<pre>
* +------+ prev +-----+ +-----+
* head | | <---- | | <---- | | tail
* +------+ +-----+ +-----+
* </pre>
import sun.misc.Unsafe;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractOwnableSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.LockSupport;
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
private static final long serialVersionUID = 7373984972572414691L;
/**
* Creates a new {@code AbstractQueuedSynchronizer} instance
* with initial synchronization state of zero.
*/
//构造器
protected AbstractQueuedSynchronizer() {
}
//Node节点
static final class Node {
/**
* Marker to indicate a node is waiting in shared mode
*/
//共享锁 newNode()相当于新建一个node节点
static final Node SHARED = new Node();
/**
* Marker to indicate a node is waiting in exclusive mode
*/
//独占锁标识 null
static final Node EXCLUSIVE = null;
/**
* waitStatus value to indicate thread has cancelled
*/
//线程waitStatus标识 1 代表任务已结束
static final int CANCELLED = 1;
/**
* waitStatus value to indicate successor's thread needs unparking
*/
//waitStatus等待被唤醒
static final int SIGNAL = -1;
/**
* waitStatus value to indicate thread is waiting on condition
*/
//waitStatus在conditon单向链表的标识
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
//waitStatus 尝试获取共享锁传播标识
static final int PROPAGATE = -3;
/**
* Status field, taking on only the values:
* SIGNAL: The successor of this node is (or will soon be)
* blocked (via park), so the current node must
* unpark its successor when it releases or
* cancels. To avoid races, acquire methods must
* first indicate they need a signal,
* then retry the atomic acquire, and then,
* on failure, block.
* CANCELLED: This node is cancelled due to timeout or interrupt.
* Nodes never leave this state. In particular,
* a thread with cancelled node never again blocks.
* CONDITION: This node is currently on a condition queue.
* It will not be used as a sync queue node
* until transferred, at which time the status
* will be set to 0. (Use of this value here has
* nothing to do with the other uses of the
* field, but simplifies mechanics.)
* PROPAGATE: A releaseShared should be propagated to other
* nodes. This is set (for head node only) in
* doReleaseShared to ensure propagation
* continues, even if other operations have
* since intervened.
* 0: None of the above
* <p>
* The values are arranged numerically to simplify use.
* Non-negative values mean that a node doesn't need to
* signal. So, most code doesn't need to check for particular
* values, just for sign.
* <p>
* The field is initialized to 0 for normal sync nodes, and
* CONDITION for condition nodes. It is modified using CAS
* (or when possible, unconditional volatile writes).
*/
//线程等待状态标识 1,-1,-2,-3
volatile int waitStatus;
/**
* Link to predecessor node that current node/thread relies on
* for checking waitStatus. Assigned during enqueuing, and nulled
* out (for sake of GC) only upon dequeuing. Also, upon
* cancellation of a predecessor, we short-circuit while
* finding a non-cancelled one, which will always exist
* because the head node is never cancelled: A node becomes
* head only as a result of successful acquire. A
* cancelled thread never succeeds in acquiring, and a thread only
* cancels itself, not any other node.
*/
//node前一个节点
volatile Node prev;
/**
* Link to the successor node that the current node/thread
* unparks upon release. Assigned during enqueuing, adjusted
* when bypassing cancelled predecessors, and nulled out (for
* sake of GC) when dequeued. The enq operation does not
* assign next field of a predecessor until after attachment,
* so seeing a null next field does not necessarily mean that
* node is at end of queue. However, if a next field appears
* to be null, we can scan prev's from the tail to
* double-check. The next field of cancelled nodes is set to
* point to the node itself instead of null, to make life
* easier for isOnSyncQueue.
*/
//指向node节点的下一个节点
volatile Node next;
/**
* The thread that enqueued this node. Initialized on
* construction and nulled out after use.
*/
//用于存储node节点中的线程
volatile Thread thread;
/**
* Link to next node waiting on condition, or the special
* value SHARED. Because condition queues are accessed only
* when holding in exclusive mode, we just need a simple
* linked queue to hold nodes while they are waiting on
* conditions. They are then transferred to the queue to
* re-acquire. And because conditions can only be exclusive,
* we save a field by using special value to indicate shared
* mode.
*/
//下一个等待者 用于condition单向链表队列
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode.
*/
//判断是否为共享锁
final boolean isShared() {
return nextWaiter == SHARED;
}
/**
* Returns previous node, or throws NullPointerException if null.
* Use when predecessor cannot be null. The null check could
* be elided, but is present to help the VM.
*
* @return the predecessor of this node
*/
//获取当前节点的前驱节点，Node prev如果为null,throw 空指针异常
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
}
Node() { // Used to establish initial head or SHARED marker
}
//构造器，用于codition中 addWaiter()队列
Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
//用于condition
Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
}
/**
* Head of the wait queue, lazily initialized. Except for
* initialization, it is modified only via method setHead. Note:
* If head exists, its waitStatus is guaranteed not to be
* CANCELLED.
