JUC之CAS比较并交换

最新推荐文章于 2025-08-05 17:34:52 发布

原创最新推荐文章于 2025-08-05 17:34:52 发布 · 414 阅读

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CC 4.0 BY-SA版权

文章标签：

#java #数据库 #服务器

本文讨论了CAS操作在Java中的运用，包括其底层实现、自旋锁示例，以及ABA问题的出现与解决策略，如使用原子引用和时间戳版本号。

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CAS

原子类引入
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加入原子整型类的操作后，无锁化的操作

CAS 比较并交换

硬件级别的保证

源码分析

引出UnSafe类

UnSafe源码分析

unsafe中的do-while保证自旋

  public final int getAndAddInt(Object var1, long var2, int var4) {
    int var5;
    do {
      var5 = this.getIntVolatile(var1, var2);
    } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));

    return var5;
  }

在这里插入图片描述

CAS是一条CPU的原子指令，不会造成所谓的数据不一致问题
在这里插入图片描述
native修饰的方法代表的是底层的方法

CAS是靠硬件实现的，从而在硬件层面提升效率。最底层还是交给硬件来保证原子性和可见性。实现方式是基于硬件平台的汇编指令，在intel的CPU中（X86机器上），使用的是汇编指令cmpxchg指令。
核心思想就是：比较要更新变量的值V和预期值E(compare),相等才会将V的值设为新值N(swap),如果不相等自旋再来。

原子引用

public class TestAutomicReference {


  @ToString
  @Data
  @AllArgsConstructor
  static class Use{
    private String name;
    private int age;
  }

  public static void main(String[] args) {
    AtomicReference<Use> useAtomicReference = new AtomicReference<>();
    Use lisi = new Use("lisi", 10);
    useAtomicReference.set(lisi);
    Use zhangsan = new Use("zhangsan", 12);
    boolean b = useAtomicReference.compareAndSet(lisi, zhangsan);
    System.out.println("result:"+b+"\t\t"+useAtomicReference.get());
  }
}

CAS与自旋锁，借鉴CAS思想

自旋锁
在这里插入图片描述

手写自旋锁demo

package com.aiguigu.juclearn.atomic;

/**
 * @author: Runqiang_Jiang
 * @Time: 2024/3/16  11:55
 */

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;

/**
 * 题目：实现一个自旋锁
 * 自旋锁的好处：循环比较获取，没有类似wait的阻塞
 * 通过CAS操作完成自旋锁，A线程先进来调用myLock方法自己持有锁5秒钟，
 * B随后进来后发现当前有线程持有锁，所以只能通过自旋等待，知道啥A释放锁后B随后抢到
 */
public class SpinLockDemo {

  AtomicReference<Thread> atomicReference=new AtomicReference<>();

  public  void lock(){
    Thread thread = Thread.currentThread();

    System.out.println(Thread.currentThread().getName()+"\t"+"------come in");

    while (!atomicReference.compareAndSet(null, thread)) {

    }

  }

  public void unlock(){
    Thread thread = Thread.currentThread();

    atomicReference.compareAndSet(thread,null);
    System.out.println(Thread.currentThread().getName()+"\t"+"-----task over unlock");

  }

  public static void main(String[] args) throws InterruptedException {

    SpinLockDemo spinLockDemo = new SpinLockDemo();

     new Thread(()->{
       spinLockDemo.lock();
       //暂停几秒钟
       try {
         TimeUnit.SECONDS.sleep(5);
       } catch (InterruptedException e) {
         e.printStackTrace();
       }
       spinLockDemo.unlock();
     },"A").start();

     //暂停500毫秒，线程A先于线程B启动
     TimeUnit.MILLISECONDS.sleep(500);


    new Thread(()->{
      spinLockDemo.lock();
      spinLockDemo.unlock();
    },"B").start();
  }

}

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CAS的两大缺点
1.循环时间长开销很大

2.引出来ABA问题？？？

解决方案：版本号时间戳原子引用
demo演示

package com.aiguigu.juclearn.atomic;

import java.util.concurrent.atomic.AtomicStampedReference;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

/**
 * @author: Runqiang_Jiang
 * @Time: 2024/3/16  12:18
 */

@AllArgsConstructor
@NoArgsConstructor
@Data
class  Book{
  private int id;
  private String bookName;
}



public class AtomicStampedDemo {


  public static void main(String[] args) {

    Book java = new Book(1, "java");

    AtomicStampedReference<Book> stampedReference=new AtomicStampedReference<>(java,1);

    System.out.println(stampedReference.getReference()+"\t"+stampedReference.getStamp());

    Book mysql = new Book(2, "mysql");

    boolean b ;
        b= stampedReference.compareAndSet(java, mysql, stampedReference.getStamp(),
        stampedReference.getStamp() + 1);

    System.out.println(b+"\t"+stampedReference.getReference()+"\t"+stampedReference.getStamp());

    b= stampedReference.compareAndSet(mysql, java, stampedReference.getStamp(),
        stampedReference.getStamp() + 1);

    System.out.println(b+"\t"+stampedReference.getReference()+"\t"+stampedReference.getStamp());

  }
}

ABA后版本号有变化
在这里插入图片描述
ABA多线程demo

package com.aiguigu.juclearn.atomic;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicStampedReference;

/**
 * @author: Runqiang_Jiang
 * @Time: 2024/3/16  12:31
 */

public class ABADemo {


  static AtomicInteger atomicInteger = new AtomicInteger(100);

  static AtomicStampedReference<Integer> stampedReference = new AtomicStampedReference<>(100,1);

  public static void main(String[] args) {

    new Thread(()->{
      int stamp = stampedReference.getStamp();
      System.out.println(Thread.currentThread().getName()+"\t"+"首次版本号："+stamp);
      //休眠保证后面的t4线程拿到的版本号与t3一样
      try {
        TimeUnit.MILLISECONDS.sleep(500);
      } catch (InterruptedException e) {

      }
      stampedReference.compareAndSet(100,101,stampedReference.getStamp(),stampedReference.getStamp()+1);
      System.out.println(Thread.currentThread().getName()+"\t"+"2次版本号："+stampedReference.getStamp());


      stampedReference.compareAndSet(101,100,stampedReference.getStamp(),stampedReference.getStamp()+1);
      System.out.println(Thread.currentThread().getName()+"\t"+"3次版本号："+stampedReference.getStamp());
    },"t3").start();

    new Thread(()->{
      int stamp = stampedReference.getStamp();
      System.out.println(Thread.currentThread().getName()+"\t"+"首次版本号："+stamp);
      //等待t3线程发生了ABA的问题
      try {
        TimeUnit.SECONDS.sleep(1);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }

      boolean b = stampedReference.compareAndSet(100, 2024, stamp, stamp + 1);


      System.out.println(b+"\t"+Thread.currentThread().getName()+"\t"+stampedReference.getReference()+"\t"+stampedReference.getStamp());

    },"t4").start();

  }

  private static void abahappens() {
    new Thread(()->{
      atomicInteger.compareAndSet(100,101);
      try {
        TimeUnit.MILLISECONDS.sleep(10);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }

      atomicInteger.compareAndSet(101,100);


    },"t1").start();

    new Thread(()->{
      try {
        TimeUnit.MILLISECONDS.sleep(200);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      boolean b = atomicInteger.compareAndSet(100, 2024);
      System.out.println(b+"\t"+atomicInteger.get());
    },"t2").start();
  }

}