JAVA并发编程中AQS核心机制与多种同步器实现原理解析
大家好，今天我们来深入探讨Java并发编程中的一个核心组件：AbstractQueuedSynchronizer (AQS)，以及基于AQS构建的各种同步器。AQS是构建锁和其他同步工具的基础框架，理解AQS的原理对于编写高效、可靠的并发程序至关重要。
1. AQS：并发的基石
AQS，即抽象队列同步器，是一个用于构建锁和相关同步器的框架。它提供了一个FIFO队列来管理竞争同步状态的线程，并提供了一套模板方法供子类实现特定的同步语义。AQS自身并没有实现任何同步接口，而是定义了同步器应该如何工作，具体的同步语义由其子类来实现。
AQS的核心思想是：
- 同步状态（State）： AQS维护一个volatile int类型的state变量，用于表示同步状态。这个状态的具体含义由子类来定义，例如，在ReentrantLock中，state为0表示锁空闲，大于0表示锁被持有。
- FIFO队列（CLH队列）： 当线程竞争同步状态失败时，AQS会将这些线程加入到一个FIFO队列中，等待被唤醒。这个队列实际上是一个CLH（Craig, Landin, and Hagersten）变体的队列。
- 模板方法： AQS提供了一系列模板方法，子类需要实现这些方法来定义如何获取和释放同步状态。这些方法包括
tryAcquire、tryRelease、isHeldExclusively等。
2. AQS的核心数据结构
AQS主要涉及以下几个核心数据结构：
state(volatile int): 代表同步状态。其值取决于同步器的具体语义。head(volatile Node): 指向队列的头部节点，代表当前持有锁的线程或者即将获取锁的线程。tail(volatile Node): 指向队列的尾部节点，新加入的线程会被添加到队列尾部。-
Node: 代表等待队列中的一个线程。包含以下主要属性：waitStatus(volatile int): 表示节点的状态。可能的状态包括：CANCELLED(1): 表示线程获取锁的请求被取消。SIGNAL(-1): 表示当前节点的后继节点需要被唤醒。CONDITION(-2): 表示节点在条件队列中等待。PROPAGATE(-3): 表示释放锁的操作需要传播到后续节点。0: 表示节点处于默认状态，等待被唤醒。
thread(Thread): 代表等待获取锁的线程。prev(Node): 指向前驱节点。next(Node): 指向后继节点。nextWaiter(Node): 指向条件队列中的下一个节点（用于ConditionObject）。
3. AQS的工作流程
AQS的工作流程大致如下：
- 线程尝试获取同步状态： 线程调用同步器的acquire方法，该方法会调用AQS的
tryAcquire模板方法。 tryAcquire的实现： 子类需要实现tryAcquire方法来尝试获取同步状态。如果获取成功，tryAcquire返回true，线程继续执行。如果获取失败，tryAcquire返回false。- 加入等待队列： 如果
tryAcquire返回false，表示获取同步状态失败，线程会被封装成一个Node节点，并加入到AQS的等待队列（CLH队列）的尾部。 - 阻塞等待： 加入队列后，线程会进入阻塞状态，等待被唤醒。
- 唤醒： 当持有同步状态的线程释放同步状态时，会调用同步器的release方法，该方法会调用AQS的
tryRelease模板方法。 tryRelease的实现： 子类需要实现tryRelease方法来释放同步状态。如果释放成功，tryRelease返回true，AQS会唤醒等待队列中的一个或多个线程。- 唤醒后重新尝试获取： 被唤醒的线程会再次尝试获取同步状态，如果获取成功，则退出等待队列，继续执行。如果获取失败，则重新进入阻塞状态。
- 中断处理： 如果线程在等待过程中被中断，AQS会处理中断请求，并将线程从等待队列中移除。
4. AQS的核心方法
AQS提供了一系列核心方法，供子类使用：
getState(): 获取当前同步状态。setState(int newState): 设置当前同步状态。compareAndSetState(int expect, int update): 使用CAS操作原子性地更新同步状态。acquire(int arg): 以独占模式获取同步状态，忽略中断。acquireInterruptibly(int arg): 以独占模式获取同步状态，响应中断。tryAcquireNanos(int arg, long nanosTimeout): 以独占模式获取同步状态，响应中断，并支持超时。release(int arg): 以独占模式释放同步状态。acquireShared(int arg): 以共享模式获取同步状态，忽略中断。acquireSharedInterruptibly(int arg): 以共享模式获取同步状态，响应中断。tryAcquireSharedNanos(int arg, long nanosTimeout): 以共享模式获取同步状态，响应中断，并支持超时。releaseShared(int arg): 以共享模式释放同步状态。isHeldExclusively(): 判断当前线程是否独占式持有同步状态。
5. AQS的模板方法
子类需要实现以下模板方法来定义同步器的具体行为：
tryAcquire(int arg): 尝试以独占模式获取同步状态。成功返回true，失败返回false。tryRelease(int arg): 尝试以独占模式释放同步状态。成功返回true，失败返回false。tryAcquireShared(int arg): 尝试以共享模式获取同步状态。成功返回非负数，失败返回负数。tryReleaseShared(int arg): 尝试以共享模式释放同步状态。如果释放成功，并且允许后续共享模式的线程获取同步状态，则返回true，否则返回false。isHeldExclusively(): 判断当前线程是否独占式持有同步状态。
6. 基于AQS的同步器实现
Java并发包中提供了许多基于AQS实现的同步器，例如：
- ReentrantLock: 可重入锁，支持独占模式。
- ReentrantReadWriteLock: 可重入读写锁，支持共享模式（读锁）和独占模式（写锁）。
- Semaphore: 信号量，用于控制同时访问特定资源的线程数量。
- CountDownLatch: 倒计时门闩，允许一个或多个线程等待其他线程完成操作。
- CyclicBarrier: 循环栅栏，允许一组线程互相等待，直到所有线程都到达某个屏障点。
