单例模式

参考链接：https://www.cnblogs.com/Ycheng/p/7169381.html

1、懒汉－－线程不安全模式

public class SingletanceDemo01{
    private static SingletanceDemo01 instance;
          
     private SingletanceDemo01(){}

     public static SingletanceDemo01 getInstance(){
          if(instance == null){
                 instance = new SingletanceDemo01();
          }
          return instance;
     }
}

2、懒汉－－线程安全

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

}    

3、饿汉

public class SingletonDemo{
    private static SingletonDemo instance = new SingletonDemo();
    private SingletonDemo(){}
    public static SingletonDemo getInstance(){
           return instance;
    }  

}