Class - java.util.concurrent.atomic.AtomicStampedReference
Created by : Mr Dk.
2020 / 06 / 25 11:39
Nanjing, Jiangsu, China
Definition
这个类将一个 int 类型的 stamp 与一个对象引用一起维护，两者将被共同地原子地更新。Stamp 的作用相当于一个版本号，每当对象引用被修改时，同时更新这个 stamp。有了 stamp，就可以解决 ABA 问题。
/**
* An {@code AtomicStampedReference} maintains an object reference
* along with an integer "stamp", that can be updated atomically.
*
* <p>Implementation note: This implementation maintains stamped
* references by creating internal objects representing "boxed"
* [reference, integer] pairs.
*
* @since 1.5
* @author Doug Lea
* @param <V> The type of object referred to by this reference
*/
public class AtomicStampedReference<V> {
}
Internal State
内部维护的状态是一个 volatile 修饰的 pair，而 pair 中包含了对象引用与 stamp：
private static class Pair<T> {
final T reference;
final int stamp;
private Pair(T reference, int stamp) {
this.reference = reference;
this.stamp = stamp;
}
static <T> Pair<T> of(T reference, int stamp) {
return new Pair<T>(reference, stamp);
}
}
private volatile Pair<V> pair;
Constructor
在构造对象时，传入对象引用和初始版本号：
/**
* Creates a new {@code AtomicStampedReference} with the given
* initial values.
*
* @param initialRef the initial reference
* @param initialStamp the initial stamp
*/
public AtomicStampedReference(V initialRef, int initialStamp) {
pair = Pair.of(initialRef, initialStamp);
}
Getter
获取内部状态。可以分别获取对象引用或 stamp，也可以直接一次性获取：
/**
* Returns the current value of the reference.
*
* @return the current value of the reference
*/
public V getReference() {
return pair.reference;
}
/**
* Returns the current value of the stamp.
*
* @return the current value of the stamp
*/
public int getStamp() {
return pair.stamp;
}
/**
* Returns the current values of both the reference and the stamp.
* Typical usage is {@code int[1] holder; ref = v.get(holder); }.
*
* @param stampHolder an array of size of at least one. On return,
* {@code stampholder[0]} will hold the value of the stamp.
* @return the current value of the reference
*/
public V get(int[] stampHolder) {
Pair<V> pair = this.pair;
stampHolder[0] = pair.stamp;
return pair.reference;
}
Setter
无条件设置内部状态：
/**
* Unconditionally sets the value of both the reference and stamp.
*
* @param newReference the new value for the reference
* @param newStamp the new value for the stamp
*/
public void set(V newReference, int newStamp) {
Pair<V> current = pair;
if (newReference != current.reference || newStamp != current.stamp)
this.pair = Pair.of(newReference, newStamp);
}
CAS Implementation
首先通过 Unsafe 类获得 pair 数据的地址偏移，然后对该数据整体进行 CAS：
// 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<V> cmp, Pair<V> 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;
}
}
CAS
当且仅当对象引用和 stamp 同时与期望值相等 (或没有发生改变) 时，CAS 才会成功：
/**
* Atomically sets the value of both the reference and stamp
* to the given update values if the
* current reference is {@code ==} to the expected reference
* and the current stamp is equal to the expected stamp.
*
* <p><a href="package-summary.html#weakCompareAndSet">May fail
* spuriously and does not provide ordering guarantees</a>, so is
* only rarely an appropriate alternative to {@code compareAndSet}.
*
* @param expectedReference the expected value of the reference
* @param newReference the new value for the reference
* @param expectedStamp the expected value of the stamp
* @param newStamp the new value for the stamp
* @return {@code true} if successful
*/
public boolean weakCompareAndSet(V expectedReference,
V newReference,
int expectedStamp,
int newStamp) {
return compareAndSet(expectedReference, newReference,
expectedStamp, newStamp);
}
/**
* Atomically sets the value of both the reference and stamp
* to the given update values if the
* current reference is {@code ==} to the expected reference
* and the current stamp is equal to the expected stamp.
*
* @param expectedReference the expected value of the reference
* @param newReference the new value for the reference
* @param expectedStamp the expected value of the stamp
* @param newStamp the new value for the stamp
* @return {@code true} if successful
*/
public boolean compareAndSet(V expectedReference,
V newReference,
int expectedStamp,
int newStamp) {
Pair<V> current = pair;
return
expectedReference == current.reference &&
expectedStamp == current.stamp &&
((newReference == current.reference &&
newStamp == current.stamp) ||
casPair(current, Pair.of(newReference, newStamp)));
}
以下函数试图通过 CAS 原子地设置 pair 中的版本号。当然前提是期望的对象引用需要与 pair 中的对象引用一致，不然说明其它线程修改了对象引用，再设置版本号是不安全的。对象引用一致后，试图将对象引用与新的版本号通过 CAS 更新到内部状态中：
/**
* Atomically sets the value of the stamp to the given update value
* if the current reference is {@code ==} to the expected
* reference. Any given invocation of this operation may fail
* (return {@code false}) spuriously, but repeated invocation
* when the current value holds the expected value and no other
* thread is also attempting to set the value will eventually
* succeed.
*
* @param expectedReference the expected value of the reference
* @param newStamp the new value for the stamp
* @return {@code true} if successful
*/
public boolean attemptStamp(V expectedReference, int newStamp) {
Pair<V> current = pair;
return
expectedReference == current.reference &&
(newStamp == current.stamp ||
casPair(current, Pair.of(expectedReference, newStamp)));
}
顺带一提
java.util.concurrent.atomic.AtomicMarkableReference 的实现与 AtomicMarkableReference 几乎一样。区别在于，AtomicMarkableReference 的 pair 内部维护的不是版本号，而是一个 boolean 类型的变量，代表对象引用是否被修改过。显然，在实现上会更简单。
AtomicStampedReference 基本上可以替代 AtomicMarkableReference。因为后者只能记录对象引用是否被修改过，而前者还可以记录对象引用被修改了几次。
/**
* An {@code AtomicMarkableReference} maintains an object reference
* along with a mark bit, that can be updated atomically.
*
* <p>Implementation note: This implementation maintains markable
* references by creating internal objects representing "boxed"
* [reference, boolean] pairs.
*
* @since 1.5
* @author Doug Lea
* @param <V> The type of object referred to by this reference
*/
public class AtomicMarkableReference<V> {
private static class Pair<T> {
final T reference;
final boolean mark;
private Pair(T reference, boolean mark) {
this.reference = reference;
this.mark = mark;
}
static <T> Pair<T> of(T reference, boolean mark) {
return new Pair<T>(reference, mark);
}
}
private volatile Pair<V> pair;
// ...
}