1. 类的定义
public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer
implements java.io.Serializable
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从类的定义中可以看出
- AbstractQueuedSynchronizer 是一个抽象类,使用了模板方法设计模式(实现了大部分功能,极少部分让子类实现)
- AbstractQueuedSynchronizer 继承了 AbstractOwnableSynchronizer
- AbstractOwnableSynchronizer 实现了java.io.Serializable接口,表示AbstractQueuedSynchronizer支持序列化
2. 字段属性
//序列化版本号
private static final long serialVersionUID = 7373984972572414691L;
//等待队列的头,延迟初始化。 仅在初始化时,使用setHead修改。 注意:如果头存在,其等待状态保证不会被取消
private transient volatile Node head;
//等待队列的尾部,延迟初始化
private transient volatile Node tail;
//同步状态, 0(默认初始值)表示没有锁,1表示上锁,可以大于1表示重入锁,有的初始值为-1
//state使用volatile修饰
private volatile int state;
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从字段属性可以看出
- AbstractQueuedSynchronizer内部维护了一个链表队列
- AbstractQueuedSynchronizer存储了锁的状态state
3. 构造方法
//默认空的构造函数,没有任何操作
protected AbstractQueuedSynchronizer() { }
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从构造方法可以看出
- AbstractQueuedSynchronizer的构造方法使用protected修饰,是一个空的构造函数,需要子类自己实现
4. 方法
getState 方法
//获取当前的同步状态
protected final int getState() {
return state;
}
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setState 方法
//设置当前的同步状态
protected final void setState(int newState) {
state = newState;
}
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compareAndSetState 方法
//使用CAS设置同步状态
protected final boolean compareAndSetState(int expect, int update) {
//使用Unsafe的compareAndSwapInt方法更新同步状态,这是一个native方法
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
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enq 方法
//添加节点到队尾
private Node enq(final Node node) {
//for无限循环,重试CAS
for (;;) {
//如果队列尾节点
Node t = tail;
if (t == null) { // Must initialize
//如果尾节点为null,进行初始化
//CAS设置一个新创建的节点为头节点,有可能其它线程先创建成功
if (compareAndSetHead(new Node()))
//创建头节点成功,把尾节点指向头节点
tail = head;
} else {
//如果尾节点不为null
//把加入的节点前驱结点的引用指向尾节点
node.prev = t;
//CAS设置node为尾节点,有可能其它节点也在设置尾节点,可能会失败,然后重试
if (compareAndSetTail(t, node)) {
//如果添加成功
//把前驱节点的后驱的引用指向加入的节点
t.next = node;
//返回加入节点的前驱节点(可能是以前的尾节点,其它线程可能会修改它)
return t;
}
}
}
}
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compareAndSetHead 方法
//设置头节点,只在enq方法中调用
private final boolean compareAndSetHead(Node update) {
//如果头节点为null,才把传入的节点设置为头节点
return unsafe.compareAndSwapObject(this, headOffset, null, update);
}
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compareAndSetTail 方法
//设置尾节点,只在enq方法中调用
private final boolean compareAndSetTail(Node expect, Node update) {
//如果尾节点为expect,才把传入的节点设置为尾节点
return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
}
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addWaiter 方法
//添加节点到队尾
private Node addWaiter(Node mode) {
//使用当前线程和传入节点创建新的节点,后面添加的是这个新创建的节点
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
//获取尾节点的副本
Node pred = tail;
if (pred != null) {
//如果尾节点不为null
//把添加节点的前驱结点引用指向尾节点
node.prev = pred;
//CAS设置node为尾节点,有可能其它节点也在设置尾节点,可能会失败,然后重试
if (compareAndSetTail(pred, node)) {