*/
//头节点
private transient volatile Node head;
/**
* Tail of the wait queue, lazily initialized. Modified only via
* method enq to add new wait node.、
* 等待队列的尾部，延迟初始化。修改只能通过
* 方法enq添加新的等待节点。
*/
//尾节点
private transient volatile Node tail;
/**
* The synchronization state.
*/
//队列中的state值 0,1,2,3,........ 0 初始化 1独占锁 2可重入锁 .....
private volatile int state;
/**
* Returns the current value of synchronization state.
* This operation has memory semantics of a {@code volatile} read.
*
* @return current state value
*/
//获取状态值
protected final int getState() {
return state;
}
/**
* Sets the value of synchronization state.
* This operation has memory semantics of a {@code volatile} write.
*
* @param newState the new state value
*/
//当acciqure(int args) or release(int args) 等 需要设置状态值
protected final void setState(int newState) {
state = newState;
}
/**
* Atomically sets synchronization state to the given updated
* value if the current state value equals the expected value.
* This operation has memory semantics of a {@code volatile} read
* and write.
*
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful. False return indicates that the actual
* value was not equal to the expected value.
*/
//CAS操作 设置state 状态 expect 内存中的值 update 要更新的值
protected final boolean compareAndSetState(int expect, int update) {
// See below for intrinsics setup to support this
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
// Queuing utilities
/**
* The number of nanoseconds for which it is faster to spin
* rather than to use timed park. A rough estimate suffices
* to improve responsiveness with very short timeouts.
*/
static final long spinForTimeoutThreshold = 1000L;
/**
* Inserts node into queue, initializing if necessary. See picture above.
*
* @param node the node to insert
* @return node's predecessor
*/
//初始化node节点 将当前node节点插入SyncQueue同步队列至尾节点
private Node enq(final Node node) {
//自旋
for (; ; ) {
//尾节点
Node t = tail;
//尾节点为Null，必须初始化同步SyncQueue队列
if (t == null) { // Must initialize
//初始化头节点
if (compareAndSetHead(new Node()))
//头尾节点相连
tail = head;
} else {
//指向node节点的前驱节点
node.prev = t;
//t为尾节点时，CAS设置node节点为尾节点
if (compareAndSetTail(t, node)) {
//t ---> node 相当于调换位置，尾节点t变成了node的前节点 node变为尾节点
t.next = node;
//返回node尾节点的前一个节点
return t;
}
}
}
}
/**
* Creates and enqueues node for current thread and given mode.
* 为当前线程和给定模式创建和排队节点。
*
* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
* @return the new node
*/
private Node addWaiter(Node mode) {
//创建指向当前线程的node节点
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
//尾节点赋值pred
Node pred = tail;
if (pred != null) {
//尾节点指向node节点， 变换位置，node节点为尾节点，pred为node节点的前节点
//相当于双向链表指针
node.prev = pred;
//node节点设置为队列的尾节点
if (compareAndSetTail(pred, node)) {
pred.next = node;
//返回node节点
return node;
}
}
//如果尾节点为空，初始化 入列
enq(node);
//返回加入等待队列的node节点
return node;
}
/**
* Sets head of queue to be node, thus dequeuing. Called only by
* acquire methods. Also nulls out unused fields for sake of GC
* and to suppress unnecessary signals and traversals.
*
* @param node the node
*/
//设置头节点 等待被唤醒
private void setHead(Node node) {
head = node;
node.thread = null;
node.prev = null;
}
/**
* Wakes up node's successor, if one exists.
*
* @param node the node
*/
//唤醒节点的后续等待节点线程争抢cpu资源
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.
*/
//获取waitStatus 0,-1,-2,-3
int ws = node.waitStatus;
//ws<0代表该节点等待被唤醒
if (ws < 0)
//CAS设置waitStatus 为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.
*/
//获取node.next node指向下一个节点
Node s = node.next;
//是否为null,或者状态waitStatus>0 清除下一个节点
if (s == null || s.waitStatus > 0) {
s = null;
//从队尾部向头部依次遍历，寻找waitStatus状态为等待被唤醒的节点
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
//等待被唤醒的节点赋值给s
s = t;
}
if (s != null)
//唤醒该节点的线程争抢资源
LockSupport.unpark(s.thread);
}
/**
* Release action for shared mode -- signals successor and ensures
* propagation. (Note: For exclusive mode, release just amounts
* to calling unparkSuccessor of head if it needs signal.)
* <p>
* *共享模式释放动作——信号的后继和保证
* *传播。(注意:对于排他模式，释放的数量是有限的
* *在需要信号时，致电unpark继承处的head。
* <p>
* 释放共享模式的锁
*/
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.
