一 UML類圖
1.1、ReentrantLock
通過類圖ReentrantLock是同步鎖，同一時間只能有一個線程獲取到鎖，其他獲取該鎖的線程會被阻塞而被放入AQS阻塞隊列中。ReentrantLock類繼承Lock接口；內部抽象類Sync實現抽象隊列同步器AbstractQueuedSynchronizer；Sync類有兩個子類NonfairSync（非公平鎖）和FairSync（公平鎖），默認構造方法使用非公平鎖，可以使用帶布爾參數的構造方法指定使用公平鎖；ReentrantLock可以創建多個條件進行綁定。
1.2、AbstractQueuedSynchronizer
AbstractQueuedSynchronizer：抽象隊列同步器，維護一個volatile int state變量標識共享資源和一個FIFO線程阻塞隊列（AQS隊列）。
protected final void setState(int newState)：設置state值
protected final int getState()：獲取state值
protected final boolean compareAndSetState(int expect, int update)：CAS設置state值
AQS有兩種共享資源類型：SHARED（共享）和EXCLUSIVE（獨佔），針對類型有不同的方法：
protected boolean tryAcquire(int arg)：獨佔類型獲取鎖
protected boolean tryRelease(int arg)：獨佔類型釋放鎖
protected int tryAcquireShared(int arg)：共享類型獲取鎖
protected boolean tryReleaseShared(int arg)：共享類型釋放鎖
protected boolean isHeldExclusively()：是否是獨佔類型
1.3、線程節點類型waitStatus
AQS隊列中節點的waitStatus枚舉值（java.util.concurrent.locks.AbstractQueuedSynchronizer.Node）含義：
static final int CANCELLED = 1; //線程被取消
static final int SIGNAL = -1; //成功的線程需要被喚醒
static final int CONDITION = -2; //線程在條件隊列中等待
static final int PROPAGATE = -3; //釋放鎖是需要通知其他節點
二 原理分析
2.1 獲取鎖
2.1.1 void lock()方法
調用線程T調用該方法嘗試獲取當前鎖。
①如果鎖未被其他線程獲取，則調用線程T成功獲取到當前鎖，然後設置當前鎖的擁有者為調用線程T，並設置AQS的狀態值state為1，然後直接返回。
②如果調用線程T之前已經獲取當前鎖，則只設置設置AQS的狀態值state加1，然後返回。
③如果當前鎖已被其他線程獲取，則調用線程T放入AQS隊列后阻塞掛起。
public void lock() { sync.lock();//委託內部公平鎖和非公平鎖獲取鎖 }
//非公平鎖
final void lock() { if (compareAndSetState(0, 1))//設置AQS狀態值為1 setExclusiveOwnerThread(Thread.currentThread());//成功設置鎖的線程擁有者 else acquire(1);//失敗加入到AQS隊列阻塞掛起 } //公平鎖 final void lock() { acquire(1); }
非公平鎖分析：
//調用父類AbstractOwnableSynchronizer方法CAS設置state值，成功返回true，失敗返回false protected final boolean compareAndSetState(int expect, int update) { return unsafe.compareAndSwapInt(this, stateOffset, expect, update); } //調用父類AbstractOwnableSynchronizer方法，設置當前線程為鎖的擁有者 protected final void setExclusiveOwnerThread(Thread thread) { exclusiveOwnerThread = thread; }
//調用AbstractQueuedSynchronizer父類方法，第一次獲取鎖失敗 public final void acquire(int arg) { if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg))//排它鎖類型 selfInterrupt(); } //NonfairSync子類，重寫父類方法 protected final boolean tryAcquire(int acquires) { return nonfairTryAcquire(acquires); }
//Sync類非公平鎖嘗試獲取鎖 final boolean nonfairTryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) {//二次獲取鎖 if (compareAndSetState(0, acquires)) { setExclusiveOwnerThread(current); return true; } } else if (current == getExclusiveOwnerThread()) {//當前線程已獲取鎖，AQS狀態值加1 int nextc = c + acquires; if (nextc < 0) // overflow throw new Error("Maximum lock count exceeded"); setState(nextc); return true; } return false; }
//AbstractQueuedSynchronizer類創建節點，添加到AQS隊列後面 private Node addWaiter(Node mode) { Node node = new Node(Thread.currentThread(), mode);//創建AQS隊列的節點，節點類型排它鎖 Node pred = tail;//尾結點 if (pred != null) { node.prev = pred;//新節點的前一個節點是尾結點 if (compareAndSetTail(pred, node)) {//CAS機制添加節點 pred.next = node;//尾結點執行新的節點 return node; } } enq(node); return node; }
//插入節點到隊列中
