ReentrantLock分為公平鎖和非公平鎖碍遍，默認的為非公平鎖
public ReentrantLock() {
sync = new NonfairSync();
}
可以手動指定
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
先從非公平鎖講起
1 lock
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
首先嘗試cas把state設(shè)置為1，如果成功則獲取獨占鎖衙解，失敗執(zhí)行acquire
2 acquire
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
2.1 tryAcquire
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
先獲取當前獨占鎖的數(shù)量：
1.如果為0，則cas獲取鎖，成功則獲取獨占鎖夷恍。
2.如果不為0，則檢查當前線程是否是重入鎖媳维，是就獨占鎖數(shù)量加一并返回酿雪。
2.2.1 addWaiter
如果tryAcquire都失敗，那么就執(zhí)行入隊操作侄刽。
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
Node pred = tail;
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
enq(node);
return node;
}
如果尾節(jié)點不為空执虹，執(zhí)行一次快速入隊操作，如果cas成功唠梨，就入隊成功袋励。
尾節(jié)點為空或者快速入隊不成功，for循環(huán)執(zhí)行入隊直到成功
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
這里有個生成空節(jié)點的操作当叭，因為在存在隊列的情況下茬故，隊列頭節(jié)點表示的是當前擁有鎖的線程
2.2.1 acquireQueued
節(jié)點入隊成功之后再執(zhí)行線程掛起
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
注意 p == head && tryAcquire(arg)這個條件是這個函數(shù)的唯一出口，先看線程掛起部分p == head && tryAcquire(arg)
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
/*
* waitStatus must be 0 or PROPAGATE. Indicate that we
* need a signal, but don't park yet. Caller will need to
* retry to make sure it cannot acquire before parking.
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
判斷前置節(jié)點的狀態(tài)蚁鳖，在這里分為三種情況：
1.Node.SIGNAL,可以喚醒后繼節(jié)點磺芭，表示可以掛起單錢節(jié)點線程，所以直接返回
2.大于0醉箕，表示取消狀態(tài)钾腺，則往前遍歷刪除節(jié)點直至非取消狀態(tài)
3.如果不是前兩者徙垫，將前置節(jié)點置為Node.SIGNAL
線程掛起
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
2.3 selfInterrupt
static void selfInterrupt() {
Thread.currentThread().interrupt();
}
這是一個比較細節(jié)的地方，結(jié)合掛起部分看放棒。
線程掛起的方式是采用LockSupport.park姻报，能夠響應(yīng)中斷，但是不會拋異常间螟。也就是說掛起的線程有可能被ReentrantLock之外的線程喚醒吴旋，這時候就需要重置中斷狀態(tài)，并且保證線程正確被喚醒并獲取鎖的時候厢破，保持中斷狀態(tài)荣瑟。
3 unlock
采用LockSupport.unpark喚醒隊列下一個線程。
再來看公平鎖
其實差別就兩點：
1.沒有直接cas獲取鎖的操作
2.在獲取鎖數(shù)量為0的時候摩泪，不再直接進行cas操作笆焰，要先判斷下隊列中是否還有節(jié)點未執(zhí)行
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
}