*
* 确保发布能够传播，即使存在其他版本
*正在进行的获得/版本。这一切照常进行
*如有需要，可尝试将头部后继器取出
*信号。但如果没有，则将状态设置为传播到
*确保发布后，传播仍在继续。
*此外，我们必须循环，以防添加新节点
当我们在做这件事的时候。而且，不像其他用法
* unpark继承者，我们需要知道CAS是否重置状态
*失败，如果是，请重新检查。
*/
//自旋
for (; ; ) {
//头节点
Node h = head;
//判断SyncQueue同步队列是否只有一个节点
if (h != null && h != tail) {
//获取头节点状态
int ws = h.waitStatus;
//状态为-1 等待被唤醒
if (ws == Node.SIGNAL) {
//设置waitStatus状态失败 跳过
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
//唤醒后续节点线程争抢资源
unparkSuccessor(h);
//节点状态为0 CAS waitStatus不能传播 跳过
} else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
//从尾部向前直至到头部 break
if (h == head) // loop if head changed
break;
}
}
/**
* Sets head of queue, and checks if successor may be waiting
* in shared mode, if so propagating if either propagate > 0 or
* PROPAGATE status was set.
* <p>
* 设置队列头，并检查是否有后续队列在等待
* *在共享模式下，如果是传播，则传播> 0或
* *设定传播状态
*
* @param node the node
* @param propagate the return value from a tryAcquireShared tryAcquireShared返回的值
*/
private void setHeadAndPropagate(Node node, int propagate) {
//记录头节点便于核对
Node h = head; // Record old head for check below
//当前节点设置为头节点
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.
*
*
如果:
*传播由主叫人指示，
*或记录(记为h。waitStatus要么之前
*或在setHead之后)通过以前的操作
*(注意:这使用了sign-check of waitStatus，因为
*传播状态可能转换为信号。)
*和
*下一个节点在共享模式下等待，
或者我们不知道，因为它看起来是空的
*
*这两种检查中的稳健性都可能导致
*不必要的起床，但只有当有多的时候
*
*
*/
//tryAcquireShared尝试获取共享锁的return value >表明获取共享锁成功
//等待状态waitStatus<0等待被唤醒 or 头节点为null
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
//指向node节点的下一个节点 s
Node s = node.next;
//s为null || 共享锁
if (s == null || s.isShared())
//共享模式释放 and 持续传播
doReleaseShared();
}
}
// Utilities for various versions of acquire
/**
* Cancels an ongoing attempt to acquire.
* 取消正在尝试获取资源的节点
*
* @param node the node
*/
private void cancelAcquire(Node node) {
// Ignore if node doesn't exist
//节点是否存在
if (node == null)
return;
//设置thread 为null
node.thread = null;
// Skip cancelled predecessors
//跳过取消的前节点
Node pred = node.prev;
//waitStatus>0说明该节点被取消或已完成状态
while (pred.waitStatus > 0)
//依次从尾向前寻找
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.
//下一个节点
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.
//完成任务标识 waitStatus=1
node.waitStatus = Node.CANCELLED;
// If we are the tail, remove ourselves.
//如果node是尾节点，remove node节点
if (node == tail && compareAndSetTail(node, pred)) {
//CAS设置尾节点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;
//如果后继者需要信号，试着设置pred的下一个链接
//所以它会得到一个signal。否则唤醒它传播
if (pred != head &&
((ws = pred.waitStatus) == Node.SIGNAL ||
(ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
pred.thread != null) {
//指向Node节点的next Node
Node next = node.next;
if (next != null && next.waitStatus <= 0)
//CAS设置pred的next节点为node,next next等待被唤醒
compareAndSetNext(pred, predNext, next);
} else {
//唤醒当前节点的后续节点
unparkSuccessor(node);
}
node.next = node; // help GC
}
}
/**
* Checks and updates status for a node that failed to acquire.
* Returns true if thread should block. This is the main signal
* control in all acquire loops. Requires that pred == node.prev.
* <p>
* 检查和更新未能获取的节点的状态。
* 如果线程阻塞，返回true。这是主要信号
* *控制所有的获取循环。要求pred == node.prev。
*
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*/
//检查更新未能获取资源的节点的状态 线程阻塞则return true
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
//pred waitStatus
int ws = pred.waitStatus;
//-1 等待被唤醒
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
* 此节点已经设置了请求发布的状态
*发出信号，以便安全停车。
*/
//可挂起阻塞状态 park 重量级锁
return true;
//等待状态>0 跳过
if (ws > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
//从队列尾部依次向头节点寻找 waitStatus<=0的节点
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
//设置pred的下个节点为node节点
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.
*
* waitStatus必须为0或PROPAGATE。表明我们
需要信号，但不要停车。来电者需要
* 停车前请务必确认无法获取。
*/
//设置状态为signal 等待被唤醒
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
/**
* Convenience method to interrupt current thread.
*/
//当前线程自我中断
static void selfInterrupt() {
Thread.currentThread().interrupt();
}
/**
* Convenience method to park and then check if interrupted
*
* @return {@code true} if interrupted
*/
//当前线程挂起并check是否中断过
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
/*
* Various flavors of acquire, varying in exclusive/shared and
* control modes. Each is mostly the same, but annoyingly
* different. Only a little bit of factoring is possible due to
* interactions of exception mechanics (including ensuring that we
* cancel if tryAcquire throws exception) and other control, at
* least not without hurting performance too much.
*/
/**
* Acquires in exclusive uninterruptible mode for thread already in
* queue. Used by condition wait methods as well as acquire.
