并发编程—AQS源码分析
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AbstractQueuedSynchronizer源码分析
类的继承关系
AbstractQueuedSynchronizer继承自AbstractOwnableSynchronizer抽象类,并且实现了Serializable接口,可以进行序列化。
public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable
中AbstractOwnableSynchronizer抽象类的源码如下:
public abstract class AbstractOwnableSynchronizer implements java.io.Serializable {
// 版本序列号
private static final long serialVersionUID = 3737899427754241961L;
// 构造方法
protected AbstractOwnableSynchronizer() { }
// 独占模式下的线程
private transient Thread exclusiveOwnerThread;
// 设置独占线程
protected final void setExclusiveOwnerThread(Thread thread) {
exclusiveOwnerThread = thread;
}
// 获取独占线程
protected final Thread getExclusiveOwnerThread() {
return exclusiveOwnerThread;
}
}
AbstractOwnableSynchronizer抽象类中,可以设置独占资源线程和获取独占资源线程。分别为setExclusiveOwnerThread与getExclusiveOwnerThread方法,这两个方法会被子类调用。
AbstractQueuedSynchronizer类有两个内部类,分别为Node类与ConditionObject类。下面分别做介绍。
static final class Node {
// 模式,分为共享与独占
// 共享模式
static final Node SHARED = new Node();
// 独占模式
static final Node EXCLUSIVE = null;
// 结点状态
// CANCELLED,值为1,表示当前的线程被取消
// SIGNAL,值为-1,表示当前节点的后继节点包含的线程需要运行,也就是unpark
// CONDITION,值为-2,表示当前节点在等待condition,也就是在condition队列中
// PROPAGATE,值为-3,表示当前场景下后续的acquireShared能够得以执行
// 值为0,表示当前节点在sync队列中,等待着获取锁
static final int CANCELLED = 1;
static final int SIGNAL = -1;
static final int CONDITION = -2;
static final int PROPAGATE = -3;
// 结点状态
volatile int waitStatus;
// 前驱结点
volatile Node prev;
// 后继结点
volatile Node next;
// 结点所对应的线程
volatile Thread thread;
// 下一个等待者
Node nextWaiter;
// 结点是否在共享模式下等待
final boolean isShared() {
return nextWaiter == SHARED;
}
// 获取前驱结点,若前驱结点为空,抛出异常
final Node predecessor() throws NullPointerException {
// 保存前驱结点
Node p = prev;
if (p == null) // 前驱结点为空,抛出异常
throw new NullPointerException();
else // 前驱结点不为空,返回
return p;
}
// 无参构造方法
Node() { // Used to establish initial head or SHARED marker
}
// 构造方法
Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
// 构造方法
Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
}
每个线程被阻塞的线程都会被封装成一个Node结点,放入队列。每个节点包含了一个Thread类型的引用,并且每个节点都存在一个状态,具体状态如下。
CANCELLED
,值为1,表示当前的线程被取消。SIGNAL
,值为-1,表示当前节点的后继节点包含的线程需要运行,需要进行unpark操作。CONDITION
,值为-2,表示当前节点在等待condition,也就是在condition queue中。PROPAGATE
,值为-3,表示当前场景下后续的acquireShared能够得以执行。值为0,表示当前节点在sync queue中,等待着获取锁。
// 内部类
public class ConditionObject implements Condition, java.io.Serializable {
// 版本号
private static final long serialVersionUID = 1173984872572414699L;
/** First node of condition queue. */
// condition队列的头结点
private transient Node firstWaiter;
/** Last node of condition queue. */
// condition队列的尾结点
private transient Node lastWaiter;
/**
* Creates a new {@code ConditionObject} instance.
*/
// 构造方法
public ConditionObject() { }
// Internal methods
/**
* Adds a new waiter to wait queue.
* @return its new wait node
*/
// 添加新的waiter到wait队列
private Node addConditionWaiter() {
// 保存尾结点
Node t = lastWaiter;
// If lastWaiter is cancelled, clean out.
if (t != null && t.waitStatus != Node.CONDITION) { // 尾结点不为空,并且尾结点的状态不为CONDITION
// 清除状态为CONDITION的结点
unlinkCancelledWaiters();
// 将最后一个结点重新赋值给t
t = lastWaiter;
}
// 新建一个结点
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null) // 尾结点为空
// 设置condition队列的头结点
firstWaiter = node;
else // 尾结点不为空
// 设置为节点的nextWaiter域为node结点
t.nextWaiter = node;
// 更新condition队列的尾结点
lastWaiter = node;
return node;
}
/**
* Removes and transfers nodes until hit non-cancelled one or
* null. Split out from signal in part to encourage compilers
* to inline the case of no waiters.
* @param first (non-null) the first node on condition queue
*/
private void doSignal(Node first) {
// 循环
do {
if ( (firstWaiter = first.nextWaiter) == null) // 该节点的nextWaiter为空
// 设置尾结点为空
lastWaiter = null;
// 设置first结点的nextWaiter域
first.nextWaiter = null;
} while (!transferForSignal(first) &&
(first = firstWaiter) != null); // 将结点从condition队列转移到sync队列失败并且condition队列中的头结点不为空,一直循环
}
/**
* Removes and transfers all nodes.
