序
做开发也有几年了, 对1.5的concurrent并发包了解并不是很深入, 近来正好有空做个深入的学习
基础
在看 AQS 源码之前, 需要对下面的知识点有个大致的了解, 看源码会快很多
- Unsafe 相关, 主要是 CAS 原子锁
- CLH 锁
- 管程模型
Unsafe CAS原子锁
CAS原子锁是基于CPU的原子指令 compareAndSet 实现的命令, 该操作是原子的, 要么成功要么失败
CLH 锁
- CLH是一种基于队列的自旋锁
- 过来获取锁的线程全部排排队
- 只有排在第一个的线程才有执行权
- 其他的线程自旋判断排在前面一个节点的状态值, 不是允许执行状态则一直自旋等待
管程模型
- 管程模型是实现线程同步锁的一种方式, synchronized 的实现模式其实也是这个
- 管程主要的组成部分是: 多线程共享资源, 限制并发的进入队列, 不满足执行条件的等待队列和一些出队, 入队的方法
- 进入管程需要先判断是否有执行条件, 没有则在进入队列中等待
- 进入管程后, 不满足执行条件后, 进入等待队列等待
- 执行完后离开管程
代码分析
源码做了详细的注释
/**
* 该处存在两个队列 <br>
* 同步队列 : 争抢不到锁的线程进入同步队列<br>
* 条件队列 : 不满足继续运行条件的await后进入条件队列
*
*/
@SuppressWarnings("restriction")
public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable {
private static final long serialVersionUID = 7373984972572414691L;
protected AbstractQueuedSynchronizer() {
}
/**
* 条件队列和同步队列的节点类
*/
static final class Node {
// 标记节点在共享模式下等待
static final Node SHARED = new Node();
// 标记节点在排他模式下等待
static final Node EXCLUSIVE = null;
// 状态值, 表示线程已经取消了, 线程超时或者被中断
static final int CANCELLED = 1;
// 节点还有一个中间状态值0, 为0是空白状态, 后面的节点也未进入park
// 状态值, 表示后续节点需要被唤醒
static final int SIGNAL = -1;
// 状态值, 表示线程在条件队列中, 即Condition队列, 调用Condition.await()后进入该状态值
static final int CONDITION = -2;
// 状态值, 表示一下个acquireShared应该是无条件传播
static final int PROPAGATE = -3;
// 节点的等待状态值
volatile int waitStatus;
// Lock.lock()获取不到锁的线程进入同步队列
// 同步队列中当前节点的前面的节点
volatile Node prev;
// 同步队列中当前节点的下一个节点
volatile Node next;
// 当前节点的线程
volatile Thread thread;
// 维持Condition条件队列的属性
// Condition.await() 等待队列中的下一个等待节点, 节点在CONDITION状态下
Node nextWaiter;
// 判断节点是否在共享模式下
final boolean isShared() {
return nextWaiter == SHARED;
}
// 获取前置节点
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
}
Node() {
}
Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
}
// 同步队列的头节点, 不为空时该节点的状态不能是CANCELLED
private transient volatile Node head;
// 同步队列的尾部节点, 只能通过enq添加一个新的同步节点
private transient volatile Node tail;
// 线程进入同步器的次数
// state>0 说明同步锁已经被某个线程线程拿了
private volatile int state;
// 获取重入次数
protected final int getState() {
return state;
}
// 设置重入次数
protected final void setState(int newState) {
state = newState;
}
// 设置重入次数
protected final boolean compareAndSetState(int expect, int update) {
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
// Queuing utilities
// 线程自旋时间
// > spinForTimeoutThreshold线程才会被park
// < spinForTimeoutThreshold的将自旋
static final long spinForTimeoutThreshold = 1000L;
/**
* for循环强制将节点插入同步队列的尾部
*
* @param 待插入的node
* @return 插入前的tail
*/
private Node enq(final Node node) {
// 因为存在并发, 需要循环执行, 直到当前把当前线程作为节点追加到同步队列尾部
for (;;) {// 循环直到将当前节点放置到等待队伍尾部
Node t = tail;
if (t == null) { // 尾结点为null, 表示等待队列没有节点, 需要创建一个虚节点
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;// node.prev=tail
