设计模式之单例模式

单例模式可以保证系统中一个类只有一个实例,下面是单例模式的几种实现:

第一种（懒汉，线程不安全）：

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

第二种（懒汉，线程安全）：

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

第三种（饿汉）：

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

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

第四种（静态内部类）：

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

第五种（双重校验锁）：

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;  
    }  
}