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#ixzz3uvGIuYWT2、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);
	}
}