单例模式

volatile变量双重判空

class Singleton{
    private Singleton(){}
    private volatile static Singleton instance;
    public static Singleton getInstance(){
        if(instance==null){
            synchronized(Singleton.class){
                if(instance==null){
                    instance=new Singleton();
                }
            }
        }
        return instance;
    }

}

静态内部类

 public class Singleton{
      private Singleton(){}//私有构造方法,避免外部创建实例
      private static class SingletonHolder
         {
          public static Singleton instance= new Singleton();
         }
      public static Singleton getInstance()
         {
          return SingletonHolder.instance;
        }
}