参考书:《java并发编程艺术》
参考博客:https://www.cnblogs.com/micrari/p/6937995.html
源码分析:活在夢裡 大佬写的很详细,也是我看过的AQS源码分析最好的博客了。
我就自己学习和理解的过程做一下记录。
1 数据存储结构
AQS的数据存储,是通过内部的Node对象存储Thread,并标识线程的状态;多个Node组成的双向链表结构;
static final class Node {
// 用于标记一个节点在共享模式下等待
static final Node SHARED = new Node();
// 用于标记一个节点在独占模式下等待
static final Node EXCLUSIVE = null;
// 等待状态:取消。由于再同步队列中等待的线程等待超时或被中断,需要从同步队列中取消等待,节点进入该状态将不会变化。
static final int CANCELLED = 1;
// 等待状态:通知。后继节点的线程处于等待状态,当前节点的线程如果释放了同步状态或者被取消,将会通知后继节点。
static final int SIGNAL = -1;
// 等待状态:条件等待。存放在条件队列,调用signal()方法或signalAll()方法,会将该节点转移至同步队列中。
static final int CONDITION = -2;
// 等待状态:传播。表示下一次同步状态的获取将会无条件的被传播下去
static final int PROPAGATE = -3;
// 等待状态
volatile int waitStatus;
// 前驱节点
volatile Node prev;
// 后继节点
volatile Node next;
// 节点对应的线程
volatile Thread thread;
// 等待队列中的后继节点
Node nextWaiter;
// 当前节点是否处于共享模式等待
final boolean isShared() {
return nextWaiter == SHARED;
}
// 获取前驱节点,如果为空的话抛出空指针异常
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null) {
throw new NullPointerException();
} else {
return p;
}
}
Node() {
}
Node(Thread thread, Node mode) {
this.nextWaiter = mode;
this.thread = thread;
}
Node(Thread thread, int waitStatus) {
this.waitStatus = waitStatus;
this.thread = thread;
}
}
2.独占锁方法
大致的过程:
-
成功获取锁,执行线程任务,释放锁,唤醒后继node;
-
没有获取到锁,放入到queue中,进行自旋。判断前驱为头节点时,如果是,设置当前节点为head节点,移除queue中。
-
当node自旋中出现iterruptException时,节点移除queue.
2.1 独占锁获取
aquire方法
-
1.tryAcquire(arg)方法获取独占锁,由AQS继承类实现
-
2.如果获取独占锁成功则返回,否则加入queue中
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();//中断当前线程
}
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);//创建独占节点
// Try the fast path of enq; backup to full enq on failure
Node pred = tail;//获取最后的节点
if (pred != null) {//末尾节点不为空的时候
node.prev = pred;//设置节点的前置节点是tail节点
if (compareAndSetTail(pred, node)) {//通过CAS将新加入的节点,设置成末尾节点
pred.next = node;
return node;
}
}
//队列初始化,node设置为tail节点
enq(node);
return node;
}
/**
* 作用:将当前节点加入到queue
* 1.死循环,只有当节点添加到末尾时才退出
* 2.通过CAS进行节点操作,类似“串行”执行
**/
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) { // 如果queue为空
if (compareAndSetHead(new Node()))//通过CAS,增加空的头节点
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {//通过CAS,结尾节点进行关联
t.next = node;
return t;
}
}
}
}
/**
* 作用:循环等待,当前节点获取到锁,或者当前节点中断;
* 1.如果前驱是头节点并且获取到了锁,则设置当前节点为头节点
* 2.如果没有获取到锁,则变更pre节点为SINGLE,当前线程进行阻塞
**/
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {//一致等待队列中的任务,直到当前任务为头节点
final Node p = node.predecessor();//当前节点的前一个节点
if (p == head && tryAcquire(arg)) {//前一个节点是头节点,并且获取到锁的时候
setHead(node);//将当前节点设置为头节点
p.next = null; // help GC
failed = false;
return interrupted;
}
/**
* 检查前一个节点的状态,如果为Single
* parkAndCheckInterrupt方法把当前线程阻塞
**/
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {// 当出现interruptedException的时候,不对异常处理,但是对当前线程所对应的当前节点的状态变更为Cancelled
if (failed)
cancelAcquire(node);//变更当前节点状态
}
}
/**
* 作用:将node节点的前置节点,状态变成SIGNAL,如果是CANCELLED则移除
**/
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)//表示这个节点锁已经释放,可以安全释放
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) {//是cancelled状态只能跳过,找到node节点前面的非Cancelled状态的节点
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {//设置前驱状态为SIGNAL,可以获取锁的状态
/*
* 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;
}
/**
* 1.阻塞当前线程
* 2.判断当前线程是否中断
**/
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
/**
* 作用:链表中取消节点,并更改节点状态
**/
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)//遍历node前节点为非cacelled状态
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;//变更当前节点的状态
// If we are the tail, remove ourselves.
