爱威尔-java设计模式----单例模式

单例模式 


	 


	其目的是控制实例化(new 一个类)次数. 


	 


	市面上的单例有饿汉式，懒汉式，双重校验锁，Lazy initialization holder class(延迟初始化容器)，最牛的要数枚举单例。


	 


	不要问我从哪里来的，我也是网上看到的，(哈哈哈哈)待我给你一一道来。


	 


	package com.weir.singleton;


	/**


	* 饿汉式--线程安全


	* @author Administrator


	*


	*/


	public class Singleton1 {


	 


	private static Singleton1 singleton = new Singleton1();


	 


	private Singleton1(){}


	 


	public static Singleton1
getInstance() {


	return singleton;


	}


	 


	public void a() {


	System.out.println("A");


	}


	}


	 


	 


	package com.weir.singleton;


	/**


	* 懒汉式--线程不安全


	* @author Administrator


	*


	*/


	public class Singleton2 {


	 


	private static Singleton2 singleton = null;


	 


	private Singleton2(){}


	 


	public static Singleton2
getInstance() {


	if (singleton==null) {


	singleton = new Singleton2();


	}


	return singleton;


	}


	 


	public void a() {


	System.out.println("A");


	}


	}


	 


	 


	package com.weir.singleton;


	/**


	* 双重检查加锁优化版--线程安全--（但是极端情况下可能出现空指针异常（编译重排序）volatile 就是用来解决这个问题的）


	* @author Administrator


	*


	*/


	public class Singleton3 {


	 


	private volatile static Singleton3 singleton = null;


	 


	private Singleton3(){}


	 


	public static Singleton3
getInstance() {


	if (singleton==null) {


	synchronized (Singleton3.class) {


	if (singleton==null) {


	singleton = new Singleton3();


	}


	}


	 


	}


	return singleton;


	}


	 


	public void a() {


	System.out.println("A");


	}


	}


	 


	 


	package com.weir.singleton;


	/**


	* Lazy initialization holder class(延迟初始化容器)--线程安全


	* @author Administrator


	*


	*/


	public class Singleton4 {


	 


	private static class Holder{


	private static Singleton4 instance = new Singleton4();


	}


	 


	private Singleton4(){}


	 


	public static Singleton4
getInstance() {


	return Holder.instance;


	}


	 


	public void a() {


	System.out.println("A");


	}


	}


	 


	package com.weir.singleton;


	/**


	* 枚举单例模式---线程安全


	* @author Administrator


	*


	*/


	public enum SingletonEnum {


	 


	instance;


	 


	public void a() {


	System.out.println("A");


	}


	}


	 


	package com.weir.singleton;


	 


	public class Client {


	 


	public static void main(String[] args) {


	SingletonEnum
s = SingletonEnum.instance;


	 


	s.a();


	 


	}


	}


	 


	 


	看上去一个比一个吊。