* @param first (non-null) the first node on condition queue
*/
private void doSignalAll(Node first) {
// condition队列的头结点尾结点都设置为空
lastWaiter = firstWaiter = null;
// 循环
do {
// 获取first结点的nextWaiter域结点
Node next = first.nextWaiter;
// 设置first结点的nextWaiter域为空
first.nextWaiter = null;
// 将first结点从condition队列转移到sync队列
transferForSignal(first);
// 重新设置first
first = next;
} while (first != null);
}
/**
* Unlinks cancelled waiter nodes from condition queue.
* Called only while holding lock. This is called when
* cancellation occurred during condition wait, and upon
* insertion of a new waiter when lastWaiter is seen to have
* been cancelled. This method is needed to avoid garbage
* retention in the absence of signals. So even though it may
* require a full traversal, it comes into play only when
* timeouts or cancellations occur in the absence of
* signals. It traverses all nodes rather than stopping at a
* particular target to unlink all pointers to garbage nodes
* without requiring many re-traversals during cancellation
* storms.
*/
// 从condition队列中清除状态为CANCEL的结点
private void unlinkCancelledWaiters() {
// 保存condition队列头结点
Node t = firstWaiter;
Node trail = null;
while (t != null) { // t不为空
// 下一个结点
Node next = t.nextWaiter;
if (t.waitStatus != Node.CONDITION) { // t结点的状态不为CONDTION状态
// 设置t节点的额nextWaiter域为空
t.nextWaiter = null;
if (trail == null) // trail为空
// 重新设置condition队列的头结点
firstWaiter = next;
else // trail不为空
// 设置trail结点的nextWaiter域为next结点
trail.nextWaiter = next;
if (next == null) // next结点为空
// 设置condition队列的尾结点
lastWaiter = trail;
}
else // t结点的状态为CONDTION状态
// 设置trail结点
trail = t;
// 设置t结点
t = next;
}
}
// public methods
/**
* Moves the longest-waiting thread, if one exists, from the
* wait queue for this condition to the wait queue for the
* owning lock.
*
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
// 唤醒一个等待线程。如果所有的线程都在等待此条件,则选择其中的一个唤醒。在从 await 返回之前,该线程必须重新获取锁。
public final void signal() {
if (!isHeldExclusively()) // 不被当前线程独占,抛出异常
throw new IllegalMonitorStateException();
// 保存condition队列头结点
Node first = firstWaiter;
if (first != null) // 头结点不为空
// 唤醒一个等待线程
doSignal(first);
}
/**
* Moves all threads from the wait queue for this condition to
* the wait queue for the owning lock.
*
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
// 唤醒所有等待线程。如果所有的线程都在等待此条件,则唤醒所有线程。在从 await 返回之前,每个线程都必须重新获取锁。
public final void signalAll() {
if (!isHeldExclusively()) // 不被当前线程独占,抛出异常
throw new IllegalMonitorStateException();
// 保存condition队列头结点
Node first = firstWaiter;
if (first != null) // 头结点不为空
// 唤醒所有等待线程
doSignalAll(first);
}
/**
* Implements uninterruptible condition wait.
* <ol>
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* </ol>
*/
// 等待,当前线程在接到信号之前一直处于等待状态,不响应中断
public final void awaitUninterruptibly() {
// 添加一个结点到等待队列
Node node = addConditionWaiter();
// 获取释放的状态
int savedState = fullyRelease(node);
boolean interrupted = false;
while (!isOnSyncQueue(node)) { //
// 阻塞当前线程
LockSupport.park(this);
if (Thread.interrupted()) // 当前线程被中断
// 设置interrupted状态
interrupted = true;
}
if (acquireQueued(node, savedState) || interrupted) //
selfInterrupt();
}
/*
* For interruptible waits, we need to track whether to throw
* InterruptedException, if interrupted while blocked on
* condition, versus reinterrupt current thread, if
* interrupted while blocked waiting to re-acquire.
*/
/** Mode meaning to reinterrupt on exit from wait */
private static final int REINTERRUPT = 1;
/** Mode meaning to throw InterruptedException on exit from wait */
private static final int THROW_IE = -1;
/**
* Checks for interrupt, returning THROW_IE if interrupted
* before signalled, REINTERRUPT if after signalled, or
* 0 if not interrupted.
*/
private int checkInterruptWhileWaiting(Node node) {
return Thread.interrupted() ?
(transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
0;
}
/**
* Throws InterruptedException, reinterrupts current thread, or
* does nothing, depending on mode.
