设计模式之单例模式

GOF对单例模式的定义：保证类仅有一个实例，并提供一个访问它的全局访问点。

单例模式的的基本代码比较简单

/** *  * @author ricardo * @Time 下午10:40:11 * @Function：定义单例模式类 * */public class Singleton {private static Singleton singleton;private Singleton() {}public static Singleton GetInstance() {if(singleton == null)singleton = new Singleton();return singleton;}}

单例模式中，Singleton类封装了它的唯一实例，以此来控制第三方如何以及何时访问它。也就是对唯一实例的受限访问。

单例模式的应用很简单，举个例子来说明，现在实行一夫一妻制，假设我们现在有一个Wife/Husband 类

/** *  * @author ricardo * @Time 下午10:17:02 * @Function：姑娘 * */public class Wife {public void Show() {System.out.println("秀外慧中的姑娘");}public void Marry() {System.out.println("姑娘要嫁人了");}}

/** *  * @author ricardo * @Time 下午10:18:22 * @Function：男孩纸 * */public class Husband {private Wife myWife = null;public void ChooseWife() {if(myWife == null)myWife = new Wife();myWife.Show();}public void GetMarry() {if(myWife == null)myWife = new Wife();myWife.Marry();}}

/** *  * @author ricardo * @Time 下午10:20:35 * @Function：客户端代码 * */public class First {public static void main(String[] args) {Husband husband = new Husband();System.out.println("男孩纸喜欢上一个姑娘");husband.ChooseWife();System.out.println("姑娘被男孩子打动了");husband.GetMarry();}}

运行截图：

但是这样会有个问题，假设男孩纸的妈妈想见见儿媳，于是：

public class Mother {Wife daughter = null;public void CheckDaughterInLaw() {if(daughter == null)daughter = new Wife();daughter.Show();}}

这里母亲会实例化一个Wife出来，很明显，这里出来的并不是男孩子心仪的姑娘了，而不实例化，则会出现空指针。so，我们使用单例来实现

/** *  * @author ricardo * @Time 下午10:17:02 * @Function：姑娘 * */public class Wife {private static Wife wife;private Wife() {}public static Wife newInstance() {if(wife == null)wife = new Wife();return wife;}public void Show() {System.out.println("秀外慧中的姑娘");}public void Marry() {System.out.println("姑娘要嫁人了");}}

/** *  * @author ricardo * @Time 下午10:18:22 * @Function：男孩纸 * */public class Husband {private Wife myWife = null;public void ChooseWife() {myWife = Wife.newInstance();//if(myWife == null)//myWife = new Wife();myWife.Show();}public void GetMarry() {myWife = Wife.newInstance();//if(myWife == null)//myWife = new Wife();myWife.Marry();}}

public class Mother {Wife daughter = null;public void CheckDaughterInLaw() {daughter = Wife.newInstance();//if(daughter == null)//daughter = new Wife();daughter.Show();}}

客户端：

/** *  * @author ricardo * @Time 下午10:20:35 * @Function：客户端代码 * */public class First {public static void main(String[] args) {Husband husband = new Husband();System.out.println("男孩纸喜欢上一个姑娘");husband.ChooseWife();System.out.println("姑娘被男孩子打动了");husband.GetMarry();System.out.println("母亲想见见儿媳");Mother mother = new Mother();mother.CheckDaughterInLaw();}}

这样子，就会看到，姑娘只有唯一一个了。

不过如果将程序扩展一下，假设婚礼准备安排现场直播，这时候就是多线程上场的时候了，但是这样的单例模式没有办法保证在不同线程进入Wife类的时候，进入myWife==null的判定，所以我们需要对Wife进行加锁。

import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;/** *  * @author ricardo * @Time 下午10:17:02 * @Function：姑娘 * */public class Wife {static Lock lock = new ReentrantLock();private static Wife wife;private Wife() {}public static Wife newInstance() {//如果不加这个判定，每次进入都会被锁上，将严重影响系统性能if(wife == null) {lock.lock();//如果不加这个判定if(wife == null)wife = new Wife();lock.unlock();}return wife;}public void Show() {System.out.println("秀外慧中的姑娘");}public void Marry() {System.out.println("姑娘要嫁人了");}}

我们使用双重判断来优化系统，但是，如果仅仅相对于单例模式来说，可以利用JVM第一次加载类的时候会初始化所有静态变量，所以可以这么来

/** *  * @author ricardo * @Time 下午10:17:02 * @Function：姑娘 * */public class Wife {static Lock lock = new ReentrantLock();private static Wife wife = new Wife();;private Wife() {}public static Wife newInstance() {//if(wife == null) {//lock.lock();//if(wife == null)//wife = new Wife();//lock.unlock();//}return wife;}public void Show() {System.out.println("秀外慧中的姑娘");}public void Marry() {System.out.println("姑娘要嫁人了");}}

理解这样的单例模式，我们也可以通过计数的方式，实现双例模式，三例模式等需要实例化有限次的类。

以上内容，整理自刘径舟，张玉华编著的《设计模式其实很简单》读书笔记，欢迎转载.