if (node == tail && compareAndSetTail(node, pred)) {//如果是node是为节点,替换尾节点为node的pre
compareAndSetNext(pred, predNext, null);//更改pre节点的后继节点为空
} 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) {//pred节点不是头节点,不是Callced状态,
Node next = node.next;
if (next != null && next.waitStatus <= 0)
compareAndSetNext(pred, predNext, next);
} else {
//pred==head,唤醒后继锁操作
unparkSuccessor(node);
}
node.next = node; // help GC
}
}
执行的逻辑大致如下图:
acquireInterruptibly方法
- 这个方法和acquire方法最大的区别在于,会抛出InterruptException
//如果中断了,抛出异常
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 {
//省略部分代码。。。。。。
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
//省略部分代码。。。。。。
}
2.2 独占锁释放
执行的过程:
-
释放锁操作,成功则唤醒后继节点
-
释放锁失败,则返回false
/**
* 1.tryRelease方法子类进行重写;
* 2.释放锁成功后,唤醒后继node
**/
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
/**
* 作用:唤醒后继节点
**/
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;//节点的状态变成0,后续等待的线程可以对状态进行调整
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.
*
* 从tail查找非null和非cancelled状态的node的后继节点
*/
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);
}
3.共享锁方法
3.1 共享锁获取
acquireShared方法
-
如果节点是取消状态,则直接跳过;如果不是取消状态,增加到链表中。
-
当前节点的前驱是头节点,获取共享锁>=0,设置当前节点为头节点。
-
当前节点是否传播(propagation>0)和下个节点是否共享来判断是否唤醒下个节点。
/**
* 返回-1表示没有获取到共享锁
* 不会捕获InterruptException
**/
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}
/**
* 和独占式锁代码类似
*/
private void doAcquireShared(int arg) {
final Node node = addWaiter(Node.SHARED);//在链表末尾增加一个共享节点
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
/**
* 当前节点的前置节点获取到锁;
* 把当前节点设置为头节点;
**/
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
/**
* 判断p前置节点状态,阻塞当前线程并判断当前线程中断装填
**/
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {//非正常结束进行移除
if (failed)
cancelAcquire(node);
}
}
//设置共享锁头节点,唤醒下一个节点
private void setHeadAndPropagate(Node node, int propagate) {
Node h = head; // Record old head for check below
setHead(node);
//
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
Node s = node.next;
if (s == null || s.isShared())
doReleaseShared();
}
}
/**
* 作用:释放头节点和唤醒下一个节点
**/
private void doReleaseShared() {
for (;;) {
Node h = head;
if (h != null && h != tail) {//存在后继节点
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))//头节点为SIGNAL,则变成0状态
continue; // loop to recheck cases
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))//如果为0状态,则变成PROPAGATE状态
continue; // loop on failed CAS
}
if (h == head) // 如果头节点发生变化了,就推出循环
break;
}
}
// 如果存在后继节点,则唤醒后继节点
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);
}
3.2 共享锁释放
//判断有无获取到锁,获取到锁则唤醒后继节点
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
//这一块代码和上面增加共享节点的一样。
doReleaseShared();
return true;
}
return false;
}
Condition子类
条件队列插入
await方法
-
当前线程中断则报错退出
-
创建节点增加到条件队列
-
释放互斥锁
-
没有在同步队列就进行阻塞
-
检测是否发生中断,是更新完状态加入到同步队列时发生或是调用signal方法更改状态
-
如果有中断状态,则抛异常或者中断当前线程
/**模式意味着在退出等待时重新中断 */
private static final int REINTERRUPT = 1;
/** 退出等待时抛出Interrupt异常 */
private static final int THROW_IE = -1;
public final void await() throws InterruptedException {
if (Thread.interrupted())//当前线程异常,抛出异常
throw new InterruptedException();
/**
* 1.最后节点lasterWaiter关联nextWaiter关联当前节点
* 2.将当前节点设置为lastWaiter
**/
Node node = addConditionWaiter();
/**
* 1.释放当前状态
* 2.如果释放成功,则调整当前节点状态为释放
**/
int savedState = fullyRelease(node);
int interruptMode = 0;
/**
* 判断是否在同步队列,如果在同步队列则跳过,不在同步队列,就阻塞
**/
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
/**
* 获取互斥锁中,如果中断,但是中断异常不抛出,设置成REINTERRUPT
**/
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
//当前节点的nextWaiter不为空,则清除cancelled状态的节点
if (node.nextWaiter != null) // clean up if cancelled
unlinkCancelledWaiters();
//THROW_IE抛出中断异常,REINTERRUPT中断当前线程
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
}
/**
* 作用:增加当前线程到条件队列
*/
private Node addConditionWaiter() {
Node t = lastWaiter;//
// If lastWaiter is cancelled, clean out.
