目次
内部クラスFairSync、NonFairSync、2つのコンストラクター
方法4acquireXXX、4 tryAcquireXXX、リリース
前書き
package java.util.concurrent;
import java.util.Collection;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
/**
* 计数信号量。从概念上讲，一个信号量维护一组许可。
* 如果有必要，每个acquire阻塞直到一个许可可用，然后获取它。
* 每次release都会增加一个许可，潜在地释放一个阻塞的获取者。
* 但是，没有使用实际的permit对象;
* Semaphore只保存可用数量的计数，并相应地采取行动。
*
* <p>信号量通常用于限制不能访问某些(物理或逻辑)资源的线程数量。
* 例如，这里的一个类使用信号量来控制对项目池的访问:
*
* <pre> {@code
* class Pool {
* private static final int MAX_AVAILABLE = 100;
* private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
*
* public Object getItem() throws InterruptedException {
* available.acquire();
* return getNextAvailableItem();
* }
*
* public void putItem(Object x) {
* if (markAsUnused(x))
* available.release();
* }
*
* // Not a particularly efficient data structure; just for demo
*
* protected Object[] items = ... whatever kinds of items being managed
* protected boolean[] used = new boolean[MAX_AVAILABLE];
*
* protected synchronized Object getNextAvailableItem() {
* for (int i = 0; i < MAX_AVAILABLE; ++i) {
* if (!used[i]) {
* used[i] = true;
* return items[i];
* }
* }
* return null; // not reached
* }
*
* protected synchronized boolean markAsUnused(Object item) {
* for (int i = 0; i < MAX_AVAILABLE; ++i) {
* if (item == items[i]) {
* if (used[i]) {
* used[i] = false;
* return true;
* } else
* return false;
* }
* }
* return false;
* }
* }}</pre>
*
* <p>在获得一个项之前，每个线程必须从信号量获得一个许可，保证一个项可以使用。
* 当线程使用完该对象后，它被返回到池中，并向信号量返回一个许可，允许另一个线程获取该对象。
* 注意，当调用acquire时，不存在同步锁，因为这会阻止一个项目被返回到池中。
* 信号量封装了限制访问池所需的同步，与维护池本身的一致性所需的任何同步分开。
*
* <p>初始化为1的信号量，并且它最多只有一个允许可用，可以作为互斥锁。
* 这通常被称为binary semaphore，因为它只有两种状态:一个允许可用，或零允许可用。
* 当以这种方式使用时，二进制信号量具有这样一个属性(不同于许多java.util.concurrent.locks.Lock实现)，
* 即“锁”可以由所有者以外的线程释放(因为信号量没有所有权的概念)。
* 这在某些特定的上下文中很有用，比如死锁恢复。
*
* <p> 这个类的构造函数可选地接受一个公平性参数。
* 当设置为false时，该类不保证线程获得许可的顺序。
* 特别地，barging是允许的，也就是说，线程调用的获取可以在等待的线程之前分配一个许可——逻辑上，新线程将自己放在等待线程队列的头。
*
* 当公平感设置为true时，这些信号量保证调用任何acquire方法的线程被选择以它们调用这些方法的处理顺序获得许可(first-in-first-out;先进先出)。
* 注意，FIFO排序必须适用于这些方法中的特定内部执行点。
* 因此，一个线程可能在另一个线程之前调用acquire，但在另一个线程之后到达排序点，从方法返回时也是如此。
* 还要注意的是，不计时的tryAcquire方法不遵守公平性设置，但会接受任何可用的许可。
*
* <p>通常，用于控制资源访问的信号量应该被初始化为公平的，以确保没有线程因为访问资源而饿死。
* 当使用信号量进行其他类型的同步控制时，非公平排序的吞吐量优势常常超过公平性考虑。
*
* <p>这个类还提供了一次获取和释放多个许可证的方便方法。
* 当这些方法使用时没有设置公平为真，要注意增加的不确定延迟的风险。
*
* <p>内存一致性效应:在一个线程中，在调用“release”方法(如release())之前的动作发生在另一个线程中，
* 在成功调用“acquire”方法(如acquire())之后的动作发生在另一个线程中。
*
* Semaphore在多线程协作中常常用来控制公共资源的访问，限制同时访问数量。
* 打个比方，Semaphore就像是一个装有令牌（permit）的黑箱子，拿到令牌的人才能去做爱做的事情，
* 谁都可以从里面拿走若干令牌，谁都可以把新的令牌扔到里面去，但Semaphore从来不记载谁拿走的令牌。
*
* 获取信号量（减小state）
* Semaphore方法 调用的AQS方法 是否阻塞 是否响应中断 是否超时机制 返回值及含义
*
* acquire() sync.acquireSharedInterruptibly(1) ✓ ✓ - void
* acquire(int permits) sync.acquireSharedInterruptibly(permits) ✓ ✓ - void
* acquireUninterruptibly() sync.acquireShared(1) ✓ - - void
* acquireUninterruptibly(int permits) sync.acquireShared(permits) ✓ - - void
*
* tryAcquire(long timeout, TimeUnit unit) sync.tryAcquireSharedNanos(1, unit.toNanos(timeout) ) ✓ ✓ ✓ boolean 返回时是否获得了锁