*/
private void reportInterruptAfterWait(int interruptMode)
throws InterruptedException {
if (interruptMode == THROW_IE)
throw new InterruptedException();
else if (interruptMode == REINTERRUPT)
selfInterrupt();
}
/**
* Implements interruptible condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled or interrupted.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*/
// // 等待,当前线程在接到信号或被中断之前一直处于等待状态
public final void await() throws InterruptedException {
if (Thread.interrupted()) // 当前线程被中断,抛出异常
throw new InterruptedException();
// 在wait队列上添加一个结点
Node node = addConditionWaiter();
//
int savedState = fullyRelease(node);
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
// 阻塞当前线程
LockSupport.park(this);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) // 检查结点等待时的中断类型
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null) // clean up if cancelled
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
}
/**
* Implements timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*/
// 等待,当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态
public final long awaitNanos(long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
final long deadline = System.nanoTime() + nanosTimeout;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (nanosTimeout <= 0L) {
transferAfterCancelledWait(node);
break;
}
if (nanosTimeout >= spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
nanosTimeout = deadline - System.nanoTime();
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return deadline - System.nanoTime();
}
/**
* Implements absolute timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* <li> If timed out while blocked in step 4, return false, else true.
* </ol>
*/
// 等待,当前线程在接到信号、被中断或到达指定最后期限之前一直处于等待状态
public final boolean awaitUntil(Date deadline)
throws InterruptedException {
long abstime = deadline.getTime();
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
boolean timedout = false;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (System.currentTimeMillis() > abstime) {
timedout = transferAfterCancelledWait(node);
break;
}
LockSupport.parkUntil(this, abstime);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return !timedout;
}
/**
* Implements timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* <li> If timed out while blocked in step 4, return false, else true.
* </ol>
*/
// 等待,当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态。此方法在行为上等效于: awaitNanos(unit.toNanos(time)) > 0
public final boolean await(long time, TimeUnit unit)
throws InterruptedException {
long nanosTimeout = unit.toNanos(time);
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
final long deadline = System.nanoTime() + nanosTimeout;
boolean timedout = false;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (nanosTimeout <= 0L) {
timedout = transferAfterCancelledWait(node);
break;
}
if (nanosTimeout >= spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
nanosTimeout = deadline - System.nanoTime();
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return !timedout;
}
// support for instrumentation
/**
* Returns true if this condition was created by the given
* synchronization object.
*
* @return {@code true} if owned
*/
final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
return sync == AbstractQueuedSynchronizer.this;
}
/**
* Queries whether any threads are waiting on this condition.
* Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
*
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
// 查询是否有正在等待此条件的任何线程
protected final boolean hasWaiters() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION)
return true;
}
return false;
}
/**
* Returns an estimate of the number of threads waiting on
* this condition.
* Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
*
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
// 返回正在等待此条件的线程数估计值
protected final int getWaitQueueLength() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
int n = 0;
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION)
++n;
}
return n;
}
/**
* Returns a collection containing those threads that may be
* waiting on this Condition.
* Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
*
* @return the collection of threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
// 返回包含那些可能正在等待此条件的线程集合
protected final Collection<Thread> getWaitingThreads() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION) {
Thread t = w.thread;
if (t != null)
list.add(t);
}
}
return list;
}
}
此类实现了Condition接口,Condition接口定义了条件操作规范,具体如下
public interface Condition {
// 等待,当前线程在接到信号或被中断之前一直处于等待状态
void await() throws InterruptedException;
// 等待,当前线程在接到信号之前一直处于等待状态,不响应中断
void awaitUninterruptibly();
//等待,当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态
long awaitNanos(long nanosTimeout) throws InterruptedException;
// 等待,当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态。此方法在行为上等效于: awaitNanos(unit.toNanos(time)) > 0
boolean await(long time, TimeUnit unit) throws InterruptedException;
// 等待,当前线程在接到信号、被中断或到达指定最后期限之前一直处于等待状态
boolean awaitUntil(Date deadline) throws InterruptedException;
// 唤醒一个等待线程。如果所有的线程都在等待此条件,则选择其中的一个唤醒。在从 await 返回之前,该线程必须重新获取锁。
void signal();
// 唤醒所有等待线程。如果所有的线程都在等待此条件,则唤醒所有线程。在从 await 返回之前,每个线程都必须重新获取锁。
void signalAll();
}
Condition接口中定义了await、signal方法,用来等待条件、释放条件。之后会详细分析CondtionObject的源码。
类的属性
public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer
implements java.io.Serializable {
// 版本号
private static final long serialVersionUID = 7373984972572414691L;
// 头结点
private transient volatile Node head;
// 尾结点
private transient volatile Node tail;
// 状态
private volatile int state;
// 自旋时间
static final long spinForTimeoutThreshold = 1000L;
// Unsafe类实例
private static final Unsafe unsafe = Unsafe.getUnsafe();
// state内存偏移地址
private static final long stateOffset;
// head内存偏移地址
private static final long headOffset;
// state内存偏移地址
private static final long tailOffset;
// tail内存偏移地址
private static final long waitStatusOffset;
// next内存偏移地址
private static final long nextOffset;
// 静态初始化块
static {
try {
stateOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("state"));
headOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("head"));
tailOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
waitStatusOffset = unsafe.objectFieldOffset
(Node.class.getDeclaredField("waitStatus"));
nextOffset = unsafe.objectFieldOffset
(Node.class.getDeclaredField("next"));
} catch (Exception ex) { throw new Error(ex); }
}
}
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