if (compareAndSetTail(t, node)) {// 设置当前节点为尾节点
t.next = node;// tail.next = node
return t;
}
}
}
}
/**
* 创建线程节点, 并且添加到同步队列尾部
*
* @param mode 同步模式 Node.EXCLUSIVE for exclusive, Node.SHARED for shared
* @return 新创建的节点
*/
private Node addWaiter(Node mode) {
// 创建当前线程的等待节点
Node node = new Node(Thread.currentThread(), mode);
Node pred = tail;
if (pred != null) {
node.prev = pred;
// 该处存在并发, 需要进行原子操作
if (compareAndSetTail(pred, node)) {// 尝试将当前节点放置到等待队伍末尾
pred.next = node;
return node;
}
}
enq(node);// 上面尝试失败, 执行下面的强制放入
return node;
}
// 设置头节点
private void setHead(Node node) {
head = node;
// 用不着的属性全部设置为null
node.thread = null;// help GC
node.prev = null;
}
/**
* 唤醒同步队列中的后续节点<br>
*
* @param node the node
*/
private void unparkSuccessor(Node node) {
// 设置当前节点的等待状态值为0
int ws = node.waitStatus;
if (ws < 0)// 如果节点状态<0, 将当前等待状态设置为0
compareAndSetWaitStatus(node, ws, 0);// waitStatus==0, 表示当前线程即将完成, 需要唤醒后面的节点
// unpark下一个节点, 如果下一个节点cancelled或者不存在则继续解锁下面的节点
Node s = node.next;
if (s == null || s.waitStatus > 0) {// 找个下一个没有Canceled的节点, 然后唤醒
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);// 唤醒
}
/**
* 共享模式下释放锁
*/
private void doReleaseShared() {
// 循环解锁头节点线程, 被解锁的线程将离开等待队列, 新的头节点将会别代替
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {// 如果节点状态是唤醒
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // 状态设置失败再次设置状态值0
unparkSuccessor(h);// 设置成功后唤醒后续节点
} else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}
/**
* 设置头节点然后传播
*
* @param node
* @param propagate
*/
private void setHeadAndPropagate(Node node, int propagate) {
Node h = head; // Record old head for check below
setHead(node);
// 传播
if (propagate > 0 || h == null || h.waitStatus < 0 || (h = head) == null || h.waitStatus < 0) {
Node s = node.next;
if (s == null || s.isShared())
doReleaseShared();
}
}
// Utilities for various versions of acquire
/**
* 取消正在进行的获取请求 <br>
* <ol>
* <li>删除当前节点</li>
* <li>设置后面节点的状态值SIGNAL或者唤醒后续节点</li>
* </ol>
*
* @param node the node
*/
private void cancelAcquire(Node node) {
if (node == null)
return;
// 节点的线程设置为null
node.thread = null;
// Skip cancelled 所有的前置节点
Node pred = node.prev;
while (pred.waitStatus > 0)
node.prev = pred = pred.prev;
// waitState不为canceled的第一个前置节点的next节点
// 可能是当前节点也可能不是
Node predNext = pred.next;
// 设置当前节点的等待状态为CANCELLED
node.waitStatus = Node.CANCELLED;
// 如果自己是tail, 则移除它
if (node == tail && compareAndSetTail(node, pred)) {
compareAndSetNext(pred, predNext, null);
} 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) {
Node next = node.next;
if (next != null && next.waitStatus <= 0)
compareAndSetNext(pred, predNext, next);// 设置前一个节点的后一个节点, 将当前节点排除在等待队列之外
} else {
// 什么情况下需要唤醒后面的节点 ?
