What does Holder refer to in java singleton mode?

Difference

1. In the declaring class, member variables do not declare instance variables, but are placed in static inner classes. This approach is similar to that of a slacker. They all use a class loading mechanism to ensure that there is only one thread to initialize the instance. The difference is that the Holder single mode puts the initialization of the instance into a static category to achieve lazy loading.

The core of the Holder mode is static variables, which are convenient enough and thread-safe; holding real examples through the static Holder class indirectly implements lazy loading.

2. Features, realizes lazy loading, good performance and thread safety.

Example

public class Singleton {
      /**
       * 类级的内部类，也就是静态的成员式内部类，该内部类的实例与外部类的实例
       * 没有绑定关系，而且只有被调用到才会装载，从而实现了延迟加载
       */
      private static class SingletonHolder{
          /**
           * 静态初始化器，由JVM来保证线程安全
           */
         private static Singleton instance = new Singleton();
     }
     /**
      * 私有化构造方法
      */
     private Singleton(){
     }
     public static  Singleton getInstance(){
         return SingletonHolder.instance;
     }
 }

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