ReetrantLock?? 內部靜態類? Sync? 繼承AbstractQueuedSynchronizer（負責阻塞隊列，線程去槍戰鎖,并且包含了node）

NonfairSync? extends Sync extends AbstractQueuedSynchronizer

FairSync extends Sync extends AbstractQueuedSynchronizer

AbstractQueuedSynchronizer?中有一個模板方法 tryacquire（模板模式）

非公平鎖NonfairSync

?final void lock() {

　　　　//直接槍占啊。基于cas保證state操作在并發的時候是正確的
??????????? if (compareAndSetState(0, 1))
??????????????? setExclusiveOwnerThread(Thread.currentThread());
??????????? else
??????????????? acquire(1);
??????? }

公平鎖FairSync
??????? final void lock() {
??????????? acquire(1);
??????? }

acquire的方法 是在AbstractQueuedSynchronizer中實現的

public final void acquire(int arg) {

//addWaiter是加入到隊列尾巴 吧線程封裝成node? acquireQueued里面又做了循環。所有線程去搶占鎖。至于里面的對線程的狀態的切換不用關系
??????? if (!tryAcquire(arg) &&
??????????? acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
??????????? selfInterrupt();
??? }

tryAcquire 在AbstractQueuedSynchronizer 只是實現了個方法

?protected boolean tryAcquire(int arg) {
??????? throw new UnsupportedOperationException();
??? }

具體的實現分別在NonfairSync? FairSync

前者

?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;
??????? }

后者

protected final boolean tryAcquire(int acquires) {
??????????? final Thread current = Thread.currentThread();
??????????? int c = getState();
??????????? if (c == 0) {

　　　　　　//判斷當前線程是否是隊列頭 保證了按順序
??????????????? 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;
??????? }

轉載于:https://www.cnblogs.com/tecnologycc/p/10174689.html

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