* <p>
* 为已经在的线程获取独占的不可中断模式
* *队列。用于条件等待方法和获取方法。
*
* @param node the node
* @param arg the acquire argument
* @return {@code true} if interrupted while waiting
*/
//使线程在等待队列中获取资源，一直获取到资源后才返回。
// 如果在整个等待过程中被中断过，则返回true，否则返回false。
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
//中断标识
boolean interrupted = false;
//自旋
for (; ; ) {
//获取node前驱节点
final Node p = node.predecessor();
//头节点 and node获取锁成功
if (p == head && tryAcquire(arg)) {
//设置node节点为头节点
setHead(node);
//p.next=null
p.next = null; // help GC
failed = false;
//不可中断
return interrupted;
}
//检查更新未能获取资源的节点的状态 park node 挂起node节点
if (shouldParkAfterFailedAcquire(p, node) &&
//线程挂起 检查是否被中断
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
//失败清除
if (failed)
//清除node节点
cancelAcquire(node);
}
}
/**
* Acquires in exclusive interruptible mode.
* 获取独占锁/排它锁的中断模式
*
* @param arg the acquire argument
*/
private void doAcquireInterruptibly(int arg)
throws InterruptedException {
//Thread.currentThread独占锁node
final Node node = addWaiter(Node.EXCLUSIVE);
//失败标识
boolean failed = true;
try {
//自旋
for (; ; ) {
//前驱节点
final Node p = node.predecessor();
//前驱节点为head头节点，并且当前节点能获取到资源
if (p == head && tryAcquire(arg)) {
//设置当前节点为头节点
setHead(node);
p.next = null; // help GC
failed = false;
return;
}
//是否可中断 interrupt and 挂起 park
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
//clean up node 清除当前节点
cancelAcquire(node);
}
}
/**
* Acquires in exclusive timed mode.
* <p>
* 在一定时间内获取独占锁
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*/
private boolean doAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
//时间<=0 false
if (nanosTimeout <= 0L)
return false;
//系统计数当前时间+获取时间 用于后面判断是否超时
final long deadline = System.nanoTime() + nanosTimeout;
//独占锁入列
final Node node = addWaiter(Node.EXCLUSIVE);
boolean failed = true;
try {
//自旋
for (; ; ) {
//前驱节点
final Node p = node.predecessor();
//前节点为头节点 and 当前线程的节点可获取锁成功
if (p == head && tryAcquire(arg)) {
//设置当前节点为头节点等待被唤醒
setHead(node);
p.next = null; // help GC
failed = false;
return true;
}
//用于判断是否超时
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;
//是否可以中断 and park挂起
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
//中断抛异常标识处理
throw new InterruptedException();
}
} finally {
//失败
if (failed)
//获取失败，清除当前节点
cancelAcquire(node);
}
}
/**
* Acquires in shared uninterruptible mode.
* 获得共享锁不可中断模式
*
* @param arg the acquire argument
*/
private void doAcquireShared(int arg) {
//共享addWaiter入列排队
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false;
for (; ; ) {
//寻找前节点
final Node p = node.predecessor();
//前驱节点为头节点
if (p == head) {
//尝试获取共享锁 返回结果可重入锁次数
int r = tryAcquireShared(arg);
if (r >= 0) {
//当前接待你设置头节点，并传播
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
//是否可以中断 and park 阻塞
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
//清除当前节点
cancelAcquire(node);
}
}
/**
* Acquires in shared interruptible mode.
* 获取共享锁的可中断模式
*
* @param arg the acquire argument
*/
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
//当前节点共享模式入列
final Node node = addWaiter(Node.SHARED);
//清除节点标识
boolean failed = true;
try {
//自旋
for (; ; ) {
//获取前驱节点
final Node p = node.predecessor();
//前驱节点==head头节点
if (p == head) {
//当前节点尝试获取共享锁
int r = tryAcquireShared(arg);
//获取重入次数
if (r >= 0) {
//设置当前节点为头节点，并传播
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
//当前线程是否阻塞 and中断
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
//清除当前节点
cancelAcquire(node);
}
}
/**
* Acquires in shared timed mode.
* 一定时间内尝试获取共享锁
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*/
private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
//获取时间<=0 不能获取
if (nanosTimeout <= 0L)
return false;
//用于计算超时
final long deadline = System.nanoTime() + nanosTimeout;
//当前线程的节点入列
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
//自旋
for (; ; ) {
//前驱节点
final Node p = node.predecessor();
//判断是否为头节点
if (p == head) {
//共享锁的可重入次数
int r = tryAcquireShared(arg);
if (r >= 0) {
//设置头节点并传播
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return true;
}
}
//计算是否超时
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;
//是否阻塞 park and interrupt中断
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
if (failed)
//清除node节点
cancelAcquire(node);
}
}
// Main exported methods
/**
* Attempts to acquire in exclusive mode. This method should query
* if the state of the object permits it to be acquired in the
* exclusive mode, and if so to acquire it.
*
* <p>This method is always invoked by the thread performing
* acquire. If this method reports failure, the acquire method
* may queue the thread, if it is not already queued, until it is
* signalled by a release from some other thread. This can be used
* to implement method {@link Lock#tryLock()}.
