线程安全的单例模式的几种实现方法分享

   private Singleton() { }

   public static Singleton getInstance() {
      return INSTANCE;
   }
}

   private static class SingletonHolder {
      private final static Singleton INSTANCE = new Singleton();
   }

   public static Singleton getInstance() {
      return SingletonHolder.INSTANCE;
   }
}

   private Singleton() { }

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

      return instance;
   }
}

   private Singleton() { }

   public static Singleton getInstance() {
      if(instance == null) {
         synchronzied(Singleton.class) {
            Singleton temp = instance;
            if(temp == null) {
               temp = new Singleton();
               instance = temp
            }
         }
      }

      return instance;
   }
}

由于指令重排序问题，所以不可以直接写成下面这样：
public class Singleton {
   private static Singleton instance = null;

   private Singleton() { }

   public static Singleton getInstance() {
      if(instance == null) {
         synchronzied(Singleton.class) {
            if(instance == null) {
               instance = new Singleton();
            }
         }
      }

      return instance;
   }
}

但是如果instance实例变量用volatile修饰就可以了，volatile修饰的话就可以确保instance = new Singleton();对应的指令不会重排序，如下的单例代码也是线程安全的：
public class Singleton {
   private static volatile Singleton instance = null;

   private Singleton() { }

   public static Singleton getInstance() {
      if(instance == null) {
         synchronzied(Singleton.class) {
            if(instance == null) {
               instance = new Singleton();
            }
         }
      }

      return instance;
   }
}