单件模式

public class Singleton {
	private static Singleton UniqueInstance;
	
	private  Singleton(){}
	
	public static Singleton getInstance(){
		if(UniqueInstance == null)
			UniqueInstance = new Singleton();
		return UniqueInstance;
	}
}

* Notes:
* 多线程的处理注意点:
* 1.可在getInstance()方法前加上synchronized关键字使方法同步,如:public static synchronized Singleton getInstance()
* 2.使用"急切(eagerly)"过创建实例,而不用延迟实例化的方法.此时的做法是:

public class Singleton {
	private static Singleton UniqueInstance = new Singleton();//这里直接实例化之
	
	private  Singleton(){}
	
	public static Singleton getInstance(){
		return UniqueInstance; //不再判断,直接返回对象.
	}
}

3.双重检查加锁(double-checked locking),使用关键字volatile,不过只对JDK1.5及之后版本有用.

public class Singleton {
	private volatile static Singleton UniqueInstance;
	
	private  Singleton(){}
	
	public static Singleton getInstance(){
		if(UniqueInstance == null)//只有第一次时才彻底执行此if代码块.
			synchronized (Singleton.class){
				if(UniqueInstance == null)//需要再次检查,为什么???因为这是多线程环境,任一时刻可能都有其他线程竞争,如前一个判断之后就已经有其他线程实例化了此单件对象,此判断非多余的.
					UniqueInstance = new Singleton();
			}
		return UniqueInstance;
	}
}