原子引用
public class AtomicReferenceDemo {
public static void main(String[] args) {
User cuzz = new User("cuzz", 18);
User faker = new User("faker", 20);
AtomicReference<User> atomicReference = new AtomicReference<>();
atomicReference.set(cuzz);
System.out.println(atomicReference.compareAndSet(cuzz, faker)); // true
System.out.println(atomicReference.get()); // User(userName=faker, age=20)
}
}
ABA问题的产生
/**
* 当有一个值从 A 改为 B 又改为 A,这就是 ABA 问题
**/
public class ABADemo {
private static AtomicReference<Integer> atomicReference = new AtomicReference<>(100);
public static void main(String[] args) {
new Thread(() -> {
atomicReference.compareAndSet(100, 101);
atomicReference.compareAndSet(101, 100);
}).start();
new Thread(() -> {
// 保证上面线程先执行
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
atomicReference.compareAndSet(100, 2019);
System.out.println(atomicReference.get()); // 2019
}).start();
}
}
时间戳原子引用
/**
* 我们先保证两个线程的初始版本为一致,后面修改是由于版本不一样就会修改失败
**/
public class ABADemo2 {
private static AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(100, 1);
public static void main(String[] args) {
new Thread(() -> {
int stamp = atomicStampedReference.getStamp();
System.out.println(Thread.currentThread().getName() + " 的版本号为:" + stamp);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
atomicStampedReference.compareAndSet(100, 101, atomicStampedReference.getStamp(), atomicStampedReference.getStamp() + 1 );
atomicStampedReference.compareAndSet(101, 100, atomicStampedReference.getStamp(), atomicStampedReference.getStamp() + 1 );
}).start();
new Thread(() -> {
int stamp = atomicStampedReference.getStamp();
System.out.println(Thread.currentThread().getName() + " 的版本号为:" + stamp);
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
boolean b = atomicStampedReference.compareAndSet(100, 2019, stamp, stamp + 1);
System.out.println(b); // false
System.out.println(atomicStampedReference.getReference()); // 100
}).start();
}
}