// 1.如果当前节点是头节点
// 2.不是头节点, 但是当前节点的前面的节点状态值不是SIGNAL, 但是设置SIGNAL失败时
// 3.前面的节点为null, 当前节点已经成为了新的头节点
unparkSuccessor(node);
}
node.next = node; // help GC
}
}
/**
* 条件: 节点的线程进入Prak状态时, 需先设置前置节点的等待状态是SIGNAL <br>
* 因为获取锁失败后需要进入Park, 所以需要强制设置前置节点的waitStatus=SIGNAL <br>
* 所以该方法需在for循环中使用, 循环直到前置节点的waitStatus=SIGNAL <br>
*
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*/
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
// 前节点状态为-1, 说明当前节点需要被唤醒
return true;
if (ws > 0) {// 状态>0是节点都是CANCELLED
// 前节点状态>0, 找到前状态<=0的节点连在它的后面
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
// 前节点状态<0时, 设置前节点的等待状态为-1
// 下一个循环就可以将当前线程Park
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
// 打断当前线程, 目的是重置中断标识位
static void selfInterrupt() {
Thread.currentThread().interrupt();
}
/**
* Park当前线程, 等待中断获取unpark
*
* @return 线程是否被中断唤醒
*/
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);// 当前线程Park, 等待被unpark获取被中断
return Thread.interrupted();// 该方法会重置中断位, 后续的操作需要重新设置中断位
}
/**
* 节点请求排队<br>
* 节点已经进入同步队列中排队, 排队中再次尝试获取锁<br>
* 如果获取不到则设置前置节点的waitStatus=SIGNAL后Park等待
*
* @param node 当前节点
* @param arg 锁重入次数
* @return 排队被中断返回true, 正常唤醒返回false
*/
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
// 第一个是虚节点, 当节点是第二个时就可以抢资源了, 如果获取锁成功了, 下面就不需要执行直接返回
// 其他情况是设置前节点的等待状态为-1, 线程Park后等待唤醒
// for循环, 该处必须要是循环之内, 线程要是被中断唤醒后, 没有抢到锁继续Park
for (;;) {
final Node p = node.predecessor();// 前节点
if (p == head && tryAcquire(arg)) {// 前面节点是头节点, 而且获取到了锁
setHead(node);// 释放前节点
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node)// 强制设置前置节点的waitStatus=SIGNAL
&& parkAndCheckInterrupt())// Park当前线程
interrupted = true;// 不是中断唤醒不执行
}
} finally {
if (failed)// 上面的操作出现异常, 取消当前线程的请求
cancelAcquire(node);
}
}
/**
* 排他模式下, 先获取锁, 获取不到则Park, 如果线程被打断则抛出InterruptedException
*
* @param arg 重入锁的次数
* @throws InterruptedException
*/
private void doAcquireInterruptibly(int arg) throws InterruptedException {
final Node node = addWaiter(Node.EXCLUSIVE);// 添加排他等待者
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return;
}
if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* 在时间限定下请求锁, 每个多少个ns时间周期请求锁
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*/
private boolean doAcquireNanos(int arg, long nanosTimeout) throws InterruptedException {
if (nanosTimeout <= 0L)
return false;
final long deadline = System.nanoTime() + nanosTimeout;
final Node node = addWaiter(Node.EXCLUSIVE);// 添加到等待队列
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {// 获取到锁则执行
setHead(node);
p.next = null; // help GC
failed = false;
return true;
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;
if (shouldParkAfterFailedAcquire(p, node)// 线程应该park
&& nanosTimeout > spinForTimeoutThreshold// 等待时间>自旋超时时间
)
LockSupport.parkNanos(this, nanosTimeout);// 线程Park
if (Thread.interrupted())// 线程被打断则抛出异常, 没有打断则继续请求锁
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* 共享模式下请求锁
*
* @param arg the acquire argument
*/
private void doAcquireShared(int arg) {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* 共享模式下请求锁, 抛出异常
*
* @param arg the acquire argument
*/
private void doAcquireSharedInterruptibly(int arg) throws InterruptedException {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* 共享模式下时间间隔请求锁
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*/
private boolean doAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {
if (nanosTimeout <= 0L)
return false;
final long deadline = System.nanoTime() + nanosTimeout;
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return true;
}
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;
if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
// Main exported methods
/**
* 请求锁, 该处实现需要注意并发情况, 使用CAS原子锁执行操作
*
* @param arg
* @return