//把前驱节点的后驱节点的引用指向加入的节点
pred.next = node;
//返回加入节点的前驱节点(可能是以前的尾节点,其它线程可能会修改它)
return node;
}
}
//如果尾节点为null,或者被其它线程先添加成功
//使用enq方法添加
enq(node);
return node;
}
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setHead 方法
//设置头节点,仅由acquire方法调用
private void setHead(Node node) {
//把头节点指向传入的节点
head = node;
//把传入节点的thread置为null
node.thread = null;
//把传入节点的prev置为null
node.prev = null;
}
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unparkSuccessor 方法
//如果有节点退出,唤醒队列下一个未被取消节点的线程执行
//注意,这个node参数是头节点
private void unparkSuccessor(Node node) {
//获取头节点的waitStatus,waitStatus大于0表示被取消
int ws = node.waitStatus;
if (ws < 0)
//如果头节点的waitStatus小于0, 把头节点的waitStatus设置为0
compareAndSetWaitStatus(node, ws, 0);
//获取头节的后驱节点
Node s = node.next;
if (s == null || s.waitStatus > 0) {
//如果后驱节点为nul 或者 后驱节点的waitStatus大于0(被取消)
s = null;
//使用for循环从尾节点开始往前遍历
//找到遍历的第一个waitStatus小于等于0(未被取消)的节点
//注意,找到的是顺序的第一个,不是倒序的第一个,这个会一直循环到头节点
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
//如果节点不为null, 唤醒节点的线程
LockSupport.unpark(s.thread);
}
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doReleaseShared 方法
//共享模式下的信号释放
private void doReleaseShared() {
/*
* Ensure that a release propagates, even if there are other
* in-progress acquires/releases. This proceeds in the usual
* way of trying to unparkSuccessor of head if it needs
* signal. But if it does not, status is set to PROPAGATE to
* ensure that upon release, propagation continues.
* Additionally, we must loop in case a new node is added
* while we are doing this. Also, unlike other uses of
* unparkSuccessor, we need to know if CAS to reset status
* fails, if so rechecking.
*/
//for无限循环
for (;;) {
//获取头节点的副本
Node h = head;
if (h != null && h != tail) {
//如果h节点不为null 并且 h节点不等于尾节点(队列中有元素)
//获取h节点的waitStatus
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
//如果h节点的状态为-1
//把h节点的waitStatus设置为0
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
//如果失败进行下一次循环(继续操作)
continue; // loop to recheck cases
//如果设置成功,唤醒h的后驱节点,头节点的状态可能会改变
unparkSuccessor(h);
}
//如果h节点的状态为0 并且 CAS设置节点的waitStatus为-3失败的话进行下一次循环
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
//如果h节点为头节点中断循环,如果head改变则继续循环
break;
}
}
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setHeadAndPropagate 方法
//设置队列头和传播方式
private void setHeadAndPropagate(Node node, int propagate) {
//获取头节点的副本
Node h = head; // Record old head for check below
//把传入的节点设置为头节点
setHead(node);
/*
* Try to signal next queued node if:
* Propagation was indicated by caller,
* or was recorded (as h.waitStatus either before
* or after setHead) by a previous operation
* (note: this uses sign-check of waitStatus because
* PROPAGATE status may transition to SIGNAL.)
* and
* The next node is waiting in shared mode,
* or we don't know, because it appears null
*
* The conservatism in both of these checks may cause
* unnecessary wake-ups, but only when there are multiple
* racing acquires/releases, so most need signals now or soon
* anyway.
