chapter15_原子变量与非阻塞同步机制_4_非阻塞算法

原子变量的compareAndSet操作__既能提供原子性, 又能提供可见性__

public class AtomicInteger extends Number implements java.io.Serializable {

...

private static final jdk.internal.misc.Unsafe U = jdk.internal.misc.Unsafe.getUnsafe();

...

public final int incrementAndGet() {
return U.getAndAddInt(this, VALUE, 1) + 1;
}

...
}

public final class Unsafe {

...

@HotSpotIntrinsicCandidate
public final int getAndAddInt(Object o, long offset, int delta) {

int v;

do {
v = getIntVolatile(o, offset);
} while (!weakCompareAndSetInt(o, offset, v, v + delta));

return v;
}

...
}

非阻塞计数器

@ThreadSafe
public class CasCounter {

private SimulatedCAS value;

public int getValue() {

return value.get();
}

public int increment() {

int v;

do {
v = value.get();
} while (v != value.compareAndSwap(v, v + 1));

return v + 1;
}
}

非阻塞的栈

@ThreadSafe
public class ConcurrentStack<E> {

private AtomicReference<Node<E>> top = new AtomicReference<Node<E>>();

public void push(E item) {

Node<E> newHead = new Node<>(item);
Node<E> oldHead;

do {
oldHead = top.get();
newHead.next = oldHead;
} while (!top.compareAndSet(oldHead, newHead));
}

public E pop() {

Node<E> oldHead;
Node<E> newHead;

do {
oldHead = top.get();
if (oldHead == null) {
return null;
}

newHead = oldHead.next;

} while (!top.compareAndSet(oldHead, newHead));

return oldHead.item;
}

private static class Node<E> {

public final E item;
public Node<E> next;

public Node(E item) {

this.item = item;
}
}
}

CAS的基本使用模式是

在更新某个值时存在不确定性, 在更新失败时重新尝试