七种单例

package com.example.demo.test01;

/**
 * 静态内部类实现单例
 */
public class Singleton01 {
    /**
     * 静态内部类
     */
    private static class Singleton01Holder{
        private static final Singleton01 INSTANCE=new Singleton01();
    }

    private Singleton01(){}

    public static Singleton01 getInstance(){
        return Singleton01Holder.INSTANCE;
    }
}

package com.example.demo.test01;
/**
 * 饿汉式单例
 */
public class Singleton02 {
    private static Singleton02 instance=new Singleton02();

    //私有构造器
    private Singleton02(){}

    public static  Singleton02 getInstance(){
        return instance;
    }

}

package com.example.demo.test01;

/**
 * 变种的饿汉式单例模式
 */
public class Singleton03 {

    private static Singleton03 instance=null;
    {
        instance=new Singleton03();
    }
    private Singleton03(){}

    public static Singleton03 getInstance(){
        return instance;
    }
}

package com.example.demo.test01;

/**
 * 枚举单例
 */
public enum Singleton04 {
    INSTANCE;
}

package com.example.demo.test01;

import java.util.concurrent.atomic.AtomicReference;

/**
 * CAS实现单例
 */
public class Singleton05 {

    private static final AtomicReference<Singleton05> INSTANCE=new AtomicReference<>();

    private Singleton05(){}

    public static Singleton05 getInstance(){
        for(;;){
            Singleton05 singleton=INSTANCE.get();
            if(null!=singleton){
                return singleton;
            }
            singleton=new Singleton05();
            if(INSTANCE.compareAndSet(null,singleton)){
                return singleton;
            }
        }
    }

}

package com.example.demo.test01;

/**
 * 使用synchronized实现单例
 */
public class Singleton06 {

    private static volatile Singleton06 singleton=null;

    //构造器私有
    private Singleton06(){}

    public static Singleton06 getInstance(){
        if(singleton!=null){
            synchronized (Singleton06.class){
                if(singleton!=null){
                    singleton=new Singleton06();
                }
            }
        }
        return singleton;
    }
}

package com.example.demo.test01;

/**
 * 使用ThreadLocal实现单例
 */
public class Singleton07 {

    private static final ThreadLocal<Singleton07> singleton=new ThreadLocal<Singleton07>(){
        @Override
        protected Singleton07 initialValue(){
            return new Singleton07();
        }
    };

    public static Singleton07 getInstance(){
        return singleton.get();
    }

    private Singleton07(){}
}