AbstractQueuedSynchronizer(AQS)实现原理
AQS中包含两种锁
- 独占锁，每次只能有一个线程能持有锁，ReentrantLock就是以独占方式实现的互斥锁
- 共享锁，则允许多个线程同时获取锁，并发访问 共享资源(只能读),但是只能有一个线程进行写操作，如：ReadWriteLock
锁队列的实现都是基于CLH的一种变体,在其队列节点的前驱上自旋
队列节点定义
static final class Node {
/** Marker to indicate a node is waiting in shared mode */
static final Node SHARED = new Node();
/** Marker to indicate a node is waiting in exclusive mode */
static final Node EXCLUSIVE = null;
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
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
*
* 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.
*
* 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).
*/
//属于初始化0,用于条件阻塞
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.
*/
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.
*/
volatile Node next;
/**
* The thread that enqueued this node. Initialized on
* construction and nulled out after use.
*/
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.
*/
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode.
*/
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
*/
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
}
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;
}
}1.独占锁实现
acquire获取独占锁
/**
*
* 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.
*/
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
//子类实现，尝试获取锁，get lock -> true or false
protected boolean tryAcquire(int arg) {
throw new UnsupportedOperationException();
}
//创建个Node A 入队
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
Node pred = tail;
//前驱节点为null,注意return
if (pred != null) {
node.prev = pred;
//tail指向创建的Node
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
//入队
enq(node);
return node;
}
//入队
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
---------------------------------------------------
acquireQueued(进行前驱节点自旋)
---------------------------------------------------
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}挂起未获得锁的线程，并检测挂起线程是否中断
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
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.
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
private final boolean parkAndCheckInterrupt() {
//挂起此线程
LockSupport.park(this);
//此代码必须等到线程被唤醒后才会执行
return Thread.interrupted();
}上述代码分析可得：
1. thread1 执行lock锁定,调用acquire方法获取锁,tryAcquire返回true,由acquire实现可以看出,!tryAcquire(arg)为false,线程获取锁成功,此时不释放锁
2. thread2 执行lock锁定,tryAcquire返回false,开始执行acquireQueued方法，此时开始初始化Node节点,node队列如下: Thread2插入的节点是B,A是队列初始化插入的头节点，且线程在其前驱节点A上自旋
| | | Node | | Node |
|thread1|---| A |--->| B |
| | | |<---| |
| | | head | | tail |
|thread2| | |可以看出thrad2绑定到nodeB上，但是其自旋在前驱节点A上,LockSupport.park挂起此线程
3.thread3 执行lock锁定，
Thread3插入的是节点C,自旋在其前驱节点B上。
| | | Node | | Node | | Node |
|thread1|---| A |--->| B |--->| C |
| | | |<---| |<-->| |
| | | head | | | | |
|thread2| |thread3| | |thread3绑定在节点C上，但是其自旋在前驱节点B上,LockSupport.park挂起此线程。
由此可以看出，前一个锁中断或者释放锁，只需要唤醒node下一个引用的节点所关联的线程就可以获取到锁
release释放独占锁
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
//唤醒下个被挂起的线程
private void unparkSuccessor(Node node) {
/*
* If status is negative (i.e., possibly needing signal) try
* to clear in anticipation of signalling. It is OK if this
* fails or if status is changed by waiting thread.
*/
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0);
/*
* Thread to unpark is held in successor, which is normally
* just the next node. But if cancelled or apparently null,
* traverse backwards from tail to find the actual
* non-cancelled successor.
*/
Node s = node.next;
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
//唤醒绑定的线程
LockSupport.unpark(s.thread);
}释放锁的过程:
| | | Node | | Node | | Node |
|thread1|---| A |--->| B |--->| C |
| | | |<---| |<-->| |
| | | head | | | | |
| | |thread3| | |thread1内执行release操作,唤醒thread2,thread2执行realse操作唤醒thread3,
实现独占锁
2.共享锁实现
acquireShared(获取共享锁)
public final void acquireShared(int arg) {
//小于0,线程进行挂起
if (tryAcquireShared(arg) < 0)
//添加到锁队列
doAcquireShared(arg);
}
//供子类实现
protected int tryAcquireShared(int arg) {
throw new UnsupportedOperationException();
}
//挂起线程
private void doAcquireShared(int arg) {
//添加到锁队列
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
//获取前驱结点
final Node p = node.predecessor();
if (p == head) {
//尝试获取许可
int r = tryAcquireShared(arg);
//获取许可成功
if (r >= 0) {
//设置队列头，传递许可到下个节点
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
//获取许可失败线程进行挂起并在唤醒后检测挂起线程是否中断
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}setHeadAndPropagate 共享锁传递许可
private void setHeadAndPropagate(Node node, int propagate) {
Node h = head; // Record old head for check below
setHead(node);
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
Node s = node.next;
//下个节点不为空并且是共享的
if (s == null || s.isShared())
//释放
doReleaseShared();
}
}
//释放锁
private void doReleaseShared() {
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus; //-1
if (ws == Node.SIGNAL) { //true
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
//唤醒挂起的线程
unparkSuccessor(h);
}
//线程还在处在准备阶段
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}releaseShared(释放共享锁)
public final boolean releaseShared(int arg) {
// >=1 = true 达到释放锁的阙值
if (tryReleaseShared(arg)) {
//见上
doReleaseShared();
return true;
}
return false;
}请求共享锁后CLSH队列跟独占锁的一样，不同的是绑定的锁类型标识不一样
如下:
| | | Node | | Node | | Node |
|thread1|---| A |--->| B |--->| C |
| | | |<---| |<-->| |
| | | head | | | | |
|thread2| |thread3| | |达到条件后进行释放操作：
执行releaseShared操作，并且达到阙值，执行doReleaseShared，此时会唤醒thread1,thread1执行setHeadAndPropagate函数唤醒下一个线程thread2,依次往下传递唤醒。
共享锁是在唤醒下一个线程后return,才开始执行锁定的代码
AQS实现的共享锁是公平锁
3.Condition实现
基于链表队列实现的wait/notify,对应方法await/signal 执行await时入队并park此线程,执行signal时唤醒一个线程，执行signal时唤醒此condition的所有阻塞线程
**一个lock可以对应多个conndition,每个conndition都有其自己的阻塞队列，
而signal和signalAll只会唤醒自己阻塞队列里绑定的线程。**
可以实现多线程间的单对单，单对多通信
signal唤醒的顺序是跟线程的入队顺序一致，FIFO