设计模式 -- 单例设计模式

在Java编程语言中，单例模式(饿汉模式)应用的例子如下述代码所示：

  public class Singleton { 

     private final static Singleton INSTANCE = new Singleton();//Privateconstructor suppresses 

  private Singleton() {}
 
    // default public constructor 

  public static Singleton getInstance() {
        return INSTANCE;
    } 

}

在Java编程语言中，单例模式(懒汉模式)应用的例子如下述代码所示 (此种方法只能用在JDK5及以后版本(注意 INSTANCE 被声明为 volatie)，之前的版本使用“双重检查锁”会发生非预期行为^[1])：

  public class Singleton { 

     private static volatile Singleton INSTANCE = null;
 
    // Private constructor suppresses 
    // default public constructor 

     private Singleton() {}
 
    //thread safe and performance  promote 

  public static  Singleton getInstance() { 

         if(INSTANCE == null){ 

              synchronized(Singleton.class){
                 //when more than two threads run into the first null check same time, to avoid instanced more than one time, it needs to be checked again.
                 if(INSTANCE == null){ 

 INSTANCE = new Singleton(); 

                    } 

                  } 

          } 

         return INSTANCE;

  } 

}