doAcquireShared()方法
該方法在共享模式以不響應中斷的方式阻塞等待獲取鎖，實現如下：
1、將當前線程封裝成節點入隊；
2、在死循環中調用park方法。第一次循環（自旋、acquire loop），或者被喚醒從park方法返回後，會判斷前驅節點是否是頭節點，以及調用tryAcquire()方法是否返回true，如果這2個條件都爲真，由當前節點自己設置爲頭節點，並將後繼節點喚醒，然後return;退出死循環。如果這2個條件不滿足，會繼續調用park方法阻塞等待。
3、在第二步中，被喚醒從park方法返回後，有一個額外操作就是會判斷線程的中斷狀態，如果中斷狀態爲true，僅僅是設置中斷標誌位interrupted，不拋出中斷異常。
4、在第二步中，在判斷那2個條件不滿足，和調用park方法阻塞等待之間，還有一個操作就是判斷在獲取失敗後是否應該調用park方法阻塞等待。即shouldParkAfterFailedAcquire方法。在acquire loop中會不斷調用該方法retry，使該方法最終總是趨向於返回true。
/**
* Acquires in shared uninterruptible mode.
* @param arg the acquire argument
*/
private void doAcquireShared(int arg) {
final Node node = addWaiter(Node.SHARED);//將當前線程封裝成節點入隊
boolean failed = true;
try {
boolean interrupted = false;
for (;;) { //死循環
final Node p = node.predecessor();
if (p == head) { //如果前驅節點是頭節點
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r); //當前節點設置爲頭節點，並喚醒後繼節點
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return; //退出死循環
}
}
if (shouldParkAfterFailedAcquire(p, node) && //獲取失敗後判斷是否應該park阻塞等待
parkAndCheckInterrupt()) //調用park方法阻塞等待，park方法返回後判斷是否中斷
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
doAcquireSharedInterruptibly方法
該方法在共享模式以響應中斷的方式阻塞獲取鎖。實現邏輯與doAcquireShared方法是基本相同的。不同的是在從park方法返回後，如果判斷線程的中斷狀態爲true，會拋出中斷異常。
/**
* Acquires in shared interruptible mode.
* @param arg the acquire argument
*/
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
shouldParkAfterFailAcquire方法
該方法在當前節點獲取失敗時判斷是否應該使線程阻塞等待。它會檢查和更新前驅結點的狀態。在acquire loop中會不斷調用該方法retry，使最終總是趨向於返回true：
1、剛入隊的節點，他們的waitStatus都是int數據類型的初始值0。
2、對於剛入隊的節點，它的前驅節點的waitStatus，要麼是0，要麼是propagate，要麼是cancelled。如果前驅節點的waitStatus是0，在第一次acquire loop中，會使用cas將前驅節點的waitStatus設置爲SIGNAL。如果前驅節點的waitStatus是propagate，說明該前驅節點是head節點，在第一次acquire loop中，會使用cas將前驅節點的waitStatus設置爲SIGNAL。如果前驅節點的waitStatus是cancelled，會從後遍歷直至找到有效的前驅節點。雖然在第一次acquire loop中調用shouldParkAfterFailAcquire方法，它的返回值是false，但是在多次acquire loop後，前驅節點的waitStatus總爲SIGNAL，該方法的返回值總是true。
/**
* Checks and updates status for a node that failed to acquire.
* Returns true if thread should block. This is the main signal
* control in all acquire loops. Requires that pred == node.prev.
*
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*/
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;
}
parkAndCheckInterrupt方法
調用park方法阻塞等待，park方法返回後判斷是否中斷
/**
* Convenience method to park and then check if interrupted
*
* @return {@code true} if interrupted
*/
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
setHeadAndPropagate方法
將當前節點設置爲頭節點，並將後繼節點喚醒，頭節點的waitStatus設爲propagate。
/**
* Sets head of queue, and checks if successor may be waiting
* in shared mode, if so propagating if either propagate > 0 or
* PROPAGATE status was set.