*
* <p>The default
* implementation throws {@link UnsupportedOperationException}.
*
* @param arg the acquire argument. This value is always the one
* passed to an acquire method, or is the value saved on entry
* to a condition wait. The value is otherwise uninterpreted
* and can represent anything you like.
* @return {@code true} if successful. Upon success, this object has
* been acquired.
* @throws IllegalMonitorStateException if acquiring would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*/
//自我实现尝试获取独占锁模式资源
protected boolean tryAcquire(int arg) {
/*
int c= getState();
if (Thread.currentThread()==getExclusiveOwnerThread()){
int i = c + arg;
if (i<0){
throw new UnsupportedOperationException("");
}
setState(i);
}else{
acquire(arg);
}*/
throw new UnsupportedOperationException();
}
/**
* Attempts to set the state to reflect a release in exclusive
* mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this object is now in a fully released
* state, so that any waiting threads may attempt to acquire;
* and {@code false} otherwise.
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*/
//尝试释放锁资源
protected boolean tryRelease(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to acquire in shared mode. This method should query if
* the state of the object permits it to be acquired in the shared
* mode, and if so to acquire it.
*
* <p>This method is always invoked by the thread performing
* acquire. If this method reports failure, the acquire method
* may queue the thread, if it is not already queued, until it is
* signalled by a release from some other thread.
*
* <p>The default implementation throws {@link
* UnsupportedOperationException}.
*
* @param arg the acquire argument. This value is always the one
* passed to an acquire method, or is the value saved on entry
* to a condition wait. The value is otherwise uninterpreted
* and can represent anything you like.
* @return a negative value on failure; zero if acquisition in shared
* mode succeeded but no subsequent shared-mode acquire can
* succeed; and a positive value if acquisition in shared
* mode succeeded and subsequent shared-mode acquires might
* also succeed, in which case a subsequent waiting thread
* must check availability. (Support for three different
* return values enables this method to be used in contexts
* where acquires only sometimes act exclusively.) Upon
* success, this object has been acquired.
* @throws IllegalMonitorStateException if acquiring would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if shared mode is not supported
*/
//尝试获取共享锁 通过继承基类，重写该方法
protected int tryAcquireShared(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to set the state to reflect a release in shared mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this release of shared mode may permit a
* waiting acquire (shared or exclusive) to succeed; and
* {@code false} otherwise
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if shared mode is not supported
*/
//释放共享锁 通过继承基类，重写该方法
protected boolean tryReleaseShared(int arg) {
throw new UnsupportedOperationException();
}
/**
* Returns {@code true} if synchronization is held exclusively with
* respect to the current (calling) thread. This method is invoked
* upon each call to a non-waiting {@link ConditionObject} method.
* (Waiting methods instead invoke {@link #release}.)
*
* <p>The default implementation throws {@link
* UnsupportedOperationException}. This method is invoked
* internally only within {@link ConditionObject} methods, so need
* not be defined if conditions are not used.
*
* @return {@code true} if synchronization is held exclusively;
* {@code false} otherwise
* @throws UnsupportedOperationException if conditions are not supported
*/
//是否为独占锁/排它锁
protected boolean isHeldExclusively() {
throw new UnsupportedOperationException();
}
/**
* Acquires in exclusive mode, ignoring interrupts. Implemented
* by invoking at least once {@link #tryAcquire},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquire} until success. This method can be used
* to implement method {@link Lock#lock}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
*/
//获取独占锁的入口
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
//使线程在等待队列中获取资源，一直获取到资源后才返回。
// 如果在整个等待过程中被中断过，则返回true，否则返回false。
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
/**
* Acquires in exclusive mode, aborting if interrupted.
* Implemented by first checking interrupt status, then invoking
* at least once {@link #tryAcquire}, returning on
* success. Otherwise the thread is queued, possibly repeatedly
* blocking and unblocking, invoking {@link #tryAcquire}
* until success or the thread is interrupted. This method can be
* used to implement method {@link Lock#lockInterruptibly}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @throws InterruptedException if the current thread is interrupted
*/
//获取可中断的独占锁入口
public final void acquireInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (!tryAcquire(arg))
doAcquireInterruptibly(arg);
}
/**
* Attempts to acquire in exclusive mode, aborting if interrupted,
* and failing if the given timeout elapses. Implemented by first
* checking interrupt status, then invoking at least once {@link
* #tryAcquire}, returning on success. Otherwise, the thread is
* queued, possibly repeatedly blocking and unblocking, invoking
* {@link #tryAcquire} until success or the thread is interrupted
* or the timeout elapses. This method can be used to implement
* method {@link Lock#tryLock(long, TimeUnit)}.
*
* @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
*/
//一定时间内尝试获取锁资源
public final boolean tryAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquire(arg) ||
doAcquireNanos(arg, nanosTimeout);
}
/**
* Releases in exclusive mode. Implemented by unblocking one or
* more threads if {@link #tryRelease} returns true.
* This method can be used to implement method {@link Lock#unlock}.
*
* @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}
*/
//释放锁资源
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
//释放锁资源
unparkSuccessor(h);
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.