*/
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
//如果传入的传播方式大于0
//如果头节点为null
//如果头节点的waitStatus小于0
//如果node为null
//如果node的waitStatus小于0
//获取node的后驱节点
Node s = node.next;
if (s == null || s.isShared())
//如果后驱节点为null 或者 后驱节点是共享模式
//释放共享模式下的信号
doReleaseShared();
}
}
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cancelAcquire 方法
//取消节点获取锁的尝试
private void cancelAcquire(Node node) {
if (node == null)
//如果node为null,直接返回
return;
//把node的线程置为null
node.thread = null;
//获取node的前一个节点
Node pred = node.prev;
//while循环,如果前驱结点的waitStatus大于0(被取消),一直往上找
//找到前面第一个未被取消的节点
while (pred.waitStatus > 0)
node.prev = pred = pred.prev;
//获取前驱结点的后驱节点
Node predNext = pred.next;
//把node节点的waitStatus设置为取消状态
node.waitStatus = Node.CANCELLED;
// 如果当前节点是尾节点,移除自己,把前驱结点设置为尾节点
if (node == tail && compareAndSetTail(node, pred)) {
//把前驱节点的后驱节点设置为null
compareAndSetNext(pred, predNext, null);
} else {
//node不是尾节点的情况
//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 {
//唤醒前驱结点
unparkSuccessor(node);
}
node.next = node; // help GC
}
}
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shouldParkAfterFailedAcquire 方法
//当前线程没有获取到锁,是否需要挂起线程
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
//获取前驱结点的waitStatus
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
//前驱节点的waitStatus为-1,说明前驱结点状态正常,可以安全的挂起当前线程,直接返回true
return true;
if (ws > 0) {
//如果前驱结点的waitStatus大于0,表示前驱结点取消了排队
//注意:进入阻塞队列排队的线程会被挂起,而唤醒的操作是由前驱节点完成的
//往前一直找,直到找到waitStatus大于等于0的节点作为node节点的前驱节点
//node的线程挂起以后后面要靠找到的前驱节点来唤醒
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
//前驱节点的waitStatus不等于1和-1,只能等于0,-2, -3
//把前驱节点的waitStatus设置为-1
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
//返回false表示不需要挂起
return false;
}
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selfInterrupt 方法
//中断当前线程
static void selfInterrupt() {
Thread.currentThread().interrupt();
}
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parkAndCheckInterrupt 方法
//挂起当前线程,并检查是否中断
private final boolean parkAndCheckInterrupt() {
//挂起当前线程
LockSupport.park(this);
//检查当前线程是否中断
return Thread.interrupted();
}
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acquireQueued 方法
//尝试获取锁,失败的话就中断等着前驱节点唤醒自己
final boolean acquireQueued(final Node node, int arg) {
//默认失败
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
//获取node的前驱结点
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
//如果p为头节点 并且 获取锁成功
//把node设置为头节点
setHead(node);
//把前驱结点的后驱节点置为null
p.next = null; // help GC
//设置为成功标志
failed = false;
//返回interrupted标志
return interrupted;
}
//找到能够唤醒自己的前驱节点,然后中断在这等着唤醒
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
//如果node需要被挂起
//interrupted标志置为true
interrupted = true;
}
} finally {
if (failed)
//如果失败,取消node获取锁的尝试
cancelAcquire(node);
}
}
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doAcquireInterruptibly 方法
//获取独占锁,如果获取失败,把线程挂起并抛出异常
private void doAcquireInterruptibly(int arg)
throws InterruptedException {
//添加独占模式节点到队尾
final Node node = addWaiter(Node.EXCLUSIVE);
//设置失败标志为true
boolean failed = true;
try {
for (;;) {
//获取node的前驱节点
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
//如果node的前驱节点是头结点,并且获取锁成功
//把node设置为头结点
setHead(node);
//把前驱节点的后驱节点引用置为null
p.next = null; // help GC
//设置失败标志为false
failed = false;
//直接返回
return;
}
//如果没获取到锁,把线程挂起
//如果挂起线程失败,抛出异常
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
//失败,取消node获取锁的尝试
cancelAcquire(node);
}
}
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doAcquireNanos 方法
//获取独占锁, 指定超时时间
private boolean doAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
//参数检查
if (nanosTimeout <= 0L)
return false;
//计算过期时间
final long deadline = System.nanoTime() + nanosTimeout;
//添加独占模式节点到队尾
final Node node = addWaiter(Node.EXCLUSIVE);
//设置失败标志为true
boolean failed = true;
try {
//for无限循环
for (;;) {
//获取node的前驱节点
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
//如果node的前驱节点是头结点,并且获取锁成功
//把node设置为头结点
setHead(node);
//把前驱节点的后驱节点引用置为null
p.next = null; // help GC
//设置失败标志为false
failed = false;