- FutureTask: 代表一个异步计算的结果。
7. ReentrantLock源码分析
ReentrantLock是一个可重入的独占锁，它基于AQS实现。下面我们分析ReentrantLock的关键源码：
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
public class ReentrantLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = -607776692572029626L;
/** Synchronizer providing all implementation mechanics */
private final Sync sync;
/**
* Base of synchronization control for this lock. Subclassed
* into fair and nonfair versions below. Uses AQS state to
* represent the number of holds on the lock.
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L;
/**
* Performs {@link Lock#lock}. The main reason for using
* separate methods is to allow limiting scope of final
* variables.
*/
abstract void lock();
/**
* Returns {@code true} if this sync is owned exclusively by
* the given thread. This is used to implement {@link
* Condition#isHeldExclusively}.
*
* @param thread the thread
* @return {@code true} if this sync is owned exclusively by
* the given thread
*/
protected final boolean isHeldExclusively() {
// While we must in general read state before owner,
// we don't need to do so to check if current thread is owner
return getExclusiveOwnerThread() == Thread.currentThread();
}
}
/**
* Synchronization control for fair locks
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = -300089878970904665L;
final void lock() {
acquire(1);
}
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
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;
}
}
/**
* Synchronization control for nonfair locks.
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 731615356378235015L;
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
/**
* Performs all needd internal locking for both fair and nonfair
* versions.
* @param acquires the number of reconcursions of acquire. Decreases
* until becomes 0.
* @return {@code true} if acquired.
*/
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
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;
}
/**
* Creates an instance of {@code ReentrantLock}.
* This is equivalent to using {@code ReentrantLock(false)}.
*/
public ReentrantLock() {
sync = new NonfairSync();
}
/**
* Creates an instance of {@code ReentrantLock} with the
* given fairness policy.
*
* @param fair {@code true} if this lock should use a fair ordering policy
*/
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
/**
* Acquires the lock.
*
* <p>Acquires the lock if it is not held by another thread and returns
* immediately, setting the lock hold count to one.
*
* <p>If the current thread already holds this lock, then the hold
* count is incremented and the method returns immediately.
*
* <p>If the lock is held by another thread, then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until the lock has been acquired, at which time the lock
* hold count is set to one.