*/
//尝试获取共享锁
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
*/
//尝试获取可中断的共享锁
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
*/
//一定时间内尝试获取共享锁资源
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}
*/
//释放共享锁 可重写 tryReleaseShared()
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
// Queue inspection methods
/**
* Queries whether any threads are waiting to acquire. Note that
* because cancellations due to interrupts and timeouts may occur
* at any time, a {@code true} return does not guarantee that any
* other thread will ever acquire.
*
* <p>In this implementation, this operation returns in
* constant time.
*
* @return {@code true} if there may be other threads waiting to acquire
*/
//队列中是否有线程
public final boolean hasQueuedThreads() {
return head != tail;
}
/**
* Queries whether any threads have ever contended to acquire this
* synchronizer; that is if an acquire method has ever blocked.
*
* <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
*/
//获取队列中等待的第一个线程
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;
//获取队列中的线程
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.
*/
//尾节点
Node t = tail;
Thread firstThread = null;
//从尾部依次向头部循环，直至找到firstThread
while (t != null && t != head) {
Thread tt = t.thread;
if (tt != null)
firstThread = tt;
t = t.prev;
}
return firstThread;
}
/**
* Returns true if the given thread is currently queued.
*
* <p>This implementation traverses the queue to determine
* presence of the given thread.
*
* @param thread the thread
* @return {@code true} if the given thread is on the queue
* @throws NullPointerException if the thread is null
*/
//当前线程是否在队列中
public final boolean isQueued(Thread thread) {
if (thread == null)
throw new NullPointerException();
//从尾部向头部依次遍历寻找当前线程是否在队列中
for (Node p = tail; p != null; p = p.prev)
if (p.thread == thread)
return true;
return false;
}
/**
* Returns {@code true} if the apparent first queued thread, if one
* exists, is waiting in exclusive mode. If this method returns
* {@code true}, and the current thread is attempting to acquire in
* shared mode (that is, this method is invoked from {@link
* #tryAcquireShared}) then it is guaranteed that the current thread
* is not the first queued thread. Used only as a heuristic in
* ReentrantReadWriteLock.
*/
//头节点是否为独占锁/排他锁
final boolean apparentlyFirstQueuedIsExclusive() {
Node h, s;
return (h = head) != null &&
(s = h.next) != null &&
!s.isShared() &&
s.thread != null;
}
/**
* Queries whether any threads have been waiting to acquire longer
* than the current thread.
*
* <p>An invocation of this method is equivalent to (but may be
* more efficient than):
* <pre> {@code
* getFirstQueuedThread() != Thread.currentThread() &&
* hasQueuedThreads()}</pre>
*
* <p>Note that because cancellations due to interrupts and
* timeouts may occur at any time, a {@code true} return does not
* guarantee that some other thread will acquire before the current
* thread. Likewise, it is possible for another thread to win a
* race to enqueue after this method has returned {@code false},
* due to the queue being empty.
*
* <p>This method is designed to be used by a fair synchronizer to
* avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
* Such a synchronizer's {@link #tryAcquire} method should return
* {@code false}, and its {@link #tryAcquireShared} method should
* return a negative value, if this method returns {@code true}
* (unless this is a reentrant acquire). For example, the {@code
* tryAcquire} method for a fair, reentrant, exclusive mode
* synchronizer might look like this:
*
* <pre> {@code
* protected boolean tryAcquire(int arg) {
* if (isHeldExclusively()) {
* // A reentrant acquire; increment hold count
* return true;
* } else if (hasQueuedPredecessors()) {
* return false;
* } else {
* // try to acquire normally
* }
* }}</pre>
*
* @return {@code true} if there is a queued thread preceding the
* current thread, and {@code false} if the current thread
* is at the head of the queue or the queue is empty
* @since 1.7
*/
//在队列中是否有前驱节点
public final boolean hasQueuedPredecessors() {
// The correctness of this depends on head being initialized
// before tail and on head.next being accurate if the current
// thread is first in queue.
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
/**
* Returns an estimate of the number of threads waiting to
* acquire. The value is only an estimate because the number of
* threads may change dynamically while this method traverses
* internal data structures. This method is designed for use in
* monitoring system state, not for synchronization
* control.
*
* @return the estimated number of threads waiting to acquire
*/
//获取队列中的长度
public final int getQueueLength() {
int n = 0;
for (Node p = tail; p != null; p = p.prev) {
if (p.thread != null)
++n;
}
return n;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire. 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. This method is
* designed to facilitate construction of subclasses that provide
* more extensive monitoring facilities.
*
* @return the collection of threads
*/
//获取队列中所有线程的集合
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;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire in exclusive mode. This has the same properties
* as {@link #getQueuedThreads} except that it only returns
* those threads waiting due to an exclusive acquire.
*
* @return the collection of threads
*/
//获取队列中独占锁/排他锁 模式下线程的集合
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;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire in shared mode. This has the same properties
* as {@link #getQueuedThreads} except that it only returns
* those threads waiting due to a shared acquire.
*
* @return the collection of threads
*/
//获取共享锁的线程集合
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;
}
/**
* Returns a string identifying this synchronizer, as well as its state.
