单例模式常用写法

目录
  1. 方法一：懒汉
  2. 方法二：饿汉
  3. 方法三：双重校验锁定
  4. 方法四：枚举
  5. 方法五：静态内部类

方法一：懒汉

在调用取得实例的方法时才会实例化对象

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public class Singleton {  
    private static Singleton instance;  
    private Singleton (){}  
    public static synchronized Singleton getInstance() {  
    if (instance == null) {  
        instance = new Singleton();  
    }  
    return instance;  
    }  
}

方法二：饿汉

在单例类被加载时候完成实例化

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public class Singleton {  
    private static final Singleton instance = new Singleton();  
    private Singleton (){}  
    public static Singleton getInstance() {  
      return instance;  
    }  
}

方法三：双重校验锁定

懒汉升级版，JDK1.5后生效，毕竟getInstance上的synchronized多数情况都没有用

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public class Singleton {  
    private volatile static Singleton singleton;  
    private Singleton (){}  
    public static Singleton getSingleton() {  
      if (singleton == null) {  
          synchronized (Singleton.class) {  
            if (singleton == null) {  
                singleton = new Singleton();  
            }  
          }  
      }  
      return singleton;  
    }  
}

方法四：枚举

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public enum Singleton {  
    INSTANCE;  
    public void mothed() {  
    }  
}

方法五：静态内部类

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public class Singleton {  
    private static class SingletonHolder {  
      private static final Singleton INSTANCE = new Singleton();  
    }  
    private Singleton (){}  
    public static final Singleton getInstance() {  
      return SingletonHolder.INSTANCE;  
    }  
}