Spring源码之bean的加载(四)获取单例

前面我们有讲过Spring是如何去获取单例的bean，获取是从缓存中来的，可是要是缓存中没有的话怎么办？Spring中使用了getSingleton的方法来重新加载。（相关资源可到这里下载：http://pan.baidu.com/s/1sjSo9a9）

下面是getSingleton的主要代码。

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(beanName, "'beanName' 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 the 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 + "'");}beforeSingletonCreation(beanName);boolean newSingleton = false;boolean recordSuppressedExceptions = (this.suppressedExceptions == null);if (recordSuppressedExceptions) {this.suppressedExceptions = new LinkedHashSet<Exception>();}try {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;}afterSingletonCreation(beanName);}if (newSingleton) {addSingleton(beanName, singletonObject);}}return (singletonObject != NULL_OBJECT ? singletonObject : null);}}

上述代码中其实是使用了回调的方法，使得程序可以在单例创建的前后做一些准备及处理操作，而真正获取单例bean的方法其实并不是在此方法中实现，其实实现逻辑是在ObjectFactory类型的实例sinfletonFactory中实现。而这些准备及处理操作包括如下内容：

1)        检查缓存是否已经加载过

2)        若没有加载，则记录beanName的正在加载状态

3)        加载单例前记录加载状态

protected void beforeSingletonCreation(String beanName) {if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}}

4)        通过调用参数传入的ObjectFactory的个体Object方法实例化bean。

5)        加载单例后的处理方法调用

 protected void afterSingletonCreation(String beanName) {if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");}}

6)        将结果记录至缓存并删除加载bean过程中所记录的各种辅助状态

  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);}}

7)        返回结果处理。

if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {@Overridepublic Object getObject() throws BeansException {try {return createBean(beanName, mbd, args);}catch (BeansException ex) {// Explicitly remove instance from singleton cache: It might have been put there// eagerly by the creation process, to allow for circular reference resolution.// Also remove any beans that received a temporary reference to the bean.destroySingleton(beanName);throw ex;}}});bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}