if (t != null && t.waitStatus != Node.CONDITION) {//如果最后一个节点的waitStatus不是CONDITION,删除CANCELLED的节点,重新定位最后一个条件节点
unlinkCancelledWaiters();
t = lastWaiter;
}
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;//将当前节点设置为尾节点
return node;
}
/**
* 作用:判断是否在同步队列中
**/
final boolean isOnSyncQueue(Node node) {
if (node.waitStatus == Node.CONDITION || node.prev == null)
return false;
if (node.next != null) // 有后继节点,一定在同步队列中
return true;
return findNodeFromTail(node);//在同步队列中查找,是否有当前节点
}
/**
* 判断中断状态:
* THROW_IE表示加入到同步队列中;
* REINTERRUPT表示调用了SINGLE()方法,状态已经更新过了;
* 0表示线程中断了
**/
private int checkInterruptWhileWaiting(Node node) {
return Thread.interrupted() ?
(transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
0;
}
/**
* 作用:变更节点状态,节点加入同步队列进行判断
**/
final boolean transferAfterCancelledWait(Node node) {
/**
* 将CONDITION状态变成准备状态,
* 将节点加到同步队列尾端
*/
if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
enq(node);
return true;
}
/**
* 调用了single()方法,状态已经被变更了,所以要等待加入到同步队列中
*/
while (!isOnSyncQueue(node))
Thread.yield();//当前线程让步执行
return false;
}
//THROW_IE抛出中断异常,REINTERRUPT中断当前线程
private void reportInterruptAfterWait(int interruptMode)
throws InterruptedException {
if (interruptMode == THROW_IE)
throw new InterruptedException();
else if (interruptMode == REINTERRUPT)
selfInterrupt();
}
signal方法
-
判断当前线程独占锁的持有
-
如果持有独占锁,则将头节点加入到同步队列中
-
头节点的同步队列中前驱节点如果是取消状态,则放行进行锁竞争
/**
* isHeldExclusively方法由子类实现。用于检查持有互斥锁的线程是不是当前线程
* 将第一个节点增加到同步队列
**/
public final void signal() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignal(first);
}
/**
* 头节点移动到同步队列
*/
private void doSignal(Node first) {
do {
if ( (firstWaiter = first.nextWaiter) == null)//如果只有一个节点,则lastWaiter变成空
lastWaiter = null;
first.nextWaiter = null;
} while (!transferForSignal(first) &&
(first = firstWaiter) != null);//头节点已经状态已经取消则结束循环
}
/**
* 作用:是将头节点增加到同步队列,如果头队列的前驱节点已经是取消状态,或无法调整为singal状态,则进行锁竞争
*/
final boolean transferForSignal(Node node) {
/*
* 如果waitStatus不能改变,说明这个节点已经是Cancelled
*/
if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
return false;
/*
* 将节点增加到同步队列,判断前驱节点的waitStatus,如果是Cancelled或者CAS自旋无法把状态设置为SINGAL,
* 放行节点线程进行锁竞争
*/
Node p = enq(node);//返回node节点的前驱
int ws = p.waitStatus;
if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
LockSupport.unpark(node.thread);
return true;
}
signalALL方法
- 和signal方法相似,不同的是将所有条件队列转移到同步队列
public final void signalAll() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignalAll(first);
}
private void doSignalAll(Node first) {
lastWaiter = firstWaiter = null;
do {
Node next = first.nextWaiter;
first.nextWaiter = null;
transferForSignal(first);
first = next;
} while (first != null);
}