//在类初始化时，已经自行实例化 
public class Singleton1 {
    private Singleton1() {}
    private static final Singleton1 single = new Singleton1();
    //静态工厂方法 
    public static Singleton1 getInstance() {
        return single;
    }
}

多线程环境下注意同步

//在第一次调用的时候实例化自己
public class Singleton {
    private Singleton() {}
    private static Singleton single=null;
    //静态工厂方法 
    public static Singleton getInstance() {
         if (single == null) {  
             single = new Singleton();
         }  
        return single;
    }