*
* @param node the node
* @param propagate the return value from a tryAcquireShared
*/
private void setHeadAndPropagate(Node node, int propagate) {
Node h = head; // Record old head for check below
setHead(node);
/*
* Try to signal next queued node if:
* Propagation was indicated by caller,
* or was recorded (as h.waitStatus either before
* or after setHead) by a previous operation
* (note: this uses sign-check of waitStatus because
* PROPAGATE status may transition to SIGNAL.)
* and
* The next node is waiting in shared mode,
* or we don't know, because it appears null
*
* The conservatism in both of these checks may cause
* unnecessary wake-ups, but only when there are multiple
* racing acquires/releases, so most need signals now or soon
* anyway.
*/
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
Node s = node.next;
if (s == null || s.isShared())
doReleaseShared(); //將後繼節點喚醒，頭節點的waitStatus設爲propagate
}
}
doReleaseShared方法
將後繼節點喚醒，頭節點的waitStatus設爲propagate。
在死循環內：
1、獲取head節點的waitStatus;
2、如果head節點的waitStatus爲SIGNAL，通過cas將head節點的waitStatus改爲0，必須確保cas執行成功，否則通過新一輪循環，再次執行cas操作直至成功爲止；（死循環與cas構成自旋鎖保證cas執行成功）
3、如果第二步執行成功，調用unparkSuccessor方法喚醒後繼節點；
4、第三步執行完成後，開始新一輪的循環，判斷如果head節點的waitStatus爲0，通過cas將head節點的waitStatus改爲propagate，必須確保cas執行成功，否則通過新一輪循環，再次執行cas操作直至成功爲止；（死循環與cas構成自旋鎖保證cas執行成功）
由此可見，head節點的waitStatus爲SIGNAL時，通過兩步改爲propagate：
compareAndSetWaitStatus(h, Node.SIGNAL, 0)
compareAndSetWaitStatus(h, 0, Node.PROPAGATE)
爲什麼要經過兩步，不直接把SIGNAL改爲propagate呢？原因在unparkSuccessor方法。如果直接把SIGNAL改爲propagate，則在unparkSuccessor方法裏又會被設置爲0。
5、在第四步完成後，判斷當前head節點是否發生改變，如果沒有發生改變，break退出死循環。在第三步喚醒後繼節點後，後繼節點（所在的線程）會將自己設置爲頭節點，此時head節點就會發生改變，對新head節點繼續執行循環，從而實現release propagate。有細心的網友可能就發現了，新head節點也會調用到doReleaseShared方法，這樣會存在多個線程同時調用doReleaseShared方法，執行死循環裏的邏輯。是的，代碼註釋也說明了：
Ensure that a release propagates, even if there are other in-progress acquires/releases.
/**
* Release action for shared mode -- signals successor and ensures
* propagation. (Note: For exclusive mode, release just amounts
* to calling unparkSuccessor of head if it needs signal.)
*/
private void doReleaseShared() {
/*
* Ensure that a release propagates, even if there are other
* in-progress acquires/releases. This proceeds in the usual
* way of trying to unparkSuccessor of head if it needs
* signal. But if it does not, status is set to PROPAGATE to
* ensure that upon release, propagation continues.
* Additionally, we must loop in case a new node is added
* while we are doing this. Also, unlike other uses of
* unparkSuccessor, we need to know if CAS to reset status
* fails, if so rechecking.
*/
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}
unparkSuccessor方法
如果節點的waitStatus爲負，通過cas設置爲0；找到有效的後繼節點，調用unpark方法
/**
* Wakes up node's successor, if one exists.
*
* @param node the node
*/
private void unparkSuccessor(Node node) {
/*
* If status is negative (i.e., possibly needing signal) try
* to clear in anticipation of signalling. It is OK if this
* fails or if status is changed by waiting thread.
*/
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0);
/*
* Thread to unpark is held in successor, which is normally
* just the next node. But if cancelled or apparently null,
* traverse backwards from tail to find the actual
* non-cancelled successor.
*/
Node s = node.next;
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
LockSupport.unpark(s.thread);
}