//获取锁成功,返回true
return true;
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
//超时,返回false
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
//如果没获取到锁,把线程挂起
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
//如果线程被中断,抛出异常
throw new InterruptedException();
}
} finally {
if (failed)
//失败,取消node获取锁的尝试
cancelAcquire(node);
}
}
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doAcquireShared 方法
//获取共享锁
private void doAcquireShared(int arg) {
//添加共享模式节点到队尾
final Node node = addWaiter(Node.SHARED);
//设置失败标志为true
boolean failed = true;
try {
//设置中断标志为true
boolean interrupted = false;
//for无限循环
for (;;) {
//获取node的前驱节点
final Node p = node.predecessor();
if (p == head) {
//如果前驱节点是头结点
//尝试获取共享锁
int r = tryAcquireShared(arg);
if (r >= 0) {
//获取成功,设置头结点为node节点, 并继续传播
setHeadAndPropagate(node, r);
//设置前驱节点的后驱节点引用置为null
p.next = null; // help GC
if (interrupted)
//如果中断标志为true,中断当前线程
selfInterrupt();
//设置失败标志为false
failed = false;
//直接返回
return;
}
}
//如果没获取到锁,把线程挂起
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
//挂起线程成功,设置中断标志为true
interrupted = true;
}
} finally {
if (failed)
//失败,取消node获取锁的尝试
cancelAcquire(node);
}
}
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doAcquireSharedInterruptibly 方法
//获取共享锁
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
//添加共享模式节点到队尾
final Node node = addWaiter(Node.SHARED);
//设置失败标志为true
boolean failed = true;
try {
//for无限循环
for (;;) {
//获取node的前驱节点
final Node p = node.predecessor();
if (p == head) {
//如果前驱节点是头结点
//尝试获取共享锁
int r = tryAcquireShared(arg);
if (r >= 0) {
//获取成功,设置头结点为node节点, 并继续传播
setHeadAndPropagate(node, r);
//设置前驱节点的后驱节点引用置为null
p.next = null; // help GC
//设置失败标志为false
failed = false;
//直接返回
return;
}
}
//如果没获取到锁,把线程挂起
//如果挂起线程失败,抛出异常
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
//失败,取消node获取锁的尝试
cancelAcquire(node);
}
}
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doAcquireSharedNanos 方法
//获取共享锁,指定超时时间
private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
//参数检查
if (nanosTimeout <= 0L)
return false;
//计算过期时间
final long deadline = System.nanoTime() + nanosTimeout;
//添加共享模式节点到队尾
final Node node = addWaiter(Node.SHARED);
//设置失败标志为true
boolean failed = true;
try {
//for无限循环
for (;;) {
//获取node的前驱节点
final Node p = node.predecessor();
if (p == head) {
//如果前驱节点是头结点
//尝试获取共享锁
int r = tryAcquireShared(arg);
if (r >= 0) {
//获取成功,设置头结点为node节点, 并继续传播
setHeadAndPropagate(node, r);
//设置前驱节点的后驱节点引用置为null
p.next = null; // help GC
//设置失败标志为false
failed = false;
//直接返回
return true;
}
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
//超时,返回false
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
//如果没获取到锁,把线程挂起
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
//如果线程被中断,抛出异常
throw new InterruptedException();
}
} finally {
if (failed)
//失败,取消node获取锁的尝试
cancelAcquire(node);
}
}
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tryAcquire 方法
//尝试获取独占锁,强制子类实现
protected boolean tryAcquire(int arg) {
throw new UnsupportedOperationException();
}
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tryRelease 方法
//尝试释放锁,强制子类实现
protected boolean tryRelease(int arg) {
throw new UnsupportedOperationException();
}
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tryAcquireShared 方法
//尝试获取共享锁,强制子类实现
protected int tryAcquireShared(int arg) {
throw new UnsupportedOperationException();
}
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tryReleaseShared 方法
//尝试释放共享锁,强制子类实现
protected boolean tryReleaseShared(int arg) {
throw new UnsupportedOperationException();
}
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isHeldExclusively 方法
//返回锁的模式, true 独占锁
protected boolean isHeldExclusively() {
throw new UnsupportedOperationException();
}
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acquire 方法
//独占模式获取锁
public final void acquire(int arg) {
//先尝试获取锁,如果失败,以独占模式添加到队尾,中断当前线程等着被唤醒
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
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acquireInterruptibly 方法