private Node enq(final Node node) { for (;;) {//循環的方式，直至創建成功 Node t = tail;//尾結點 if (t == null) { //尾結點為空，初始化 if (compareAndSetHead(new Node()))//第一步：CAS創建頭結點（哨兵節點）一個空節點 tail = head; } else { node.prev = t; if (compareAndSetTail(t, node)) {//第二步：CAS設置尾結點 t.next = node; return t; } } } }
// final boolean acquireQueued(final Node node, int arg) { boolean failed = true; try { boolean interrupted = false; for (;;) { final Node p = node.predecessor();//前向節點 if (p == head && tryAcquire(arg)) {//如果p節點是頭結點，node作為隊列第二個節點 setHead(node);//將頭節點設置為node節點，node節點出隊列 p.next = null; //原頭結點設置為空，便於GC回收 failed = false; return interrupted; } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) interrupted = true; } } finally { if (failed) cancelAcquire(node);//失敗解鎖 } }
private void setHead(Node node) { head = node; node.thread = null; node.prev = null; }
//阻塞掛起當前線程
static void selfInterrupt() { Thread.currentThread().interrupt(); }
//
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);//大於0代表線程被取消 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; } //線程阻塞，打斷線程 private final boolean parkAndCheckInterrupt() { LockSupport.park(this); return Thread.interrupted(); }
公平鎖分析：
protected final boolean tryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) { if (!hasQueuedPredecessors() && compareAndSetState(0, acquires)) {//與非公平鎖相比，區別就在標紅的位置 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; }
public final boolean hasQueuedPredecessors() { // The correctness of this depends on head being initialized // before tail and on head.next being accurate if the current // thread is first in queue. Node t = tail; // Read fields in reverse initialization order Node h = head; Node s;
//①h != t：表示AQS隊列頭結點和尾結點不相同，隊列不為空；
//②(s = h.next) == null || s.thread != Thread.currentThread()：頭結點（哨兵節點）為空或者next節點不等於當前線程 return h != t && ((s = h.next) == null || s.thread != Thread.currentThread()); }
2.1.2 void lockInterruptibly()方法
與2.2.1方法相似，不同之處在於：該方法對中斷進行響應，其他線程調用當前線程中斷方法，拋出InterruptedException。
2.1.3 boolean tryLock()方法
嘗試獲取鎖。注意：該方法不會引起當前線程阻塞。
2.1.4 boolean tryLock(long timeout, TimeUnit unit)方法
與2.1.3方法相似，不同之處在於：設置了超時時間。
2.2 釋放鎖
嘗試釋放鎖。
如果當前線程T已獲取鎖，則調用該方法更新AQS狀態值減1。如果當前狀態值為0，則釋放鎖；如果當前狀態值部位0，則只是減1操作。
如果當前線程T未獲取鎖，則調用該方法是會拋出IllegalMonitorStateException異常。
2.2.1 void unlock()方法
public void unlock() { sync.release(1); } //調用AbstractQueuedSynchronizer方法 public final boolean release(int arg) { if (tryRelease(arg)) { Node h = head; if (h != null && h.waitStatus != 0) unparkSuccessor(h);//喚醒線程 return true; } return false; } //回調Sync類釋放鎖 protected final boolean tryRelease(int releases) { int c = getState() - releases; if (Thread.currentThread() != getExclusiveOwnerThread()) throw new IllegalMonitorStateException(); boolean free = false; if (c == 0) { free = true; setExclusiveOwnerThread(null);//設置鎖的擁有線程為空 } setState(c); return free; } // 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;//線程阻塞等待狀態 if (ws < 0) compareAndSetWaitStatus(node, ws, 0);//CAS設置狀態 /* * 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)//遍歷AQS隊列 if (t.waitStatus <= 0) s = t; } if (s != null) LockSupport.unpark(s.thread);//喚醒線程 }
h != t
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