* tryAcquire(int permits, long timeout, TimeUnit unit) sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout) ) ✓ ✓ ✓ boolean 返回时是否获得了锁
* tryAcquire() sync.nonfairTryAcquireShared(1) >= 0 - boolean 返回时是否获得了锁
*
* 上面表格中，所有没有参数的Semaphore方法，实质上都是在获得单位1的信号量。
* 调用的AQS方法，其实只有三种，acquireShared（共享锁的获取与释放中已经进行了讲解），
* acquireSharedInterruptibly和tryAcquireSharedNanos（CountDownLatch源码解析中已经进行了讲解）。这三者的区别已经在表格列出。
* 需要响应中断，方法声明会抛出中断异常。
* 有超时机制，就需要用返回值区别 获得锁返回 和 超时都没获得到锁 两种情况。
* 这三个方法都需要调用到AQS子类实现的tryAcquireShared，该方法用来获取共享锁，子类可以将其实现公平锁或是非公平锁。
*
* nonfairTryAcquireShared其实不大应该放在上表里面，因为它根本不是AQS的方法，只是AQS子类的新加方法。
* 因为它根本没有阻塞等待的过程，只是简单的try一次，成功失败听天由命，所以它根本不会阻塞。
* 表格中，带permits参数的方法可以获得单位不为1的信号量，但是方法中对permits参数的检查要求是>=0，
* 也就是说，允许线程获得0单位的信号量，虽然我感觉这样没有任何意义。
*
* 这里我们再复习一下tryAcquireShared返回值的含义：
*
* 如果返回值大于0，说明获取共享锁成功，并且后续获取也可能获取成功。
* 如果返回值等于0，说明获取共享锁成功，但后续获取可能不会成功。
* 如果返回值小于0，说明获取共享锁失败。
*
* Semaphore的AQS子类是一个很标准的共享锁实现。
* 获得信号量 == 减小AQS的state。
* 释放信号量 == 增加AQS的state。
* 共享锁的AQS子类实现方法需要自旋，这一点在Semaphore和CountDownLatch都有体现。独占锁的AQS子类实现方法不需要自旋。
* 获得信号量失败一定是因为信号量已经减到0了，且获得失败就会阻塞。
*
* @since 1.5
* @author Doug Lea
*/
public class Semaphore implements java.io.Serializable 
フィールド同期、内部クラス同期
private static final long serialVersionUID = -3222578661600680210L;
/** 所有机制都是通过AbstractQueuedSynchronizer子类实现的 */
private final Sync sync;
/**
* 信号量的同步实现。使用aq状态来表示许可。子类分为公平版和非公平版。
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 1192457210091910933L;
// 首先看到Sync的构造器，看来参数permits是代表共享锁的数量。
Sync(int permits) {
setState(permits);
}
final int getPermits() {
return getState();
}
/**
* 方法使用了自旋，这是合理且必要的。共享锁是共享的，自然可能有多个线程正在同时执行上面的代码，
* 即使失败了也不能退出循环，而是应该失败后再得到当前值，然后再次CAS尝试。
*
* 如果remaining算出来小于0，说明剩余信号量已经不够拿的了，那就直接返回remaining这个负数（表达获取共享锁失败），不做CAS操作。
* 如果remaining算出来大于等于0，说明剩余信号量够拿的，紧接着如果CAS设置成功，就返回remaining这个大于等于0的数（表达获取共享锁成功）。
* 这个方法想要退出，只有当前线程拿到了想要数量的信号量，或剩余信号量已经不够拿。
*
* @param acquires
* @return
*/
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
/**
*释放信号量就不用区别什么公平不公平了。
*考虑多线程，使用自旋保证releases单位的信号量能够释放到位。
*只有CAS设置成功，或溢出int型的范围，才能退出这个循环。
*/
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;
}
}
/**
* 这个方法的作用也是获得信号量，只不过这个函数相比nonfairTryAcquireShared的实现，它允许改变后的信号量是负数。
* 自旋的过程。想要退出函数，只有CAS操作成功或者向下溢出了。
* if (next > current)分支进入的原因只能是int型变量向下溢出，因为reductions被保证是一个>=0的数。
* 是protected方法，用起来不是那么方便。
*
* @param reductions
*/
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;
}
}
/**
* 自旋的过程，直到信号量被清空为0。
* @return
*/
final int drainPermits() {
for (;;) {
int current = getState();
if (current == 0 || compareAndSetState(current, 0))
return current;
}
}
}内部クラスFairSync、NonFairSync、2つのコンストラクター
/**
* NonFair version
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = -2694183684443567898L;
NonfairSync(int permits) {
super(permits);
}
protected 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);
}
/**
* 观察tryAcquireShared的公平和非公平锁的逻辑，
* 发现区别只是 公平锁里面每次循环都会判断hasQueuedPredecessors()的返回值。
*
* 如果有那同步队列中的节点属于是排在当前线程之前的，所以只好直接返回-1。
*/
protected int tryAcquireShared(int acquires) {
for (;;) {
if (hasQueuedPredecessors())
return -1;
