获取单例

如果缓存中不存在已经加载的单例bean就需要从头开始bean的加载过程了，而spring中使用getSingleton的重载方法实现bean的加载过程。

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 recordSuppressedExceptions = (this.suppressedExceptions == null);
				if (recordSuppressedExceptions) {
					this.suppressedExceptions = new LinkedHashSet<Exception>();
				}
				try {
					singletonObject = singletonFactory.getObject();
				}
				catch (BeanCreationException ex) {
					if (recordSuppressedExceptions) {
						for (Exception suppressedException : this.suppressedExceptions) {
							ex.addRelatedCause(suppressedException);
						}
					}
					throw ex;
				}
				finally {
					if (recordSuppressedExceptions) {
						this.suppressedExceptions = null;
					}
					afterSingletonCreation(beanName);
				}
				addSingleton(beanName, singletonObject);
			}
			return (singletonObject != NULL_OBJECT ? singletonObject : null);
		}
	}

上述代码中其实是使用了回调方法，使得程序可以在单例创建的前后做一些准备及处理操作，而真正的获取单例bean的方法其实不是在此方法中实现的，其实现逻辑是在ObjectFactory类型的实例singletonFactory中实现的。

（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

（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

（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

（7）返回处理结果