*/
protected boolean tryAcquire(int arg) {
throw new UnsupportedOperationException();
}
/**
* 释放排他锁 <br>
* 1. 设置同步器state=0 <br>
* 2. 设置同步器排他线程为null
*
* @param arg
* @return
*/
protected boolean tryRelease(int arg) {
throw new UnsupportedOperationException();
}
/**
* 共享模式下获取锁
*
* @param arg
* @return
*/
protected int tryAcquireShared(int arg) {
throw new UnsupportedOperationException();
}
/**
* 共享模式下释放锁
*
* @param arg
* @return
*/
protected boolean tryReleaseShared(int arg) {
throw new UnsupportedOperationException();
}
// 判断当前线程是否持有锁
protected boolean isHeldExclusively() {
throw new UnsupportedOperationException();
}
/**
* 先请求锁, 请求不到则到等待队列
*
* @param arg
*/
public final void acquire(int arg) {
// tryAcquire请求锁, 请求成功离开
// 请求不成功继续执行acquireQueued, 请求线程入同步队列末等待
if (!tryAcquire(arg)// 抢锁
&& acquireQueued(addWaiter(Node.EXCLUSIVE), arg)// 抢不到排队, 返回值是精心设计的
)
// 锁请求失败, 线程Park被中断时执行下面的方法
selfInterrupt();// 重置中断
}
/**
* 先请求锁, 请求不到则到等待队列
*
* @param arg
* @throws InterruptedException
*/
public final void acquireInterruptibly(int arg) throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (!tryAcquire(arg))
doAcquireInterruptibly(arg);
}
/**
* 尝试获取排他锁, 获取不到则间隔获取
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*/
public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquire(arg) || doAcquireNanos(arg, nanosTimeout);
}
/**
* 释放锁, 释放完成后唤醒后续节点
*
* @param arg the release argument. This value is conveyed to
* {@link #tryRelease} but is otherwise uninterpreted and can
* represent anything you like.
* @return the value returned from {@link #tryRelease}
*/
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
// 需要判断当前节点的waitStatus值
// 后面节点在进入Park是会将前面的waitStatus设置为-1, 如果是-1时就需要唤醒后面的线程节点
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);// 唤醒后面的节点
return true;
}
return false;
}
/**
* Acquires in shared mode, ignoring interrupts. Implemented by first invoking
* at least once {@link #tryAcquireShared}, returning on success. Otherwise the
* thread is queued, possibly repeatedly blocking and unblocking, invoking
* {@link #tryAcquireShared} until success.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted and can
* represent anything you like.
*/
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}
/**
* Acquires in shared mode, aborting if interrupted. Implemented by first
* checking interrupt status, then invoking at least once
* {@link #tryAcquireShared}, returning on success. Otherwise the thread is
* queued, possibly repeatedly blocking and unblocking, invoking
* {@link #tryAcquireShared} until success or the thread is interrupted.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted and can
* represent anything you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireSharedInterruptibly(int arg) throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
/**
* Attempts to acquire in shared mode, aborting if interrupted, and failing if
* the given timeout elapses. Implemented by first checking interrupt status,
* then invoking at least once {@link #tryAcquireShared}, returning on success.
* Otherwise, the thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread is interrupted
* or the timeout elapses.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted
* and can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*/
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquireShared(arg) >= 0 || doAcquireSharedNanos(arg, nanosTimeout);
}
/**
* Releases in shared mode. Implemented by unblocking one or more threads if
* {@link #tryReleaseShared} returns true.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryReleaseShared} but is otherwise uninterpreted and can
* represent anything you like.
