java单例模式的实现

1、使用内部类，而且它是线程安全的，而且只创建一次：

/**
Prof Bill Pugh, University of Maryland, was the main force behind java memory
model changes. His principle Initialization-on-demand holder idiom also uses
static block but in different way. It suggest to use static inner class.
*/
public class Singleton {

/**
* Returns Singleton instance
* @return Singleton 
*/
public static Singleton getInstance() {
return InstanceHolder.INSTANCE;
}

// private constructor prevents instantiation from other classes
private Singleton () {

}

// Maintains single instance of class
private static class InstanceHolder {
private static final Singleton INSTANCE = new Singleton();
}

}

Read more: http://javarevisited.blogspot.com/2014/05/double-checked-locking-on-singleton-in-java.html#ixzz3uvGIuYWT

2、使用enum，因为它是线程安全的，而且只创建一次

/**
* 枚举单例
* @author Administrator
*
*/
public enum EnumSingleton {

INSTANCE;

/**
* 私有构造方法
*/
private EnumSingleton(){
System.out.println("-----构造方法");
}

/**
* 实现函数
* @param i
*/
public void say(int i){
System.out.println("i :" + i);
}

/**
* 测试
* @param args
*/
public static void main(String[] args) {
EnumSingleton.INSTANCE.say(1);
EnumSingleton.INSTANCE.say(2);
EnumSingleton.INSTANCE.say(3);
}
}