线程安全的单例模式的几种实现方法分享

时间:2021-10-18 20:50:01

1、饿汉式单例

 

 1 public class Singleton {
 2    private final static Singleton INSTANCE = new Singleton();
 3  
 4 
 5    private Singleton() { }
 6 
 7    public static Singleton getInstance() {
 8       return INSTANCE;
 9    }
10 }

 

2、借助内部类
属于懒汉式单例,因为Java机制规定,内部类SingletonHolder只有在getInstance()方法第一次调用的时候才会被加载(实现了lazy),而且其加载过程是线程安全的。内部类加载的时候实例化一次instance。

 1  public class Singleton {
 2 
 3 
 4    private Singleton() { }
 5  
 6 
 7    private static class SingletonHolder {
 8       private final static Singleton INSTANCE = new Singleton();
 9    }
10 
11    public static Singleton getInstance() {
12       return SingletonHolder.INSTANCE;
13    }
14 }

3、普通加锁解决

 1  public class Singleton {
 2 
 3 
 4    private Singleton() { }
 5  
 6 
 7    private static class SingletonHolder {
 8       private final static Singleton INSTANCE = new Singleton();
 9    }
10 
11    public static Singleton getInstance() {
12       return SingletonHolder.INSTANCE;
13    }
14 }

 

虽然解决了线程安全问题,但是每个线程调用getInstance都要加锁,我们想要只在第一次调用getInstance时加锁,请看下面的双重检测方案

4、双重检测,但要注意写法

 public class Singleton {

   private static Singleton instance = null;
 

   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:

 1 public class Singleton {
 2    private static Singleton instance = null;
 3 
 4    private Singleton() { }
 5 
 6    public static Singleton getInstance() {
 7       if(instance == null) {
 8          synchronzied(Singleton.class) {
 9             if(instance == null) {
10                instance = new Singleton();
11             }
12          }
13       }
14 
15       return instance;
16    }
17 }

 

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:

 1 public class Singleton {
 2    private static volatile Singleton instance = null;
 3 
 4    private Singleton() { }
 5 
 6    public static Singleton getInstance() {
 7       if(instance == null) {
 8          synchronzied(Singleton.class) {
 9             if(instance == null) {
10                instance = new Singleton();
11             }
12          }
13       }
14 
15       return instance;
16    }
17 }

 

 

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