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

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;
     }
     
 }