线程安全的单例模式的几种实现方法分享_java

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1、饿汉式单例


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

   private Singleton() { }

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

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。


public class Singleton {

   private Singleton() { }

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

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

3、普通加锁解决


public class Singleton {
   private static Singleton instance = null;

   private Singleton() { }

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

      return instance;
   }
}

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:
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;
   }
}

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