* @return the value returned from {@link #tryReleaseShared}
*/
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
// Queue inspection methods
/**
* 等待队列中是否还有等待线程
*
* <p>
* In this implementation, this operation returns in constant time.
*
* @return {@code true} if there may be other threads waiting to acquire
*/
public final boolean hasQueuedThreads() {
return head != tail;
}
/**
* 等待队列中是否还有内容
*
* <p>
* In this implementation, this operation returns in constant time.
*
* @return {@code true} if there has ever been contention
*/
public final boolean hasContended() {
return head != null;
}
/**
* Returns the first (longest-waiting) thread in the queue, or {@code null} if
* no threads are currently queued.
*
* <p>
* In this implementation, this operation normally returns in constant time, but
* may iterate upon contention if other threads are concurrently modifying the
* queue.
*
* @return the first (longest-waiting) thread in the queue, or {@code null} if
* no threads are currently queued
*/
public final Thread getFirstQueuedThread() {
// handle only fast path, else relay
return (head == tail) ? null : fullGetFirstQueuedThread();
}
/**
* Version of getFirstQueuedThread called when fastpath fails
*/
private Thread fullGetFirstQueuedThread() {
/*
* The first node is normally head.next. Try to get its thread field, ensuring
* consistent reads: If thread field is nulled out or s.prev is no longer head,
* then some other thread(s) concurrently performed setHead in between some of
* our reads. We try this twice before resorting to traversal.
*/
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;
/*
* Head's next field might not have been set yet, or may have been unset after
* setHead. So we must check to see if tail is actually first node. If not, we
* continue on, safely traversing from tail back to head to find first,
* guaranteeing termination.
*/
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;
}
/**
* 判断线程是否已经进入等待队列
*
* <p>
* 遍历等待队列检查线程是否存在
*
* @param thread the thread
* @return {@code true} if the given thread is on the queue
* @throws NullPointerException if the thread is null
*/
public final boolean isQueued(Thread thread) {
if (thread == null)
throw new NullPointerException();
for (Node p = tail; p != null; p = p.prev)
if (p.thread == thread)
return true;
return false;
}
/**
* 判断排队个第一个是否是排他模式节点
*/
final boolean apparentlyFirstQueuedIsExclusive() {
Node h, s;
return (h = head) != null && (s = h.next) != null && !s.isShared() && s.thread != null;
}
/**
* 检查我前面是否还有别的线程在等待
*
* @return
*/
public final boolean hasQueuedPredecessors() {
Node t = tail; // Read fields in reverse initialization order
Node h = head;
Node s;
return h != t && ((s = h.next) == null || s.thread != Thread.currentThread());
}
// Instrumentation and monitoring methods
/**
* 获取节点的排队的队伍长度
*
* @return the estimated number of threads waiting to acquire
*/
public final int getQueueLength() {
int n = 0;
for (Node p = tail; p != null; p = p.prev) {
if (p.thread != null)
++n;
}
return n;
}
/**
* 获取所有的排队线程
*
* @return the collection of threads
*/
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;
}
/**
* 获取所有的排他模式的排队线程
*
* @return the collection of threads
*/
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;
}
/**
* 获取所有的共享模式下的排队线程
*
* @return the collection of threads
*/
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;
}
/**
* AQS toString
*
* @return a string identifying this synchronizer, as well as its state
*/
public String toString() {
int s = getState();
String q = hasQueuedThreads() ? "non" : "";
return super.toString() + "[State = " + s + ", " + q + "empty queue]";
}
// Internal support methods for Conditions
/**
* 当前节点是否在同步队列中
*
* @param node the node
* @return true if is reacquiring
*/
final boolean isOnSyncQueue(Node node) {
// 节点状态是CONDITION, 或者节点没有前置节点, 说明节点不在队列里面
if (node.waitStatus == Node.CONDITION// 节点状态是await, 那肯定是不在同步队列中, 应该在等待队列里面
|| node.prev == null// 前一个节点是null, 则是头节点, 那肯定不需要进行同步
)
return false;
// 节点有后置节点, 说明节点肯定在
if (node.next != null) // If has successor, it must be on queue
return true;
return findNodeFromTail(node);
}
/**
* 从等待队列的最后往前遍历, 检测节点是的在排队
*
* @return true if present
*/
private boolean findNodeFromTail(Node node) {
Node t = tail;
for (;;) {
if (t == node)
return true;
if (t == null)
return false;
t = t.prev;
}
}
/**
* 将节点从条件队列转换到同步队列
*
* @param node the node
* @return true if successfully transferred (else the node was cancelled before
* signal)
*/
final boolean transferForSignal(Node node) {
// 节点状态转换为0
if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
// 节点状态转换失败
return false;
// 节点加入到同步队列尾
Node p = enq(node);// 前置节点
int ws = p.waitStatus;
if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
// 前面的节点取消了, 或者前面节点设置SIGNAL失败, 当前节点unpark
LockSupport.unpark(node.thread);
return true;
}
/**
* Transfers node, if necessary, to sync queue after a cancelled wait. Returns
* true if thread was cancelled before being signalled.
