Java单例，

Java单例，

求职面试的时候被问到了Java单例，于是文面试官是不是线程安全的，得到肯定后，写了一个双重锁的单例。总结了四种单例

/**
 * 饿汉式单例模式
 * @author zhouzhihu
 *
 */
public class Singleton {
	private static final Singleton INSTANCE=new Singleton();
	private Singleton(){}

	public Singleton getInstance(){
		return INSTANCE;
	}
}

/**
 * 懒汉式单例
 * @author zhouzhihu
 *
 */
public class Singleton2 {

	private Singleton2(){}
	
	private static class SingletonHolder{
		private final static Singleton2 INSTANCE=new Singleton2();
	}
	public Singleton2 getInstance(){
		return SingletonHolder.INSTANCE;
	}
}
--------------------------------------------------------------------------------------------------------------------------------------/**
 * 普通加锁
 * @author zhouzhihu
 *
 */
public class Singleton3 {

	private static Singleton3 instance=null;
	private Singleton3(){}
	
	public static synchronized Singleton3 getInstance(){
		if(instance==null)
			instance=new Singleton3();
		return instance;
	}
}
--------------------------------------------------------------------------------------------------------------------------------------/**
 * 双重检测(加锁)
 * @author zhouzhihu
 *
 */

public class Singleton4 {
	private static volatile Singleton4 instance=null;
	private Singleton4(){}
	
	public static Singleton4 getInstance(){
		if(instance==null){
			synchronized(Singleton4.class){
				if(instance==null){
					instance=new Singleton4();
				}
			}
		}
		return instance;
	}
}