jdk10 ReentrantLock阅读笔记1（构造方法、锁获取、释放）
一般用法
略过不表
构造方法
public ReentrantLock() {
sync = new NonfairSync();
}
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}无参构造函数默认是非公平锁，有参数的根据参数判断是公平的还是非公平的。那实际上ReentrantLock就是把Sync包了一下罢了，两种锁的区别会在下面方法代码中体现
加锁操作
lock()
public void lock() {
sync.lock();
}没什么可看的，直接看两种Sync的实现吧：
非公平锁：
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}把stateOffset（AbstractQueuedSynchronizer）对应的原子变量设置为1，理解一下，0应该代表没有线程持有锁，1表示有线程持有锁。如果持有成功，将AbstractOwnableSynchronizer中的exclusiveOwnerThread设置为当前线程，就是在锁对象中记录一下拿到锁的那个线程。如果没有成功，那么执行acquire。
看一下aquire的实现
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}非公平锁中,tryAcquire执行的是nonfairTryAcquire
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;
}如果状态是0的话，再次执行一遍更改状态和设置持有锁的线程，其实就是看一看锁是否被释放了，重新获取一下。如果state不等于0，而且当前的线程就是持有锁的线程，那么将state设置成 原 state+重入数量。如果都不满足，那么返回false，表示获取锁失败了。获取失败的话，执行acquireQueued(addWaiter(Node.EXCLUSIVE), arg)，看一下这个方法
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);
Node pred = tail;
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
enq(node);
return node;
}
新建一个node，保存当前线程和锁模式，这里是排它锁。tail不为null，也就是当前新建的node不是第一个节点的时候，会cas的将这个node添加到队列里。如果成功了，直接返回node，如不成功，那么就执行enq入队
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;
}
}
}
}如果是第一个元素，那么cas 的设置一个头节点，当前节点为头节点的下一节点，当前节点的prev为头节点，这样的话，除了头结点，prev都不为null。返回当前节点的前一节点。如果tail已经存在，就cas添加到队列。addWaiter执行完了，接下来执行外层的acquireQueued
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);
}
}先node.predecessor()获取当前节点的前一节点。如果前一节点是头结点，并且可以拿到锁，将head设置为当前节点，返回false。如果需要挂起，执行挂起操作，否则循环执行。最后如果获取失败，那么撤销掉这次获取
private void cancelAcquire(Node node) {
// Ignore if node doesn't exist
if (node == null)
return;
node.thread = null;
// Skip cancelled predecessors
Node pred = node.prev;
while (pred.waitStatus > 0)
node.prev = pred = pred.prev;
// predNext is the apparent node to unsplice. CASes below will
// fail if not, in which case, we lost race vs another cancel
// or signal, so no further action is necessary.
Node predNext = pred.next;
// Can use unconditional write instead of CAS here.
// After this atomic step, other Nodes can skip past us.
// Before, we are free of interference from other threads.
node.waitStatus = Node.CANCELLED;
// If we are the tail, remove ourselves.
if (node == tail && compareAndSetTail(node, pred)) {
compareAndSetNext(pred, predNext, null);
} else {
// If successor needs signal, try to set pred's next-link
// so it will get one. Otherwise wake it up to propagate.
int ws;
if (pred != head &&
((ws = pred.waitStatus) == Node.SIGNAL ||
(ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
pred.thread != null) {
Node next = node.next;
if (next != null && next.waitStatus <= 0)
compareAndSetNext(pred, predNext, next);
} else {
unparkSuccessor(node);
}
node.next = node; // help GC
}
}
上面是非公平的实现方式，下面看一下公平锁的实现
final void lock() {
acquire(1);
}比偏向锁简单了很多，少了偏向相关的判断；代码追进去看了一下，只有这一个方法和偏向锁不同
// 公平锁
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;
}
// 偏向锁
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;
}区别的逻辑：公平锁在state=0的时候，如果没有前置节点并且cas设置成功后，会将持有锁的线程，设置为当前线程
- tryLock()
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}不管是不是偏向锁，都偏向拿锁
- tryLock(long timeout, TimeUnit unit)
private boolean doAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (nanosTimeout <= 0L)
return false;
final long deadline = System.nanoTime() + nanosTimeout;
final Node node = addWaiter(Node.EXCLUSIVE);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return true;
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}循环的时候，会做一下时间判断
lockInterruptibly()
public final void acquireInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (!tryAcquire(arg))
doAcquireInterruptibly(arg);
}
private void doAcquireInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.EXCLUSIVE);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}挂起的时候，会直接抛出异常
释放锁操作
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
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);
}
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;
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);
}