by shihang.mai
1. AQS
JUC包里的同步组件主要实现了AQS的方法:
独占锁:acquire()、release()
共享锁:acquireShare()、releaseShare()
2. ReentranLock
ReentranLock是一把独占锁
AQS
- ReentranLock.lock(),默认是非公平锁，所以调用到内部类NonfairSync.lock()
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
/**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
- NonfairSync.lock(),先用cas设置state(将0改为1),如果设置成功，那么当前线程获得锁(第一个线程调用ReentranLock.lock(),肯定获得到锁)。如果cas设置state不成功，那么调用acquire(1)获取锁，而acquire(1)调用的是AQS的acquire()
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
public final void acquire(int arg) {
//1. tryAcquire(1)
//2. addWaiter(Node.EXCLUSIVE)
//3. acquireQueued(addWaiter(Node.EXCLUSIVE), 1))
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
}
- tryAcquire(1)方法调用回NonfairSync.tryAcquire()，请查看第1步的源码,然后再调用Sync.nonfairTryAcquire(),首先获取state
- 如果state==0，那么cas设置state，成功即获得锁，返回true。如果不成功返回false。返回第2步源码判断
- 如果state!=0，那么判断是否是已经上锁的线程，即可重入锁,state+1，返回true，返回第2步源码判断
- 如果state!=0，并且当前线程也不是已经上锁的线程,那么返回false，返回第2步源码判断
abstract static class Sync extends AbstractQueuedSynchronizer {
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;
}
}
- 我们来看第2步的源码，已经看完tryAcquire,当tryAcquire返回true，表明获得锁，不再进行接下来的操作。如果返回false，进行下面的addWaiter(),先将当前线程和mode包装成Node
- 当第2个线程来获取锁(第1个线程已经设置了state，占用了锁),tail==null，所以直接进入enq(node),下面叫Node2
- 当第3个线程来获取锁(第1个线程已经设置了state，占用了锁,tail指向上第2个线程的Node),tail!=null,那么将Node3的的pre指向Node2，然后cas设置设置tail为当前Node,设置成功，那么将Node2的next指向Node3。如果cas设置失败，那么进入enq(node)
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
public final void acquire(int arg) {
//1. tryAcquire(1)
//2. addWaiter(Node.EXCLUSIVE)
//3. acquireQueued(addWaiter(Node.EXCLUSIVE), 1))
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
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;
}
}
我们接下来看enq(node)
- 如果是Node2进来，一个死循环，这是tail==null，cas设置head,设置成功，那么初始化双向链表,tail和head都指向一个新new的Node(),再循环一次，这时tail!=null,然后设置Node2的pre指向刚new出来的Node(),再cas设置Node2为tail，new出来的Node()的next指向Node2.说白了就是将Node2加入双向链表
- 如果是Node3进来，tail!=null，直接和上面一样，只是缺少了初始化
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
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;
}
}
}
}
}
- 我们返回来看第2步的源码，已经看了tryAcquire和addWaiter，此时双向链表已经有数据，接下来看acquireQueued()
- 当Node2进来，那么它的pre是head，就会去tryAcquire,如果tryAcquire成功，那么设置head==Node2,原来的new出来的Node()的next=null，failed变量为false，所以finally逻辑不执行。如果tryAcquire失败,那么也进入shouldParkAfterFailedAcquire
- 当Node3进来，它的pre是Node2，那就直接进入shouldParkAfterFailedAcquire()
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
public final void acquire(int arg) {
//1. tryAcquire(1)
//2. addWaiter(Node.EXCLUSIVE)
//3. acquireQueued(addWaiter(Node.EXCLUSIVE), 1))
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
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);
}
}
}
- 我们来看shouldParkAfterFailedAcquire
- 当Node2进来，pre是head,waitStatus初始值为0,进入compareAndSetWaitStatus(pred, ws, Node.SIGNAL);然后返回false,那么看回acquireQueued方法，不继续执行后面逻辑，继续循环一次，如果Node2继续获取步了锁,那么继续进入shouldParkAfterFailedAcquire，这次进来pred.waitStatus== -1，那么return true
- 当Node3进来，pre是Node2，而node2的waitStatus是初始值0,之后和上面一样的操作。
说白了，就是死循环一直设置当前Node的前一个的waitStatus为-1
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
// Node.SIGNAL== -1
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;
}
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);
}
}
}
- 我们继续来看parkAndCheckInterrupt,这个直接park。
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
}
AQS的等待队列
- 如果此时,线程1调用ReentranLock.unlock(),然后调用AQS的release(1), tryRelease(就是将state-1，直到state==0，才会释放成功)会调用ReentranLock. tryRelease(1),成功的话,调用AQS的unparkSuccessor()
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
}
public class ReentrantLock implements Lock, java.io.Serializable {
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;
}
}
- 我们来看AQS的unparkSuccessor，传入来的node为head,waitStatus==-1,那么唤醒线程，继续竞争
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
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);
}
}
- 我们来看回唤醒逻辑,当线程是被唤醒的，这个返回false,然后Node2就去获取锁
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
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);
}
}
}
3. ReentrantLock总结
- 先通过CAS尝试获取锁。如果此时已经有线程占据了锁，那就加入CLH队列并且被挂起。
- 当锁被释放之后，排在CLH队列队首的线程会被唤醒，然后CAS再次尝试获取锁。在这个时候，如果：
非公平锁：如果同时还有另一个线程进来尝试获取，那么有可能会让这个线程抢先获取；
公平锁：如果同时还有另一个线程进来尝试获取，当它发现自己不是在队首的话，就会排到队尾，由队首的线程获取到锁。
4. 重入原理
Volatile int state+1