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
}
/**
* 创建一个具有给定许可证数量和不公平公平设置的信号量。
*
* 默认构造器使用非公平锁，可以通过参数fair来得到公平锁。参数permits最终会赋值给AQS的state成员。
*
* @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 Semaphore(int permits) {
sync = new NonfairSync(permits);
}
/**
* 创建一个具有给定许可数量和给定公平设置的信号量。
*
* @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} if this semaphore will guarantee
* first-in first-out granting of permits under contention,
* else {@code false}
*/
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}方法4acquireXXX、4 tryAcquireXXX、リリース
/**
* 从这个信号量获得一个许可，阻塞直到有一个可用，或者线程被中断。
*
* <p>获得一个许可证，如果有，立即返回，减少一个许可证的数量。
*
* <p>如果没有可用的许可，那么当前线程就会因为线程调度的目的而被禁用，并且处于休眠状态，直到以下两件事情发生:
* <ul>
* <li>其他线程调用这个信号量的release方法，当前线程将被分配一个许可证;或
* <li>其他线程中断当前线程。
* </ul>
*
* <p>如果当前线程:
* <ul>
* <li>在进入该方法时设置中断状态;或
* <li>在等待许可时被中断，
* </ul>
* 然后抛出InterruptedException，并清除当前线程的中断状态。
*
* @throws InterruptedException if the current thread is interrupted
*/
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
/**
* 从这个信号量获得一个许可，阻塞直到一个可用。
*
* <p>获得一个许可证，如果有，立即返回，减少一个许可证的数量。
*
* <p>如果没有可用的许可，那么当前线程就会因为线程调度的目的而被禁用，并且处于休眠状态，
* 直到其他线程调用这个信号量的release方法，并且当前线程下一个被分配一个许可。
*
* <p>如果当前线程在等待许可时被中断，那么它将继续等待，
* 但是与没有发生中断时收到许可的时间相比，给线程分配许可的时间可能会发生变化。
* 当线程确实从这个方法返回时，它的interrupt status将被设置。
*/
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
/**
* 只有在调用时该信号量是可用的，才会获得该信号量的许可。
*
* <p>获得一个许可(如果一个许可是可用的)，并立即返回，值为true，将可用许可的数量减少一。
*
* <p>如果没有可用的许可，则此方法将立即返回值为false。
*
* <p>即使这个信号量被设置为使用公平排序策略，
* 调用tryAcquire()将立即获得一个许可，如果一个许可是可用的，无论其他线程是否正在等待。
* 这种“冲撞”行为在某些情况下是有用的，即使它破坏了公平。
* 如果你想遵守公平性设置，那么使用tryAcquire(0, TimeUnit.SECONDS)，这几乎是等价的(它也检测中断)。
*
* 上面表格里，一直没有好好讲tryAcquire，因为它在里面属于一个异类，没有阻塞等待的过程。
*
* 而nonfairTryAcquireShared的实现也看过了，里面根本没有调用过LockSupport.park，确实没有阻塞。
* 不要被自旋所迷惑，自旋也不是阻塞，而且这个自旋过程一般情形下很快就会结束。
*
* 简而言之，Semaphore#tryAcquire的作用就是尝试 一次性的、非公平的 获得锁动作。
* 注意这种一次性动作一定要是非公平实现的，不然大部分情况下（同步队列中只要有一个线程在等待），这种一次性动作肯定不能成功。
* 这也是为什么要把非公平实现放到NonfairSync和FairSync的父类里的一个公共方法里。
*
* 注意，返回值进行了处理，如果获得共享锁成功（nonfairTryAcquireShared返回值>=0），返回true；
* 如果获得共享锁失败（nonfairTryAcquireShared返回值<0），返回false。
*
* @return {@code true} if a permit was acquired and {@code false}
* otherwise
*/
public boolean tryAcquire() {
return sync.nonfairTryAcquireShared(1) >= 0;
}
/**
* Acquires a permit from this semaphore, if one becomes available
* within the given waiting time and the current thread has not
* been {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires a permit, if one is available and returns immediately,
* with the value {@code true},
* reducing the number of available permits by one.
*
* <p>If no permit is available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of three things happens:
* <ul>
* <li>Some other thread invokes the {@link #release} method for this
* semaphore and the current thread is next to be assigned a permit; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If a permit is acquired then the value {@code true} is returned.
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* to acquire a permit,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all.
