ABA問(wèn)題：

假如有兩個(gè)線程1，2；
cas下：1.線程取值完等待，2線程取值并把A改成B，有把B改成A，這是1線程執(zhí)行會(huì)任務(wù)A還是原來(lái)的A沒(méi)有發(fā)生改變，如果不在乎中間結(jié)果，只看收尾，那么沒(méi)必要解決ABA問(wèn)題

如何解決？

使用AtomicStampedRefererce，加版本號(hào)

代碼演示如下：

package Volatile;import java.util.concurrent.TimeUnit; import java.util.concurrent.atomic.AtomicReference; import java.util.concurrent.atomic.AtomicStampedReference;/*** ABA問(wèn)題的解決辦法 AtomicStampedReference 時(shí)間戳原子引用·*/ public class ABA {static AtomicReference<Integer> atomicReference = new AtomicReference<>(100);static AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(100,1);public static void main(String []args){System.out.println("==========以下是ABA問(wèn)題的產(chǎn)生=======");new Thread(()->{atomicReference.compareAndSet(100,101);atomicReference.compareAndSet(101,100);},"t1").start();new Thread(()->{//暫停1秒t2線程，保證t1完成一次ABAtry {TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace();}System.out.println(atomicReference.compareAndSet(100,2019)+"\t"+atomicReference.get());},"t2").start();try {TimeUnit.SECONDS.sleep(2); } catch (InterruptedException e) { e.printStackTrace();}System.out.println("=========以下是ABA問(wèn)題的解決==========");new Thread(()->{int stamp = atomicStampedReference.getStamp();System.out.println(Thread.currentThread().getName()+"\t第一次版本號(hào)："+atomicStampedReference.getStamp());//暫停1秒t3線程,讓t4也獲取到一樣的stamptry {TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace();}atomicStampedReference.compareAndSet(100,101,atomicStampedReference.getStamp(),atomicStampedReference.getStamp()+1);System.out.println(Thread.currentThread().getName()+"\t第二次版本號(hào)："+atomicStampedReference.getStamp());atomicStampedReference.compareAndSet(101,100,atomicStampedReference.getStamp(),atomicStampedReference.getStamp()+1);System.out.println(Thread.currentThread().getName()+"\t第三次版本號(hào)："+atomicStampedReference.getStamp());},"t3").start();new Thread(()->{int stamp = atomicStampedReference.getStamp();System.out.println(Thread.currentThread().getName()+"\t第一次版本號(hào)："+atomicStampedReference.getStamp());//暫停1秒t3線程try {TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace();}boolean result = atomicStampedReference.compareAndSet(100, 2019, stamp, stamp + 1);System.out.println(Thread.currentThread().getName()+"\t修改功否："+result);System.out.println(Thread.currentThread().getName()+"\t當(dāng)前實(shí)際最新值："+atomicStampedReference.getReference());},"t4").start();} }

總結(jié)

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