写给自己的单例模式手撕代码

import java.util.concurrent.atomic.AtomicReference;

/**
 * Created by ql on 2019/5/7.
 * 手撕单例模式饿汉式，懒汉式，双重校验锁，枚举类型，静态内部类，cas模式
 */
public class Danli {//懒汉式，但是线程不安全
    private static Danli instance = null;
    private Danli(){}
    public Danli getInstance(){
        if (instance==null){
            instance=new Danli();
        }
        return instance;
    }
}
class Singleton2{//饿汉式
    private Singleton2(){}
    private static final Singleton2 instance=new Singleton2();
    public Singleton2 getInstance(){
        return instance;
    }
}
class Singleton3{//双重校验锁
    private static volatile Singleton3 instance;
    private Singleton3(){}
    public Singleton3 getInstance(){
        if (instance==null){
            synchronized (this){
                if (instance==null){
                    instance=new Singleton3();
                }
            }
        }
        return instance;
    }
}
enum Singleton4{//枚举类型
    //private Singleton4(){}
    instance;
    public Singleton4 getInstance(){
        return Singleton4.instance;
    }
}
class Singleton5{//静态内部类
    private Singleton5(){}
    private static class Singleton5Handler{
        public static Singleton5 instance=new Singleton5();
    }
    public Singleton5 getInstance(){
        return Singleton5Handler.instance;
    }
}
class Singleton6{//CAS方法实现单例模式
    private static final AtomicReference<Singleton6>Instance=new AtomicReference<>();//实现cas操作的类
    private Singleton6(){}
    public Singleton6 getInstance(){
        while (true){
           Singleton6 instance=Instance.get();
           if (instance!=null){
               return instance;
           }
           instance=new Singleton6();
           if (Instance.compareAndSet(null,instance)){
               return Instance.get();
           }
        }
    }
}