//独占模式获取锁,获取失败抛出异常
public final void acquireInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (!tryAcquire(arg))
doAcquireInterruptibly(arg);
}
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tryAcquireNanos 方法
//独占模式获取锁,设置超时时间
public final boolean tryAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquire(arg) ||
doAcquireNanos(arg, nanosTimeout);
}
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release 方法
//释放独占锁
public final boolean release(int arg) {
if (tryRelease(arg)) {
//释放独占锁成功
//拷贝头节点的副本
Node h = head;
if (h != null && h.waitStatus != 0)
//如果头节点不为null 并且 头节点的状态正常
//唤醒下一个节点
unparkSuccessor(h);
return true;
}
return false;
}
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acquireShared 方法
//共享模式获取锁
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}
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acquireSharedInterruptibly 方法
//共享模式获取锁,获取失败抛出异常
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
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tryAcquireSharedNanos 方法
//共享模式获取锁,设置超时时间
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquireShared(arg) >= 0 ||
doAcquireSharedNanos(arg, nanosTimeout);
}
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releaseShared 方法
//释放共享锁
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
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hasQueuedThreads 方法
//队列中是否含有线程
public final boolean hasQueuedThreads() {
return head != tail;
}
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hasContended 方法
//当前是否有线程在使用锁
public final boolean hasContended() {
return head != null;
}
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getFirstQueuedThread 方法
//获取队列中第一个线程(等待时间最长的线程)
public final Thread getFirstQueuedThread() {
// handle only fast path, else relay
//为空的话返回null
//调用fullGetFirstQueuedThread方法返回队列中第一个线程
return (head == tail) ? null : fullGetFirstQueuedThread();
}
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fullGetFirstQueuedThread 方法
//获取队列中第一个线程
private Thread fullGetFirstQueuedThread() {
//一般情况下,head的后驱节点就是队列中的第一个元素
//如果head不为null,并且head的后驱节点不为null,并且head的后驱节点的前驱节点是head节点
//直接返回head的后驱节点
Node h, s;
Thread st;
if (((h = head) != null && (s = h.next) != null &&
s.prev == head && (st = s.thread) != null) ||
((h = head) != null && (s = h.next) != null &&
s.prev == head && (st = s.thread) != null))
return st;
//有可能还没来的及设置head的后驱节点(多个线程操作)
//这种情况下,从tail往前找
Node t = tail;
Thread firstThread = null;
while (t != null && t != head) {
Thread tt = t.thread;
if (tt != null)
firstThread = tt;
t = t.prev;
}
return firstThread;
}
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isQueued 方法
//查看给定线程是否正在排队
public final boolean isQueued(Thread thread) {
if (thread == null)
throw new NullPointerException();
//for循环遍历,从tail往前找
for (Node p = tail; p != null; p = p.prev)
if (p.thread == thread)
return true;
return false;
}
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getQueueLength 方法
//获取队列的长度
public final int getQueueLength() {
int n = 0;
for (Node p = tail; p != null; p = p.prev) {
if (p.thread != null)
++n;
}
return n;
}
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getQueuedThreads 方法
//获取队列中的所有线程,以集合的方式返回
public final Collection<Thread> getQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
return list;
}
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getExclusiveQueuedThreads 方法
//获取队列中的所有独占模式的线程,以集合的方式返回
public final Collection<Thread> getExclusiveQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
if (!p.isShared()) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
}
return list;
}
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getSharedQueuedThreads 方法
//获取队列中的所有共享模式的线程,以集合的方式返回
public final Collection<Thread> getSharedQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
if (p.isShared()) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
}
return list;
}
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