多线程单例 可以概括为两中模式(饿汉模式和懒汉模式)

如何保证多线程下的单例。

1多线程安全单例模式一(不使用同步锁).

 1  1 public class Singleton {
 2  2     private Singleton()
 3  3     {}
 4  4     private static Singleton singleton;
 5  5
 6  6     public static Singleton getInstance()
 7  7     {
 8  8         if(singleton ==null)
 9  9         {
10 10             singleton =new Singleton();
11 11         }
12 12         return singleton;
13 13     }
14 14
15 15
16 16 }

2.多线程安全单例模式一(使用同步锁).

public class Singleton {
    private Singleton()
    {}
    private static Singleton singleton;
    //sychronized 同步
    public static synchronized Singleton getInstance()
    {
        if(singleton ==null)
        {
            singleton =new Singleton();
        }
        return singleton;
    }

}

3.多线程安全单例模式一(使用双重同步锁).

public class Singleton {
     private static Singleton instance;
     private Singleton (){
     }
     public static Singleton getInstance(){    //对获取实例的方法进行同步
       if (instance == null){
           synchronized(Singleton.class){
               if (instance == null)
                   instance = new Singleton();
           }
       }
       return instance;
     }

 }