* The state, in brackets, includes the String {@code "State ="}
* followed by the current value of {@link #getState}, and either
* {@code "nonempty"} or {@code "empty"} depending on whether the
* queue is empty.
*
* @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
/**
* Returns true if a node, always one that was initially placed on
* a condition queue, is now waiting to reacquire on sync queue.
*
* @param node the node
* @return true if is reacquiring
*/
//是否在syncQueue队列中
final boolean isOnSyncQueue(Node node) {
if (node.waitStatus == Node.CONDITION || node.prev == null)
return false;
if (node.next != null) // If has successor, it must be on queue
return true;
/*
* node.prev can be non-null, but not yet on queue because
* the CAS to place it on queue can fail. So we have to
* traverse from tail to make sure it actually made it. It
* will always be near the tail in calls to this method, and
* unless the CAS failed (which is unlikely), it will be
* there, so we hardly ever traverse much.
*/
//从尾部至头部依次查找
return findNodeFromTail(node);
}
/**
* Returns true if node is on sync queue by searching backwards from tail.
* Called only when needed by isOnSyncQueue.
*
* @return true if present
*/
//从尾部至头部依次查找
private boolean findNodeFromTail(Node node) {
Node t = tail;
for (; ; ) {
if (t == node)
return true;
if (t == null)
return false;
t = t.prev;
}
}
/**
* Transfers a node from a condition queue onto sync queue.
* Returns true if successful.
*
* @param node the node
* @return true if successfully transferred (else the node was
* cancelled before signal)
*/
//转换当前节点状态为signal
final boolean transferForSignal(Node node) {
/*
* If cannot change waitStatus, the node has been cancelled.
*/
if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
return false;
/*
* Splice onto queue and try to set waitStatus of predecessor to
* indicate that thread is (probably) waiting. If cancelled or
* attempt to set waitStatus fails, wake up to resync (in which
* case the waitStatus can be transiently and harmlessly wrong).
*/
//增加Node节点为尾节点
Node p = enq(node);
//p node的前驱节点
int ws = p.waitStatus;
//不能转换，唤醒当前线程
if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
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
*/
//转换当前节点为condition状态 标识 可入列condition单向链表队列
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.
*/
//不在SyncQueue中的node节点
while (!isOnSyncQueue(node))
//当前线程放弃资源，重新争抢资源
Thread.yield();
return false;
}
/**
* Invokes release with current state value; returns saved state.
* Cancels node and throws exception on failure.
*
* @param node the condition node for this wait
* @return previous sync state
*/
//释放
final int fullyRelease(Node node) {
boolean failed = true;
try {
//获取状态 0,1,2,...
int savedState = getState();
if (release(savedState)) {
failed = false;
return savedState;
} else {
throw new IllegalMonitorStateException();
}
} finally {
if (failed)
//如果释放失败，则设置状态为已完成
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
*/
//是否为owns
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
*/
//condition队列中是否有下一个等待者node节点
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
*/
//获取condition队列中的node大小
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
*/
//获取condition队列中的线程集合
public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitingThreads();
}
/**
* Condition implementation for a {@link
* AbstractQueuedSynchronizer} serving as the basis of a {@link
* Lock} implementation.
*
* <p>Method documentation for this class describes mechanics,
* not behavioral specifications from the point of view of Lock
* and Condition users. Exported versions of this class will in
* general need to be accompanied by documentation describing
* condition semantics that rely on those of the associated
* {@code AbstractQueuedSynchronizer}.
*
* <p>This class is Serializable, but all fields are transient,
* so deserialized conditions have no waiters.
*/
public class ConditionObject implements Condition, java.io.Serializable {
private static final long serialVersionUID = 1173984872572414699L;
/**
* First node of condition queue.
*/
//firstWaiter 相当于头
private transient Node firstWaiter;
/**
* Last node of condition queue.
*/
//lastWaiter 尾
private transient Node lastWaiter;
/**
* Creates a new {@code ConditionObject} instance.
*/
public ConditionObject() {
}
// Internal methods
/**
* Adds a new waiter to wait queue.
*
* @return its new wait node
*/
//进入condition单向链表队列 @return 新的等待节点
private Node addConditionWaiter() {
//尾节点
Node t = lastWaiter;
// If lastWaiter is cancelled, clean out.
if (t != null && t.waitStatus != Node.CONDITION) {
//清除waitStatus状态不为condition的node
unlinkCancelledWaiters();
t = lastWaiter;
}
//新建当前线程入列condition的node节点
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
//尾节点为null,则node为firstWaiter
firstWaiter = node;
else
//尾节点不为null，则指向node节点 node为lastWaiter
t.nextWaiter = node;
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
*/
//唤醒队列中的firstWaiter
private void doSignal(Node first) {
do {
if ((firstWaiter = first.nextWaiter) == null)
lastWaiter = null;
first.nextWaiter = null;
} while (!transferForSignal(first) &&
(first = firstWaiter) != null);
}
/**
* Removes and transfers all nodes.