*
* @param timeout the maximum time to wait for a permit
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if a permit was acquired and {@code false}
* if the waiting time elapsed before a permit was acquired
* @throws InterruptedException if the current thread is interrupted
*/
public boolean tryAcquire(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
* 释放一个许可，并将其返回给信号量。
*
* <p>释放一个许可证，增加一个可用许可证的数量。
* 如果有任何线程试图获得一个许可，那么将选择一个线程并给予刚刚释放的许可。
* 为线程调度目的启用了该线程(re)。
*
* <p>没有要求释放许可的线程必须通过调用acquire获得该许可。
* 正确使用信号量是通过应用程序的编程约定来建立的。
*/
public void release() {
sync.releaseShared(1);
}
/**
* Acquires the given number of permits from this semaphore,
* blocking until all are available,
* or the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the given number of permits, if they are available,
* and returns immediately, reducing the number of available permits
* by the given amount.
*
* <p>If insufficient permits are available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of two things happens:
* <ul>
* <li>Some other thread invokes one of the {@link #release() release}
* methods for this semaphore, the current thread is next to be assigned
* permits and the number of available permits satisfies this request; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* for a permit,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
* Any permits that were to be assigned to this thread are instead
* assigned to other threads trying to acquire permits, as if
* permits had been made available by a call to {@link #release()}.
*
* @param permits the number of permits to acquire
* @throws InterruptedException if the current thread is interrupted
* @throws IllegalArgumentException if {@code permits} is negative
*/
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireSharedInterruptibly(permits);
}
/**
* Acquires the given number of permits from this semaphore,
* blocking until all are available.
*
* <p>Acquires the given number of permits, if they are available,
* and returns immediately, reducing the number of available permits
* by the given amount.
*
* <p>If insufficient permits are available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* some other thread invokes one of the {@link #release() release}
* methods for this semaphore, the current thread is next to be assigned
* permits and the number of available permits satisfies this request.
*
* <p>If the current thread is {@linkplain Thread#interrupt interrupted}
* while waiting for permits then it will continue to wait and its
* position in the queue is not affected. When the thread does return
* from this method its interrupt status will be set.
*
* @param permits the number of permits to acquire
* @throws IllegalArgumentException if {@code permits} is negative
*/
public void acquireUninterruptibly(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireShared(permits);
}
/**
* Acquires the given number of permits from this semaphore, only
* if all are available at the time of invocation.