*
* @throws InterruptedException if the current thread is
* interrupted while waiting to acquire the lock
*/
public void lock() {
sync.lock();
}
/**
* Attempts to acquire the lock.
*
* <p>Acquires the lock if it is not held by another thread and
* returns immediately with the value {@code true}, setting the lock hold
* count to one. Even when this lock has been set to use a
* fair ordering policy, a call to {@code tryLock()} <em>will</em>
* immediately acquire the lock if it is available, whether or not
* waiting threads are fair ordering policy. This
* "barging" behavior can be useful in certain
* circumstances, even though it breaks fairness. If you want to honor
* the fairness setting for this lock, then use
* {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
* <p>If the current thread already holds this lock then the hold
* count is incremented and the method returns {@code true}.
*
* <p>If the lock is held by another thread then this method will return
* immediately with the value {@code false}.
*
* @return {@code true} if the lock was acquired
* {@code false} otherwise
*/
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}
/**
* Releases the lock.
*
* <p>If the current thread is the holder of this lock, then the hold
* count is decremented. If the hold count is now zero, then the
* lock is released. If the current thread is not the holder of this
* lock, then {@link IllegalMonitorStateException} is thrown.
*
* <p>Release of the lock happens in one of two ways:
*
* <ul>
*
* <li> If the current thread is the holder of this lock and the hold
* count is decremented to zero, then the lock is released.
*
* <li> If the current thread is not the holder of this lock, then
* {@link IllegalMonitorStateException} is thrown.
*
* </ul>
*
* @throws IllegalMonitorStateException if the current thread does not
* hold this lock
*/
public void unlock() {
sync.release(1);
}
/**
* Returns a {@link Condition} instance for use with this {@link
* Lock} instance.
*
* <p>The returned {@link Condition} instance supports the same
* usages as do the {@link Object} monitor methods ({@link
* Object#wait() wait}, {@link Object#notify() notify} and {@link
* Object#notifyAll() notifyAll}) when used with the built-in
* monitor lock.
*
* <ul>
*
* <li>If this lock is not held when any of the {@link
* Condition} {@linkplain #await() waiting} methods are called,
* then an {@link IllegalMonitorStateException} is thrown.
*
* <li>When the {@link Condition} {@linkplain #signal() signaling}
* methods are called, then the current thread must be the holder
* of the lock.
*
* </ul>
*
* <p>The returned {@link Condition} instance supports the same
* usages as do the {@link Object} monitor methods ({@link
* Object#wait() wait}, {@link Object#notify() notify} and {@link
* Object#notifyAll() notifyAll}) when used with the built-in
* monitor lock.
*
* @return a {@link Condition} instance for use with this {@link
* Lock} instance
* @throws UnsupportedOperationException if this {@link Lock}
* implementation does not support conditions
*/
public Condition newCondition() {
return sync.newCondition();
}
/**
* Queries the number of holds on this lock by the current thread.
*
* <p>A thread has a hold on a lock for each lock action that is not
* matched by an unlock action.
*
* <p>The hold count information is only available for locks which are
* held by the current thread. When the lock is not held by the
* current thread, this method returns zero.
*
* <p>This operation is intended for use in monitoring of the lock and
* is not intended for use in synchronization control.
*
* @return the number of holds on this lock by the current thread,
* or zero if this lock is not held by the current thread
*/
public int getHoldCount() {
return sync.getHoldCount();
}
/**
* Queries if this lock is held by the current thread.
*
* <p>Analogous to a call to {@link #getHoldCount()}{@code > 0}.
*
* @return {@code true} if this lock is held by the current thread,
* and {@code false} otherwise
*/
public boolean isHeldByCurrentThread() {
return sync.isHeldExclusively();
}
/**
* Queries if this lock is held by any thread. This method is
* designed for use in monitoring of the system state, not for
* synchronization control.
*
* @return {@code true} if this lock is held by any thread,
* and {@code false} otherwise
*/
public boolean isLocked() {
return sync.isLocked();
}
/**
* Returns {@code true} if this lock implements fairness.
