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;    }  }