1、私有化构造器，不能直接创建，调用方法创建对象；
public class Singleton { private static Singleton instance; private Singleton (){} public static Singleton getInstance() { if (instance == null) { instance = new Singleton(); } return instance; } }
2、饿汉式：instance在类装载时就实例化
public class Singleton { private static Singleton instance = new Singleton(); private Singleton (){} public static Singleton getInstance() { return instance; } }
3、静态内部类：
public class Singleton { private static class SingletonHolder { private static final Singleton INSTANCE = new Singleton(); } private Singleton (){} public static final Singleton getInstance() { return SingletonHolder.INSTANCE; } }
原文：http://www.cnblogs.com/zilanghuo/p/5330023.html