*
* @return {@code true} if this lock implements fairness
*/
public final boolean isFair() {
return sync instanceof FairSync;
}
/**
* Return the thread that currently owns this lock, or
* {@code null} if not owned. When this method is called by a
* thread that is not the owner, the return value reflects a
* "best-effort" snapshot of the current owner. The returned value
* is not guaranteed to be accurate, and may not reflect subsequent
* changes in ownership. This method is designed for use in
* monitoring of the system state, not for synchronization
* control.
*
* @return the owner, or {@code null} if not owned
*/
protected Thread getOwner() {
return sync.getExclusiveOwnerThread();
}
/**
* Queries whether any threads are waiting to acquire this lock.
* Note that because cancellations due to interrupts and timeouts
* may occur at any time, a {@code true} return does not guarantee
* that any thread will ever acquire this lock. This method is
* designed primarily for use in monitoring of the system state.
*
* @return {@code true} if there may be waiting threads
*/
public boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
/**
* Queries whether the given thread is waiting to acquire this
* lock. Note that because cancellations due to interrupts and
* timeouts may occur at any time, a {@code true} return does not
* guarantee that this thread will ever acquire this lock. This
* method is designed primarily for use in monitoring of the
* system state.
*
* @param thread the thread
* @return {@code true} if the given thread is queued waiting for this lock
* @throws NullPointerException if the thread is null
*/
public boolean hasQueuedThread(Thread thread) {
return sync.isQueued(thread);
}
/**
* Returns an estimate of the number of threads waiting to acquire
* this lock. The value is only an estimate because the actual
* number of threads may change dynamically while this method
* traverses internal data structures. This method is designed
* for use in monitoring of the system state, not for
* synchronization control.
*
* @return the estimated number of threads waiting for this lock
*/
public int getQueueLength() {
return sync.getQueueLength();
}
/**
* Returns a collection containing threads that may be waiting to
* acquire this lock. Because the actual set of threads may change
* dynamically while constructing this collection, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed primarily for use in monitoring of the system state.
*
* @return the collection of threads
*/
protected Collection<Thread> getQueuedThreads() {
return sync.getQueuedThreads();
}
/**
* Queries whether any threads are waiting on the given condition
* associated with this lock. Note that because timeouts and
* interrupts may occur at any time, a {@code true} return does not
* guarantee that a future {@link Condition#signal} will awaken
* any particular thread. 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 this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
public boolean hasWaiters(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns an estimate of the number of threads waiting on the
* given condition associated with this lock. 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 this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
public int getWaitQueueLength(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns a collection containing those threads that may be
* waiting on the given condition associated with this lock.
* Because the actual set of threads may change dynamically while
* constructing this collection, the returned collection is only a
* best-effort estimate. The elements of the returned collection
* are in no particular order. This method is designed primarily
* for use in monitoring of the system state.