*
* @param node the node
* @return true if cancelled before the node was signalled
*/
final boolean transferAfterCancelledWait(Node node) {
if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
enq(node);
return true;
}
/*
* If we lost out to a signal(), then we can't proceed until it finishes its
* enq(). Cancelling during an incomplete transfer is both rare and transient,
* so just spin.
*/
while (!isOnSyncQueue(node))
Thread.yield();
return false;
}
/**
* 释放所有的可重入的锁
*
* @param node
* @return
*/
final int fullyRelease(Node node) {
boolean failed = true;
try {
int savedState = getState();// 锁的重入数量
if (release(savedState)) {// 释放全部
failed = false;
return savedState;
} else {
throw new IllegalMonitorStateException();
}
} finally {
if (failed)
node.waitStatus = Node.CANCELLED;
}
}
// Instrumentation methods for conditions
/**
* Queries whether the given ConditionObject uses this synchronizer as its lock.
*
* @param condition the condition
* @return {@code true} if owned
* @throws NullPointerException if the condition is null
*/
public final boolean owns(ConditionObject condition) {
return condition.isOwnedBy(this);
}
/**
* Queries whether any threads are waiting on the given condition associated
* with this synchronizer. Note that because timeouts and interrupts may occur
* at any time, a {@code true} return does not guarantee that a future
* {@code signal} will awaken any threads. This method is designed primarily for
* use in monitoring of the system state.
*
* @param condition the condition
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if exclusive synchronization is not held
* @throws IllegalArgumentException if the given condition is not associated
* with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final boolean hasWaiters(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.hasWaiters();
}
/**
* Returns an estimate of the number of threads waiting on the given condition
* associated with this synchronizer. Note that because timeouts and interrupts
* may occur at any time, the estimate serves only as an upper bound on the
* actual number of waiters. This method is designed for use in monitoring of
* the system state, not for synchronization control.
*
* @param condition the condition
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if exclusive synchronization is not held
* @throws IllegalArgumentException if the given condition is not associated
* with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final int getWaitQueueLength(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitQueueLength();
}
/**
* Returns a collection containing those threads that may be waiting on the
* given condition associated with this synchronizer. Because the actual set of
* threads may change dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the returned
* collection are in no particular order.
*
* @param condition the condition
* @return the collection of threads
* @throws IllegalMonitorStateException if exclusive synchronization is not held
* @throws IllegalArgumentException if the given condition is not associated
* with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitingThreads();
}
/**
* 条件对象, 获取到锁的线程的进入await的队列
*/
public class ConditionObject implements Condition, java.io.Serializable {
private static final long serialVersionUID = 1173984872572414699L;
// 条件队列的第一个元素
private transient Node firstWaiter;
// 条件队列的最后一个元素
private transient Node lastWaiter;
public ConditionObject() {
}
// Internal methods
/**
* 将当前线程添加到条件队列中
*
* @return 当前线程的队列节点
*/
private Node addConditionWaiter() {
Node t = lastWaiter;
// If lastWaiter is cancelled, clean out.
