1.LockSupport工具
- LockSupport类是构建同步组件的基础工具
- 主要作用是阻塞线程和唤醒线程
park开头的方法：负责阻塞线程
unpark(Thread thread)方法：负责唤醒线程
2.AbstractQueuedSynchronizer深入分析
2.1 AQS用处以及包含的方法
AbstractQueuedSynchronizer类是用来实现显式锁等
CountDownLatch
Semaphore
ReentrantLock （独占锁）
ReentrantReadWriteLock （读锁-共享锁）
使用AQS实现Semaphore和ReentrantLock本质上是一样的，都是获取锁的时候，都对state减1，当减完后，后续的线程就要进入等待队列。
这个跟《现代操作系统》上讲的本质是一模一样的。AQS使用方式和其中的设计模式
通过基础来使用，因为AQS使用了Template Method设计模式。
Template Method定义一个操作中的算法骨架，而将一些步骤延迟到子类中。Template Method使得子类可以不改变一个算法的结构即可重定义该算法的某些特定步骤。
Template Method设计模式实例-
AQS中的方法
模板方法：
独占式获取
accquire
acquireInterruptibly
tryAcquireNanos
共享式获取
acquireShared
acquireSharedInterruptibly
tryAcquireSharedNanos
独占式释放锁
release
共享式释放锁
releaseShared
需要子类覆盖的流程方法：
独占式获取 tryAcquire
独占式释放 tryRelease
共享式获取 tryAcquireShared
共享式释放 tryReleaseShared
这个同步器是否处于独占模式 isHeldExclusively
同步状态state：
getState:获取当前的同步状态
setState：设置当前同步状态
compareAndSetState 使用CAS设置状态，保证状态设置的原子性
2.2 AQS中的数据结构
- Node结点的状态
竞争失败的线程会打包成Node放到同步队列，Node可能的状态里：
CANCELLED：结点线程等待超时或者被中断了，需要从队列中移走
SIGNAL：当前节点完成工作释放，通知后面的节点去运行
CONDITION：当前节点处于等待队列
PROPAGATE：共享，表示状态要往后面的节点传播
0： 表示初始状态
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).
*/
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;
-
结点入队和出队
这里入队需要使用CAS，是因为可能很多线程竞争插入；出队无需，是因为无竞争，直接将队首第一个出队即可。
入队
出队 -
独占式同步状态获取与释放
ReentrantLock.java
public void lock() {
sync.acquire(1);
}
AbstractQueuedSynchronizer.java
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
//以CAS的方式加入到队列的尾部
private Node addWaiter(Node mode) {
Node node = new Node(mode);
for (;;) {
Node oldTail = tail;
if (oldTail != null) {
node.setPrevRelaxed(oldTail);
if (compareAndSetTail(oldTail, node)) {
oldTail.next = node;
return node;
}
} else {
initializeSyncQueue();
}
}
}
final boolean acquireQueued(final Node node, int arg) {
boolean interrupted = false;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node))
interrupted |= parkAndCheckInterrupt();
}
} catch (Throwable t) {
cancelAcquire(node);
if (interrupted)
selfInterrupt();
throw t;
}
}
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
问题：为什么要在自己的前一个结点是head结点时，就去尝试拿锁？
这个跟队列的设计有关？队列的设计是，在队列中的头结点持有锁，并且在运行，但此时并未从队列中显式清除。
这个地方的尝试拿锁和其他线程释放锁并显式唤醒下一个线程有没有冲突和重合？问题：是怎么实现线程本身已经park，而acquireQueued还能一直循环检测？
这个地方本质上是要弄清楚阻塞机制本身是怎样实现，尤其是LockSupport.park(this);释放锁
public void unlock() {
sync.release(1);
}
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
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)
node.compareAndSetWaitStatus(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 p = tail; p != node && p != null; p = p.prev)
if (p.waitStatus <= 0)
s = p;
}
if (s != null)
LockSupport.unpark(s.thread);
}
2.3 共享与超时
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}
private void doAcquireShared(int arg) {
final Node node = addWaiter(Node.SHARED);
boolean interrupted = false;
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
return;
}
}
if (shouldParkAfterFailedAcquire(p, node))
interrupted |= parkAndCheckInterrupt();
}
} catch (Throwable t) {
cancelAcquire(node);
throw t;
} finally {
if (interrupted)
selfInterrupt();
}
}
- 超时机制的实现
public final boolean tryAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquire(arg) ||
doAcquireNanos(arg, nanosTimeout);
}
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);
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
return true;
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L) {
cancelAcquire(node);
return false;
}
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
throw new InterruptedException();
}
} catch (Throwable t) {
cancelAcquire(node);
throw t;
}
}
2.4 ReentrantLock非公平锁的可重入实现
@ReservedStackAccess
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) { //直接去抢
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
@ReservedStackAccess
protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
}
2.5 ReentrantLock公平锁的实现
@ReservedStackAccess
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() && //先判定一下是有线程在等待锁
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
2.6 ReentrantReadWriteLock读写锁的实现
- 位状态来标识读、写的状态，将state 32位切位两半。
abstract static class Sync extends AbstractQueuedSynchronizer {
static final int SHARED_SHIFT = 16;
static final int SHARED_UNIT = (1 << SHARED_SHIFT);
static final int MAX_COUNT = (1 << SHARED_SHIFT) - 1;
static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;
}
- 每个线程重入的次数使用线程局部变量ThreadLocalHoldCounter
3.Condition分析
-
一个Condition包含一个等待队列（单链表）
-
同步队列与等待队列
-
调用await方法
-
调用signal方法
await/signal对于每个Condition都有一个队列，而wait/nofifyAll只有一个队列
4.Lock的实现
4.1 不可重入锁的实现
根据2中提示，实现相应的方法即可实现自己的锁
不可重入锁的实现 SelfLock.java
测试代码 TestMyLock.javaLock实现的一个说明
tryAcquire使用CAS，而tryRelease直接设置，是为什么？
因为获取锁的时候，是有很多线程竞争，所以需要保证原子性。
而释放锁时，是持有锁的线程进行释放，这时是独占的，没有竞争，所以直接释放即可。
protected boolean tryAcquire(int arg) {
if(compareAndSetState(0,1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
protected boolean tryRelease(int arg) {
if(getState()==0) {
throw new UnsupportedOperationException();
}
setExclusiveOwnerThread(null);
setState(0);
return true;
}
4.2 三元共享锁的实现——信号量
参考
- 1）享学课堂Mark老师笔记