*
* @param condition the condition
* @return the collection of threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
protected Collection<Thread> getWaitingThreads(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* ReentrantLock的内部类，用于实现公平锁和非公平锁
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L;
/**
* 尝试获取锁，由子类实现
*/
abstract void lock();
/**
* 判断当前线程是否独占式持有锁
*/
protected final boolean isHeldExclusively() {
return getExclusiveOwnerThread() == Thread.currentThread();
}
/**
* 创建一个与此锁关联的Condition对象
*/
final ConditionObject newCondition() {
return new ConditionObject(this);
}
/**
* 获取当前线程持有锁的次数
*/
final int getHoldCount() {
return isHeldExclusively() ? getState() : 0;
}
/**
* 判断锁是否被锁定
*/
final boolean isLocked() {
return getState() != 0;
}
/**
* 获取独占式持有锁的线程
*/
final Thread getExclusiveOwnerThread() {
return getExclusiveOwnerThread();
}
/**
* 判断是否有线程在等待获取锁
*/
final boolean hasQueuedThreads() {
return hasQueuedThreads();
}
/**
* 判断给定的线程是否在等待获取锁
*/
final boolean hasQueuedThread(Thread thread) {
return isQueued(thread);
}
/**
* 获取等待获取锁的线程数
*/
final int getQueueLength() {
return getQueueLength();
}
/**
* 获取等待获取锁的线程集合
*/
final Collection<Thread> getQueuedThreads() {
return getQueuedThreads();
}
/**
* 判断是否有线程在等待给定的Condition
*/
final boolean hasWaiters(ConditionObject condition) {
if (condition == null)
throw new NullPointerException();
return hasWaiters(condition);
}
/**
* 获取等待给定Condition的线程数
*/
final int getWaitQueueLength(ConditionObject condition) {
if (condition == null)
throw new NullPointerException();
return getWaitQueueLength(condition);
}
/**
* 获取等待给定Condition的线程集合
*/
final Collection<Thread> getWaitingThreads(ConditionObject condition) {
if (condition == null)
throw new NullPointerException();
return getWaitingThreads(condition);
}
}
/**
* 公平锁的实现
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = -300089878970904665L;
/**
* 获取锁
*/
final void lock() {
acquire(1);
}
/**
* 尝试获取锁，如果队列中有等待线程，则不获取
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
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;
}
}
/**
* 非公平锁的实现
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 731615356378235015L;
/**
* 获取锁，尝试直接获取锁，如果获取失败，则进入队列等待
*/
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
/**
* 尝试获取锁
*/
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
/**
* 非公平尝试获取锁
*/
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (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;
}
/**
* 释放锁
*/
public void unlock() {
sync.release(1);
}
/**
* 尝试释放锁
*/
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;
}
public boolean isFair() {
return sync instanceof FairSync;
}
}- Sync抽象类： ReentrantLock内部定义了一个Sync抽象类，继承自AQS。Sync类定义了lock()方法，并实现了isHeldExclusively()方法。
- FairSync和NonfairSync： Sync类有两个子类，分别是FairSync和NonfairSync，分别对应公平锁和非公平锁。
tryAcquire(int acquires): FairSync和NonfairSync都实现了tryAcquire方法。公平锁在tryAcquire方法中会先判断是否有其他线程在等待，如果有，则不获取锁，直接返回false，保证公平性。非公平锁则会直接尝试获取锁，如果获取成功，则返回true，否则返回false。release(int releases): ReentrantLock的unlock方法调用了sync.release(1)。Sync类重写了AQS的tryRelease方法。tryRelease方法会将state减1，如果state减为0，表示锁被释放，则将exclusiveOwnerThread设置为null，并返回true。
8. Semaphore源码分析
Semaphore 信号量是用来控制同时访问特定资源的线程数量，它也基于AQS实现。
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.Semaphore;
public class MySemaphore {
private final Sync sync;
/**
* Synchronization mechanism for semaphore. Uses AQS state
* to represent permits. Subclass manages fairness.
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 1192457210091910933L;
Sync(int permits) {
setState(permits);
}
final int getPermits() {
return getState();
}
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))
return true;
}
}
final void reducePermits(int reductions) {
for (;;) {
int current = getState();
int next = current - reductions;
if (next > current) // underflow
throw new Error("Permit count underflow");
if (compareAndSetState(current, next))
return;
}
}
}
/**
* NonFair version
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = -2694183684443567898L;
NonfairSync(int permits) {
super(permits);
}
protected final int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
}
/**
* Fair version
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = 2014338818796000944L;
FairSync(int permits) {
super(permits);
}
protected final int tryAcquireShared(int acquires) {
for (;;) {
if (hasQueuedPredecessors())
return -1;
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
}
/**
* Creates a {@code Semaphore} with the given number of
* permits and nonfair fairness setting.
*
* @param permits the initial number of permits available.
* This value may be negative, in which case releases
* must occur before any acquires will be granted.
*/
public MySemaphore(int permits) {
sync = new NonfairSync(permits);
}
/**
* Creates a {@code Semaphore} with the given number of
* permits and the indicated fairness setting.
*
* @param permits the initial number of permits available.
* This value may be negative, in which case releases
* must occur before any acquires will be granted.
* @param fair {@code true}