在翻閱AQS(AbstractQueuedSynchronizer)類的過程中，發現其進行原子操作的時候采用的是CAS。涉及的代碼如下：
1: private static final Unsafe unsafe = Unsafe.getUnsafe();
2: private static final long stateOffset;
3: private static final long headOffset;
4: private static final long tailOffset;
5: private static final long waitStatusOffset;
6: private static final long nextOffset;
7: ?
8: static {
9: try {
10: stateOffset = unsafe.objectFieldOffset
11: (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
12: headOffset = unsafe.objectFieldOffset
13: (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
14: tailOffset = unsafe.objectFieldOffset
15: (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
16: waitStatusOffset = unsafe.objectFieldOffset
17: (Node.class.getDeclaredField("waitStatus"));
18: nextOffset = unsafe.objectFieldOffset
19: (Node.class.getDeclaredField("next"));
20: ?
21: } catch (Exception ex) { throw new Error(ex); }
22: }
23: ?
24: /**
25: * CAS head field. Used only by enq.
26: */
27: private final boolean compareAndSetHead(Node update) {
28: return unsafe.compareAndSwapObject(this, headOffset, null, update);
29: }
30: ?
31: /**
32: * CAS tail field. Used only by enq.
33: */
34: private final boolean compareAndSetTail(Node expect, Node update) {
35: return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
36: }
37: ?
38: /**
39: * CAS waitStatus field of a node.
40: */
41: private static final boolean compareAndSetWaitStatus(Node node,
42: int expect,
43: int update) {
44: return unsafe.compareAndSwapInt(node, waitStatusOffset,
45: expect, update);
46: }
47: ?
48: /**
49: * CAS next field of a node.
50: */
51: private static final boolean compareAndSetNext(Node node,
52: Node expect,
53: Node update) {
54: return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
55: }
?
可以看到用到了compareAndSwapObject和compareAndSwapInt方法，那么究竟是怎么用其來實現原子操作的呢？
我們以compareAndSwapObject方法為例，其源碼大致如下：
1: UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapObject(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jobject e_h, jobject x_h))
2: UnsafeWrapper("Unsafe_CompareAndSwapObject");
3: oop x = JNIHandles::resolve(x_h); //待更新的新值，也就是UpdateValue
4: oop e = JNIHandles::resolve(e_h); //期望值,也就是ExpectValue
5: oop p = JNIHandles::resolve(obj); //待操作對象
6: HeapWord* addr = (HeapWord *)index_oop_from_field_offset_long(p, offset);//根據操作的對象和其在內存中的offset，計算出內存中具體位置
7: oop res = oopDesc::atomic_compare_exchange_oop(x, addr, e, true);// 如果操作對象中的值和e期望值一致，則更新存儲值為x，反之不更新
8: jboolean success = (res == e);
9: if (success) //滿足更新條件
10: update_barrier_set((void*)addr, x); // 更新存儲值為x
11: return success;
12: UNSAFE_END
從上述源碼可以看到，compareAndSwapObject方法中的第一個參數和第二個參數，用于確定待操作對象在內存中的具體位置的，然后取出值和第三個參數進行比較，如果相等，則將內存中的值更新為第四個參數的值，同時返回true，表明原子更新操作完畢。反之則不更新內存中的值，同時返回false，表明原子操作失敗。
同樣的，compareAndSwapInt方法也是相似的道理，第一個，第二個參數用來確定當前操作對象在內存中的存儲值，然后和第三個expect value比較，如果相等，則將內存值更新為第四個updaet value值。
由于原始的方法使用比較麻煩，所以在AQS中進行了封裝，大大簡化了操作：
1: private static final Unsafe unsafe = Unsafe.getUnsafe();
2: private static final long stateOffset;
3: private static final long headOffset;
4: private static final long tailOffset;
5: private static final long waitStatusOffset;
6: private static final long nextOffset;
7: ?
8: static {
9: try {
10: stateOffset = unsafe.objectFieldOffset
11: (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
12: headOffset = unsafe.objectFieldOffset
13: (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
14: tailOffset = unsafe.objectFieldOffset
15: (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
16: waitStatusOffset = unsafe.objectFieldOffset
17: (Node.class.getDeclaredField("waitStatus"));
18: nextOffset = unsafe.objectFieldOffset
19: (Node.class.getDeclaredField("next"));
20: ?
21: } catch (Exception ex) { throw new Error(ex); }
22: }
23: ?
24: /**
25: * CAS head field. Used only by enq.
26: */
27: private final boolean compareAndSetHead(Node update) {
28: return unsafe.compareAndSwapObject(this, headOffset, null, update);
29: }
30: ?
31: /**
32: * CAS tail field. Used only by enq.
33: */
34: private final boolean compareAndSetTail(Node expect, Node update) {
35: return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
36: }
37: ?
38: /**
39: * CAS waitStatus field of a node.
40: */
41: private static final boolean compareAndSetWaitStatus(Node node,
42: int expect,
43: int update) {
44: return unsafe.compareAndSwapInt(node, waitStatusOffset,
45: expect, update);
46: }
47: ?
48: /**
49: * CAS next field of a node.
50: */
51: private static final boolean compareAndSetNext(Node node,
52: Node expect,
53: Node update) {
54: return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
55: }
可以在其他項目中作為小模塊進行引入并使用。這樣使用起來就非常方便了：
1: ?
2: /**
3: * Creates and enqueues node for current thread and given mode.
4: *
5: * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
6: * @return the new node
7: */
8: private Node addWaiter(Node mode) {
9: Node node = new Node(Thread.currentThread(), mode);
10: // Try the fast path of enq; backup to full enq on failure
11: Node pred = tail;
12: if (pred != null) {
13: node.prev = pred;
14: if (compareAndSetTail(pred, node)) {
15: pred.next = node;
16: return node;
17: }
18: }
19: enq(node);
20: return node;
21: }
?
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