JUC之CAS比较并交换

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;}

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CAS是一条CPU的原子指令，不会造成所谓的数据不一致问题
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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@AllArgsConstructorstatic 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
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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
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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();}}