*
* @param first (non-null) the first node on condition queue
*/
//唤醒队列中的所有节点
private void doSignalAll(Node first) {
lastWaiter = firstWaiter = null;
do {
//取出next
Node next = first.nextWaiter;
//nextWaiter赋值为null
first.nextWaiter = null;
//唤醒
transferForSignal(first);
//指向next 从头至尾依次遍历唤醒
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.
*/
//清除队列中Node节点waitStatus不为condition的节点
//从条件队列中取消已取消的服务员节点的链接。
private void unlinkCancelledWaiters() {
Node t = firstWaiter;
Node trail = null;
while (t != null) {
Node next = t.nextWaiter;
if (t.waitStatus != Node.CONDITION) {
t.nextWaiter = null;
//第一次trail==null
if (trail == null)
firstWaiter = next;
else
trail.nextWaiter = next;
if (next == null)
lastWaiter = trail;
} else
trail = t;
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
public final void signal() {
//是否为独占锁/排他锁
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
//唤醒队列中第一个node节点
Node first = firstWaiter;
if (first != null)
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}
*/
//唤醒队列中所有等待节点
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>
*/
//等待不中断
public final void awaitUninterruptibly() {
//创建当前线程node 节点入列condition单向链表
Node node = addConditionWaiter();
//释放 已加入队列，则可以释放当前节点
int savedState = fullyRelease(node);
boolean interrupted = false;
//是否在SyncQueue同步队列中
while (!isOnSyncQueue(node)) {
//阻塞 重量级锁 park
LockSupport.park(this);
if (Thread.interrupted())
interrupted = true;
}
//使线程在等待队列中获取资源，一直获取到资源后才返回。
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
*
* 退出等待时重新中断 标识 1
*/
private static final int REINTERRUPT = 1;
/**
* Mode meaning to throw InterruptedException on exit from wait
*
* 退出等待时抛异常中断标识
*/
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.
*/
//node节点等待等待期间是否被中断过
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.
*/
//重新中断该线程 or 抛异常中断 or do nothing
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>
*/
//线程等待 like object.wait()
public final void await() throws InterruptedException {
//线程是否中断
if (Thread.interrupted())
throw new InterruptedException();
//当前线程入列 condition
Node node = addConditionWaiter();
//释放node节点
int savedState = fullyRelease(node);
int interruptMode = 0;
//当前节点是否在SyncQueue FIFO 队列中
while (!isOnSyncQueue(node)) {
//不在 阻塞挂起
LockSupport.park(this);
//检查是否在等待中被中断
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
// acquireQueued(node, savedState) 使线程在等待队列中获取资源，一直获取到资源后才返回。
// 如果在整个等待过程中被中断过，则返回true，否则返回false。
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null) // clean up if cancelled
unlinkCancelledWaiters();
//重新中断
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>
*/
//一定时间内等待
public final long awaitNanos(long nanosTimeout)
throws InterruptedException {
//线程是否中断
if (Thread.interrupted())
throw new InterruptedException();
//创建当前线程的node节点 并入列
Node node = addConditionWaiter();
//释放node节点
int savedState = fullyRelease(node);
//用于判断是否超时
final long deadline = System.nanoTime() + nanosTimeout;
int interruptMode = 0;
//是否在SyncQueue同步队列中
while (!isOnSyncQueue(node)) {
//超时
if (nanosTimeout <= 0L) {
//设置waitStatus 为1 cancelled取消状态
transferAfterCancelledWait(node);
break;
}
if (nanosTimeout >= spinForTimeoutThreshold)
//阻塞挂起
LockSupport.parkNanos(this, nanosTimeout);
//线程等待期间是否被中断
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
//是否超时标识
nanosTimeout = deadline - System.nanoTime();
}
//队列中尝试获取资源 直至获取资源返回结果 false 获取成功，true表示在获取资源时被中断
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
//重新获取中断标识
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
//清除waitStatus!=condition -2的节点链接
unlinkCancelledWaiters();
if (interruptMode != 0)
//中断
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}
*/
//队列中是否有等待node
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}
*/
//获取condition队列中等待node的个数
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}
*/
//获取condition队列中的线程集合
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;
}
}
/**
* Setup to support compareAndSet. We need to natively implement
* this here: For the sake of permitting future enhancements, we
* cannot explicitly subclass AtomicInteger, which would be
* efficient and useful otherwise. So, as the lesser of evils, we
* natively implement using hotspot intrinsics API. And while we
* are at it, we do the same for other CASable fields (which could
* otherwise be done with atomic field updaters).
*/
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);
}
}
/**
* CAS head field. Used only by enq.
*/
private final boolean compareAndSetHead(Node update) {
return unsafe.compareAndSwapObject(this, headOffset, null, update);
}
/**
* CAS tail field. Used only by enq.
*/
private final boolean compareAndSetTail(Node expect, Node update) {
return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
}
/**
* CAS waitStatus field of a node.
*/
private static final boolean compareAndSetWaitStatus(Node node,
int expect,
int update) {
return unsafe.compareAndSwapInt(node, waitStatusOffset,
expect, update);
}
/**
* CAS next field of a node.
*/
private static final boolean compareAndSetNext(Node node,
Node expect,
Node update) {
return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
}
}