【JUC系列】ReentrantLock实现本地锁的源码分析
使用场景
public class ReentrantLockTest {
private static ReentrantLock lock = new ReentrantLock();
public static void main(String[] args) {
new Thread(()->{
lock.lock();
// do something
System.out.println("111");
try {
Thread.sleep(Integer.MAX_VALUE);
} catch (InterruptedException e) {
e.printStackTrace();
}
lock.unlock();
}).start();
new Thread(()->{
lock.lock();
// do something
try {
Thread.sleep(Integer.MAX_VALUE);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("222");
lock.unlock();
}).start();
}
}
源码分析
默认使用非公平锁。
public ReentrantLock() {
sync = new NonfairSync();
}
加锁
ReentrantLock#lock加锁，使用sync进行加锁。
public void lock() {
sync.lock();
}
如果当前的状态为0，则设置exclusiveOwnerThread = thread;，表示当前线程占住当前锁对象。
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
AbstractQueuedSynchronizer#acquire。
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
非公平锁的实现，ReentrantLock.NonfairSync#tryAcquire。
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
ReentrantLock.Sync#nonfairTryAcquire。当前状态值为0，直接获取锁，如果锁是当前线程占用，则是可重复锁，state++。
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;
}
如果没有获取到锁，在队列中等待，执行AbstractQueuedSynchronizer#addWaiter。末尾节点不为空，设置末尾节点为node的前置节点，node为新的末尾结点。
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;
}
末尾节点为空，执行AbstractQueuedSynchronizer#enq。进行末尾节点的初始化，再插入node节点。
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;
}
}
}
}
AbstractQueuedSynchronizer#acquireQueued。如果上一个节点是头结点，尝试获取锁。
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);
}
}
AbstractQueuedSynchronizer#shouldParkAfterFailedAcquire，前置节点可唤醒，返回true，否则设置前置节点的等待状态为-1。
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;
}
AbstractQueuedSynchronizer#parkAndCheckInterrupt，找到唤醒自己的，进入休眠。
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
解锁
ReentrantLock#unlock。
public void unlock() {
sync.release(1);
}
AbstractQueuedSynchronizer#release。如果成功释放锁，且头节点不为空，waitStatus不为0，执行AbstractQueuedSynchronizer#unparkSuccessor。
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
ReentrantLock.Sync#tryRelease，修改state-1，如果state=0，，则释放exclusiveOwnerThread，返回true，否则返回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;
}
AbstractQueuedSynchronizer#unparkSuccessor。从尾结点找到waitStatus<=0的节点，唤醒
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);
}
公平锁和非公平锁的不同在于tryAcquire方法。核心在于AbstractQueuedSynchronizer#hasQueuedPredecessors，该方法判断锁同步队列没有多余的节点。
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;
}
总结概括
- 加锁的逻辑：
- 没有锁，state由0变成1，设置
exclusiveOwnerThread = thread;； - 有锁，且拥有锁的线程和加锁线程是同一个，state++；
- 有锁，但拥有锁的线程和加锁线程不是用一个，队尾添加节点，设置前置节点的
waitStatus=-1，进行休眠。
- 没有锁，state由0变成1，设置
- 解锁的逻辑。state–，当state=0，释放锁成功，设置
exclusiveOwnerThread = null;，唤醒最后一个waitStatus<=0。