关于JAVA单例模式

//懒汉模式 
public class Singleton {
	private static Singleton st;
	private Singleton(){
	}
	public static Singleton getInstance() {
		if (null == st) {
			//处理多线程的问题，如果第一次访问枷锁 之后不在枷锁
			synchronized (Singleton.class){
				st = new Singleton();
			}
		}
		return st;
	}
}

//饿汉模式 
 class Singleton1 {
	private static Singleton1 st = new Singleton1();
	private Singleton1(){
	}
	public static Singleton1 getInstance() {
		return st;
	}
}

/**
 * @author lee
 * @date Mar 5, 201210:09:12 AM
 * @version V1.0
 * @todo 单例模式属于对象创建型模式，其意图是保证一个类仅有一个实例,并提供一个访问它的全局访问点
 */
public class Singleton {
	private static Singleton instance;
	private static Singleton instance1=new Singleton();
	private static Singleton instance2;
	private static Singleton instance3;
	private volatile static Singleton instance4;
	//私有构造方法,避免外部创建实例
	private Singleton(){
	}
	//懒汉式单例>当需要的时候，并且发现没有实例的时候,才去初始化.
	public static Singleton getInstance(){
		if(instance==null){
			threadSleep();
			instance=new Singleton();
		}
		return instance;
	}
	//饿汉式单例>管你需要不需要，反正我是饿了，类加载的时候就已经初始化,没有起到延迟加载的效果
	public static Singleton getInstance1(){
		return instance1;
	}
	//对懒汉式进行同步,牺牲了性能，同步的原则应该是尽量对代码块同步，不要对整个方法同步
	public static synchronized Singleton getInstance2(){
		if(instance2==null){
			threadSleep();
			instance2=new Singleton();
		}
		return instance2;
	}
	//双重检查加锁DCL(double checking lock),只在第一次调用getInstance()时才要同步，提高性能
	public static Singleton getInstance3(){
		if(instance3==null){
			threadSleep();
			synchronized (Singleton.class) {
				if(instance3==null){
					instance3=new Singleton();
				}
			}
		}
		return instance3;
	}
	
	public static Singleton getInstance4(){
		if(instance4==null){
			threadSleep();
			synchronized (Singleton.class) {
				if(instance4==null){
					instance4=new Singleton();
				}
			}
		}
		return instance4;
	}
	//让当前线程休眠1秒，模拟多个线程同时访问instance==null之后的临界情况
	public static void threadSleep(){
		try {
			System.out.println("当前线程休眠1秒!");
			Thread.currentThread().sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

public class ThreadTest extends Thread {
	
	@Override
	public void run() {
		Singleton s=Singleton.getInstance();
		System.out.println(s.toString());
		
		/*Singleton s1=Singleton.getInstance1();
		System.out.println(s1.toString());*/
		
		/*Singleton s2=Singleton.getInstance2();
		System.out.println(s2.toString());*/
		
		/*Singleton s3=Singleton.getInstance3();
		System.out.println(s3.toString());*/
	}
	public static void main(String[] args) {	
		for(int i=0;i<3;i++){
			ThreadTest test=new ThreadTest();
			test.start();
		}		
	}
}