java编程题：编写一个单例模式

/**
* java编程题：编写一个单例模式
*/
public class Singleton {

/*
//1. 饿汉式，饿汉式,线程安全 但效率比较低 。。。一上来就new对象，真够饿的
private Singleton(){}

private static final Singleton Singleton = new Singleton();

public static Singleton getInstance(){
return Singleton;
}
*/

/*
//2. 懒汉式，非线程安全 。。。等老板来叫了，才去做事，小子真懒
private Singleton(){}

private static Singleton Singleton = null;

public static Singleton getInstance(){
if(Singleton==null){
Singleton = new Singleton();
}
return Singleton;
}
*/

/*
//3. 懒汉式，线程安全，但效率很低 。。。99%情况下不需要同步
private Singleton(){}

private static Singleton Singleton = null;

public static synchronized Singleton getInstance(){ //方法上加synchronized同步
if(Singleton==null){
Singleton = new Singleton();
}
return Singleton;
}
*/

/*
//4. 双重校验锁
private Singleton(){}

//JDK5以后，为避免双重检查锁存在的问题，加入了volatile，是保证变量修改的可见性
private static volatile Singleton Singleton = null;

public static Singleton getInstance(){
if(Singleton==null){
synchronized(Singleton.class){ //加synchronized关键字同步
if(Singleton==null){
Singleton = new Singleton();
}
}
}
return Singleton;
}

*/

/*
//5. 静态内部类
private Singleton(){}

private static class SingletonHolder {
private static final Singleton singleton = new Singleton();
}

public static final Singleton getInstance(){
return SingletonHolder.singleton;
}
*/

//6. 枚举
private static enum EnumSingleton{
INSTANCE;

private Singleton singleton;

//JVM会保证此方法绝对只调用一次
private EnumSingleton(){
singleton = new Singleton();
}

public Singleton getInstance(){
return singleton;
}
}

}

参考： http://cantellow.iteye.com/blog/838473 http://www.cnblogs.com/yinxiaoqiexuxing/p/5605338.html http://blog.csdn.net/zhanlanmg/article/details/49944991