前几天被问到了单例模式对构造函数有什么要求吗？答曰：没什么要求吧？

回来查了下具体的资料才发现，原来单例模式的实现private 的一个构造函数，目的是不让这个单例的类能够new一个对象出来。

思考了下，其实不加应该问题也不大，毕竟如果代码上都是一个人在写的话，其实这种问题还是可以规避的。但是又发现了其他新的一些想法。

先说下以前写的单例吧，其实也很简单，无非就是一个private static然后用getInstance返回，现在感觉想法确实有点幼稚。

上面说的那个private 的构造函数就不提了，个人感觉有当然最好，但是无也未尝不可。

贴一段spring的源码看看spring的sigleton如何写的先

/**
	 * 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} if none found
	 */
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			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);
	}

按照spring的做法，对于配置文件中的类进行注册到singletonObjects 中

代码如下：

/**
	 * 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);
		}
	}