如果A和B都是单例，那么在Spring中的处理方式就是当创建B的时候，涉及//自动注入A的步骤时，并不是直接去再次创建A，而是通过放入缓存中的ObjectFactory来创建实例，//这样就解决了循环依赖的问题。addSingletonFactory(beanName, new ObjectFactory<Object>() {public Object getObject() throws BeansException {//对bean再一次依赖引用，主要应用SmartInstantiationAware BeanPostProcessor,//其中我们熟知的AOP就是在这里将advice动态植入bean中，若没有则直接返回bean，不做任何处理return getEarlyBeanReference(beanName, mbd, bean);}});}// Initialize the bean instance.Object exposedObject = bean;try {//对bean进行填充，将各个属性值注入，其中，可能存在依赖于其他bean的属性，则会递归初始依赖beanpopulateBean(beanName, mbd, instanceWrapper);if (exposedObject != null) {//调用初始化方法，比如init-method.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) {//由于earlySingletonObjects是用于存储在创建bean早期对创建的原始bean的一个引用，//所以返回的earlySingletonReference只有在检测到检测到循环依赖的情况下才会不为空Object earlySingletonReference = getSingleton(beanName, false);if (earlySingletonReference != null) {//如果exposedObject没有在初始化方法中被改变，也就是没有被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) {//检测依赖//之前说过，在Spring中解决循环依赖只对单例有效，而对于prototype的bean，Spring没有好的解决//办法，唯一要做的就是抛出异常。在这个步骤里面会检测已经加载的bean是否已经出现了依赖循环，并//判断是否需要抛出异常if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {actualDependentBeans.add(dependentBean);}}/* * 因为bean创建后其所依赖的bean一定是已经创建的。 * actualDependentBeans不为空则表示当前bean创建后其依赖的bean却没有全部 * 创建完，也就是存在循环依赖。 */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 {//根据scope注册beanregisterDisposableBeanIfNecessary(beanName, bean, mbd);}catch (BeanDefinitionValidationException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);}return exposedObject;}

上面的代码非常的繁琐，每一个步骤都用了大量代码来完成其功能，最复杂也是最难以理解的当属循环依赖的处理，在真正进入doCreateBean前我们有必要了解下循环依赖。关于循环依赖，我也只能说是理解了个大概，有兴趣的话，可以去网上搜索些好的文章加强这方面的理解