1.AtomicReference对“对象”进行原子性操作。
// 使用 null 初始值创建新的 AtomicReference。
AtomicReference()
// 使用给定的初始值创建新的 AtomicReference。
AtomicReference(V initialValue)
// 如果当前值 == 预期值,则以原子方式将该值设置为给定的更新值。
boolean compareAndSet(V expect, V update)
// 获取当前值。
V get()
// 以原子方式设置为给定值,并返回旧值。
V getAndSet(V newValue)
// 最终设置为给定值。
void lazySet(V newValue)
// 设置为给定值。
void set(V newValue)
// 返回当前值的字符串表示形式。
String toString()
// 如果当前值 == 预期值,则以原子方式将该值设置为给定的更新值。
boolean weakCompareAndSet(V expect, V update)
AtomicReference源码:
package java.util.concurrent.atomic;
public class AtomicStampedReference {
private static class Pair {
final T reference;
final int stamp;
private Pair(T reference, int stamp) {
this.reference = reference;
this.stamp = stamp;
}
static Pair of(T reference, int stamp) {
return new Pair(reference, stamp);
}
}
private volatile Pair pair;
public AtomicStampedReference(V initialRef, int initialStamp) {
pair = Pair.of(initialRef, initialStamp);
}
public V getReference() {
return pair.reference;
}
public int getStamp() {
return pair.stamp;
}
public V get(int[] stampHolder) {
Pair pair = this.pair;
stampHolder[0] = pair.stamp;
return pair.reference;
}
public boolean weakCompareAndSet(V expectedReference,
V newReference,
int expectedStamp,
int newStamp) {
return compareAndSet(expectedReference, newReference,
expectedStamp, newStamp);
}
public boolean compareAndSet(V expectedReference,
V newReference,
int expectedStamp,
int newStamp) {
Pair current = pair;
return
expectedReference == current.reference &&
expectedStamp == current.stamp &&
((newReference == current.reference &&
newStamp == current.stamp) ||
casPair(current, Pair.of(newReference, newStamp)));
}
public void set(V newReference, int newStamp) {
Pair current = pair;
if (newReference != current.reference || newStamp != current.stamp)
this.pair = Pair.of(newReference, newStamp);
}
public boolean attemptStamp(V expectedReference, int newStamp) {
Pair current = pair;
return
expectedReference == current.reference &&
(newStamp == current.stamp ||
casPair(current, Pair.of(expectedReference, newStamp)));
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long pairOffset =
objectFieldOffset(UNSAFE, "pair", AtomicStampedReference.class);
private boolean casPair(Pair cmp, Pair val) {
return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val);
}
static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
String field, Class> klazz) {
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
}
}
}
AtomicReference 通过"volatile"和"Unsafe提供的CAS函数实现"原子操作。
2.AtomicReference存在ABA问题,而AtomicStampedRerence可以解决ABA问题。
ABA问题:
线程1准备用CAS将变量的值由A替换为B,在此之前,线程2将变量的值由A替换为C,又由C替换为A,然后线程1执行CAS时发现变量的值仍然为A,所以CAS成功。但实际上这时的现场已经和最初不同了,尽管CAS成功,但可能存在潜藏的问题。
AtomicStampedRerence增加一个时间戳,进行CAS操作时不仅需要维护对象值,还需要维护时间戳。
对象值和时间戳都必须满足期望值,才能更新新值。
测试案例:
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicStampedReference;
/**
* Created by Administrator on 2017/10/18.
*/
public class ABA {
private static AtomicInteger atomicInt = new AtomicInteger(100);
private static AtomicStampedReference atomicStampedRef = new AtomicStampedReference(100, 0);
public static void main(String[] args) throws InterruptedException {
Thread intT1 = new Thread(() -> {
atomicInt.compareAndSet(100, 101);
atomicInt.compareAndSet(101, 100);
}
);
Thread intT2 = new Thread(() -> {
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
}
boolean c3 = atomicInt.compareAndSet(100, 101);
System.out.println(c3); // true
}
);
intT1.start();
intT2.start();
intT1.join();
intT2.join();
Thread refT1 = new Thread(() -> {
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
}
atomicStampedRef.compareAndSet(100, 101, atomicStampedRef.getStamp(), atomicStampedRef.getStamp() + 1);
atomicStampedRef.compareAndSet(101, 100, atomicStampedRef.getStamp(), atomicStampedRef.getStamp() + 1);
}
);
Thread refT2 = new Thread(() -> {
int stamp = atomicStampedRef.getStamp();
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
}
boolean c3 = atomicStampedRef.compareAndSet(100, 101, stamp, stamp + 1);
System.out.println(c3); // false
}
);
refT1.start();
refT2.start();
}
}