ReentrantLock原始碼之二unlock方法解析(鎖的釋放)
阿新 • • 發佈：2019-01-23
在前面一節中，我們分析了ReentrantLock.lock()方法，接下來我們接著分析ReentrantLock.unlock()方法。
java.util.concurrent.locks.ReentrantLock
2
public void unlock() {
3
sync.release(1);
4
}
5
6java.util.concurrent.locks.AbstractQueuedSynchronizer
7public final boolean release(int arg) {
8
if (tryRelease(arg)) {
9 Node h = head;
10
11 unparkSuccessor(h);
12 return true;
13
}
14 return false;
15 }
16
17
18 protected final boolean tryRelease(int releases) {
19 int c = getState() - releases; //重入鎖加鎖的次數-釋放數量20 if (Thread.currentThread() != getExclusiveOwnerThread()) //
22 boolean free = false;
23 if (c == 0) { //加鎖次數=釋放數量24 free = true;
25 setExclusiveOwnerThread(null); //清除鎖擁有者標識26 }
27 setState(c); //設定加鎖狀態28 return free;
29 }
30
31
32 /**33 * Wakes up node's successor, if one exists.
34 *
35 * @param node the node
36 */
37 private void unparkSuccessor(Node node) {
38 /*39 * Try to clear status in anticipation of signalling. It is
40 * OK if this fails or if status is changed by waiting thread.
41 */
42 compareAndSetWaitStatus(node, Node.SIGNAL, 0); //清除頭節點signal狀態43
44 /*45 * Thread to unpark is held in successor, which is normally
46 * just the next node. But if cancelled or apparently null,
47 * traverse backwards from tail to find the actual
48 * non-cancelled successor.
49 */
50 Node s = node.next;
51 if (s == null || s.waitStatus > 0) { //等待佇列喚醒的競爭滿足FIFO，本段程式碼主要是尋找最靠近頭節點的，且waitStatus為signal、condition的連結串列節點52 s = null;
53 for (Node t = tail; t != null && t != node; t = t.prev)
54 if (t.waitStatus <= 0)
55 s = t;
56 }
57 if (s != null)
58 LockSupport.unpark(s.thread);
59 }
1.ReentrantLock.unlock()分析
(1)首先嚐試釋放鎖，如果要求釋放數等於鎖狀態數，那麼將鎖狀態位清0，清除鎖所有者，返回true；否則返回false;
(2)如果(1)返回的是true，說明鎖完全釋放。接下來將檢查等待佇列，並選擇下一個節點恢復（該節點必須是waitStatus)，
如果該節點不存在或者被取消，則選擇從尾節點開始，選取最靠近頭節點的等待節點,同時清理佇列中執行緒被取消的節點；
大家回憶下lock()實現機制，新節點是從尾部插入的。這就說明AQS裡面的 CLH其實是一個FIFO
(3)如果(1)返回false，說明鎖只是部分釋放，當前執行緒仍舊持有該鎖；
1java.util.concurrent.locks.ReentrantLock2
public void unlock() {3
sync.release(1);4
}5
6
java.util.concurrent.locks.AbstractQueuedSynchronizer7
public final boolean release(int arg) {8
if (tryRelease(arg)) { 9
Node h = head;10
if (h != null && h.waitStatus != 0)11
unparkSuccessor(h);12
return true;13
}14
return false;15
}16
17
18
protected final boolean tryRelease(int releases) {19
int c = getState() - releases; //重入鎖加鎖的次數-釋放數量20 if (Thread.currentThread() != getExclusiveOwnerThread()) // throw new IllegalMonitorStateException();22
boolean free = false;23
if (c == 0) { //加鎖次數=釋放數量24 free = true;25
setExclusiveOwnerThread(null); //清除鎖擁有者標識26 }27
setState(c); //設定加鎖狀態28 return free;29
}30
31
32
/**33 * Wakes up node's successor, if one exists.34
*35
* @param node the node36
*/37
private void unparkSuccessor(Node node) {38
/*39 * Try to clear status in anticipation of signalling. It is40
* OK if this fails or if status is changed by waiting thread.41
*/42
compareAndSetWaitStatus(node, Node.SIGNAL, 0); //清除頭節點signal狀態4344
/*45 * Thread to unpark is held in successor, which is normally46
* just the next node. But if cancelled or apparently null,47
* traverse backwards from tail to find the actual48
* non-cancelled successor.49
*/50
Node s = node.next;51
if (s == null || s.waitStatus > 0) { //等待佇列喚醒的競爭滿足FIFO，本段程式碼主要是尋找最靠近頭節點的，且waitStatus為signal、condition的連結串列節點52 s = null;53
for (Node t = tail; t != null && t != node; t = t.prev)54
if (t.waitStatus <= 0)55
s = t;56
}57
if (s != null)58
LockSupport.unpark(s.thread);59
}