Java的单例模式实现

    只能生成一个实例的类是实现了Singleton(单例)模式的类。以下为C#实现单例模式的方式

方式一只使用于单线程环境

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// 把构造函数设为私有函数以禁止他人创建实例 // 定义一个静态的实例，在需要的时候创建该实例 // 在Singleton的静态属性Instance中，只有在instance为null的时候才创建一个实例以避免重复创建 // 把构造函数定义为私有函数 public final class Singleton1 {     private Singleton1() {     }       private static Singleton1 _instance = null;       public static Singleton1 getInstance() {         if (_instance == null)             _instance = new Singleton1();         return _instance;     }       public static void main(String[] args) {         Singleton1 s1 = Singleton1.getInstance();         Singleton1 s2 = Singleton1.getInstance();           System.out.println(s1.hashCode());         System.out.println(s2.hashCode());     } }

方式二 加同步锁

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import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;   public final class Singleton2 {     private Singleton2(){}           static Lock lock = new ReentrantLock();           private static Singleton2 _instance = null;           public static Singleton2 getInstance(){         lock.lock();         try {             if(_instance == null)                 _instance = new Singleton2();         } finally {             lock.unlock();         }         return _instance;     }       public static void main(String[] args) {         Singleton2 s1 = Singleton2.getInstance();         Singleton2 s2 = Singleton2.getInstance();           System.out.println(s1.hashCode());         System.out.println(s2.hashCode());     } }

可行的解法 加同步锁前后两次判断实例是否已存在

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import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;   public final class Singleton3 {     private Singleton3(){}           static Lock lock = new ReentrantLock();           private static Singleton3 _instance = null;           public static Singleton3 getInstance(){         if(_instance == null){             lock.lock();             try{                 if(_instance == null)                     _instance = new Singleton3();             }finally {                 lock.unlock();             }           }         return _instance;     }       public static void main(String[] args) {         Singleton3 s1 = Singleton3.getInstance();         Singleton3 s2 = Singleton3.getInstance();           System.out.println(s1.hashCode());         System.out.println(s2.hashCode());     } }

推荐的解法一利用静态构造函数

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public final class Singleton4 {     private Singleton4(){}           private static Singleton4 _instance = new Singleton4();           public static Singleton4 getInstance()     {         return _instance;     }       public static void main(String[] args) {         Singleton4 s1 = Singleton4.getInstance();         Singleton4 s2 = Singleton4.getInstance();           System.out.println(s1.hashCode());         System.out.println(s2.hashCode());     }   }

推荐的解法二 实现按需创建实例

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public class Singleton5 {     private Singleton5(){}           public static Singleton5 getInstance(){         return Nested.instance;     }           private static class Nested{         private Nested(){}         public static final Singleton5 instance = new Singleton5();     }       public static void main(String[] args) {         Singleton5 s1 = Singleton5.getInstance();         Singleton5 s2 = Singleton5.getInstance();           System.out.println(s1.hashCode());         System.out.println(s2.hashCode());     }   }

1. 扩展 

    定义一个表示总统的类型President可以从该类型继承出FrenchPresident和AmericanPresident等类型。这些派生类型都只能产生一个实例        

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public class President {     private String _name;           public President(){}           public String getName(){         return _name;     }           public void setName(String name){         _name = name;     }           public static void main(String[] args) {     }   }   public class FrenchPresident extends President{     private FrenchPresident(){}           public static FrenchPresident getInstance()     {         return Nested.instance;     }           private static class Nested{         private Nested(){}                   public static final FrenchPresident instance = new FrenchPresident();     }       public static void main(String[] args) {         FrenchPresident s1 = FrenchPresident.getInstance();         FrenchPresident s2 = FrenchPresident.getInstance();           System.out.println(s1.hashCode());         System.out.println(s2.hashCode());       }   }     public class AmericanPresident {     private AmericanPresident() {     }       public static AmericanPresident getInstance() {         return Nested.instance;     }       private static class Nested {         private Nested() {         }           public static final AmericanPresident instance = new AmericanPresident();     }       public static void main(String[] args) {         AmericanPresident s1 = AmericanPresident.getInstance();         AmericanPresident s2 = AmericanPresident.getInstance();           System.out.println(s1.hashCode());         System.out.println(s2.hashCode());       }   }

本文转自 许大树 51CTO博客，原文链接：http://blog.51cto.com/abelxu/1965630，如需转载请自行联系原作者