if (t != null && t.waitStatus != Node.CONDITION) {
unlinkCancelledWaiters();
t = lastWaiter;
}
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;
return node;
}
/**
* 移除canceled节点, 将条件队列转换为同步队列
*
* @param first (non-null) the first node on condition queue
*/
private void doSignal(Node first) {
do {
if ((firstWaiter = first.nextWaiter) == null)// 下一个waiter设置为firstWaiter
lastWaiter = null;// 下一个waiter为null, 设置lastWaiter=null
first.nextWaiter = null;// 清除nextWaiter
} while (!transferForSignal(first) && (first = firstWaiter) != null);
}
/**
* 将所有的条件队列转换为同步队列
*
* @param first (non-null) the first node on condition queue
*/
private void doSignalAll(Node first) {
lastWaiter = firstWaiter = null;
do {
Node next = first.nextWaiter;
first.nextWaiter = null;
transferForSignal(first);
first = next;
} while (first != null);
}
/**
* 删除条件队列中所有waitStatus!=CONDITION的节点
*/
private void unlinkCancelledWaiters() {
Node t = firstWaiter;
Node trail = null;// 追踪遍历过程中节点状态是CONDITION的最新的节点
while (t != null) {
Node next = t.nextWaiter;// 下一个节点
if (t.waitStatus != Node.CONDITION) {// 当前节点的状态不是 CONDITION
t.nextWaiter = null;// 清除
if (trail == null)//
firstWaiter = next;
else
trail.nextWaiter = next;
if (next == null)// await队列尾
lastWaiter = trail;
} else
trail = t;
t = next;
}
}
// public methods
/**
* 唤醒等待队列的第一个线程
*/
public final void signal() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignal(first);
}
/**
* 唤醒所有条件队列的等待节点
*
* @throws IllegalMonitorStateException if {@link #isHeldExclusively} returns
* {@code false}
*/
public final void signalAll() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
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>
*/
public final void awaitUninterruptibly() {
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
boolean interrupted = false;
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if (Thread.interrupted())
interrupted = true;
}
if (acquireQueued(node, savedState) || interrupted)
selfInterrupt();
}
// 出队列后重新打断
private static final int REINTERRUPT = 1;
// 出队列后throw InterruptedException
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>如果当前线程被打断则抛出异常
* <li>Save lock state returned by {@link #getState}.
* <li>请求Invoke {@link #release} 释放上面获取lock状态, 失败则抛出IllegalMonitorStateException
* <li>阻塞等待唤醒或者打断
* <li>重新争抢锁, 抢到则继续运行
* <li>If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*/
public final void await() throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();// 将线程添加到await队列
int savedState = fullyRelease(node);// 释放所有的重入锁
int interruptMode = 0;//
while (!isOnSyncQueue(node)) {// 检查线程是否在同步队列中
// 不在同同步队列中就应该阻塞
LockSupport.park(this);// park
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
// 请求锁, 请求不到进入同步队列等待
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>
*/
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>
*/
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>
*/
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}
*/
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}
*/
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}
*/
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;
}
}
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long stateOffset;
private static final long headOffset;
private static final long tailOffset;
private static final long waitStatusOffset;
private static final long nextOffset;
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);
}
}
// 设置头节点
private final boolean compareAndSetHead(Node update) {
return unsafe.compareAndSwapObject(this, headOffset, null, update);
}
// 设置等待队列的尾节点
private final boolean compareAndSetTail(Node expect, Node update) {
return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
}
// 设置节点的等待状态值
private static final boolean compareAndSetWaitStatus(Node node, int expect, int update) {
return unsafe.compareAndSwapInt(node, waitStatusOffset, expect, update);
}
// 设置当前节点的下一个节点值
private static final boolean compareAndSetNext(Node node, Node expect, Node update) {
return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
}
}