*
* <p>Acquires the given number of permits, if they are available, and
* returns immediately, with the value {@code true},
* reducing the number of available permits by the given amount.
*
* <p>If insufficient permits are available then this method will return
* immediately with the value {@code false} and the number of available
* permits is unchanged.
*
* <p>Even when this semaphore has been set to use a fair ordering
* policy, a call to {@code tryAcquire} <em>will</em>
* immediately acquire a permit if one is available, whether or
* not other threads are currently waiting. This
* "barging" behavior can be useful in certain
* circumstances, even though it breaks fairness. If you want to
* honor the fairness setting, then use {@link #tryAcquire(int,
* long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
* @param permits the number of permits to acquire
* @return {@code true} if the permits were acquired and
* {@code false} otherwise
* @throws IllegalArgumentException if {@code permits} is negative
*/
public boolean tryAcquire(int permits) {
if (permits < 0) throw new IllegalArgumentException();
return sync.nonfairTryAcquireShared(permits) >= 0;
}
/**
* Acquires the given number of permits from this semaphore, if all
* become available within the given waiting time and the current
* thread has not been {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the given number of permits, if they are available and
* returns immediately, with the value {@code true},
* reducing the number of available permits by the given amount.
*
* <p>If insufficient permits are available then
* the current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happens:
* <ul>
* <li>Some other thread invokes one of the {@link #release() release}
* methods for this semaphore, the current thread is next to be assigned
* permits and the number of available permits satisfies this request; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If the permits are acquired then the value {@code true} is returned.
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* to acquire the permits,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
* Any permits that were to be assigned to this thread, are instead
* assigned to other threads trying to acquire permits, as if
* the permits had been made available by a call to {@link #release()}.
*
* <p>If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all. Any permits that were to be assigned to this
* thread, are instead assigned to other threads trying to acquire
* permits, as if the permits had been made available by a call to
* {@link #release()}.
*
* @param permits the number of permits to acquire
* @param timeout the maximum time to wait for the permits
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if all permits were acquired and {@code false}
* if the waiting time elapsed before all permits were acquired
* @throws InterruptedException if the current thread is interrupted
* @throws IllegalArgumentException if {@code permits} is negative
*/
public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
}
/**
* Releases the given number of permits, returning them to the semaphore.
*
* <p>Releases the given number of permits, increasing the number of
* available permits by that amount.
* If any threads are trying to acquire permits, then one
* is selected and given the permits that were just released.
* If the number of available permits satisfies that thread's request
* then that thread is (re)enabled for thread scheduling purposes;
* otherwise the thread will wait until sufficient permits are available.
* If there are still permits available
* after this thread's request has been satisfied, then those permits
* are assigned in turn to other threads trying to acquire permits.
*
* <p>There is no requirement that a thread that releases a permit must
* have acquired that permit by calling {@link Semaphore#acquire acquire}.
* Correct usage of a semaphore is established by programming convention
* in the application.
*
* @param permits the number of permits to release
* @throws IllegalArgumentException if {@code permits} is negative
*/
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.releaseShared(permits);
}方法availablePermits、drainPermits、reducePermits、isFair、hasQueuedThreads、getQueueLength、getQueuedThreads、toString
/**
* 返回该信号量中可用的当前许可数量。
*
* <p>此方法通常用于调试和测试目的。
*
* @return the number of permits available in this semaphore
*/
public int availablePermits() {
return sync.getPermits();
}
/**
* 获取并返回所有可立即获得的许可。
*
* @return the number of permits acquired
*/
public int drainPermits() {
return sync.drainPermits();
}
/**
* 按指示减少可用许可证的数量。
* 在使用semaphores跟踪不可用资源的子类中，此方法非常有用。
* 这种方法不同于获取，因为它不会阻塞等待许可变得可用。
*
* @param reduction the number of permits to remove
* @throws IllegalArgumentException if {@code reduction} is negative
*/
protected void reducePermits(int reduction) {
if (reduction < 0) throw new IllegalArgumentException();
sync.reducePermits(reduction);
}
/**
* 如果该信号量的公平性设置为true则返回true。
*
* @return {@code true} if this semaphore has fairness set true
*/
public boolean isFair() {
return sync instanceof FairSync;
}
/**
* 查询是否有等待获取的线程。
* 注意，因为取消可能在任何时候发生，返回true并不保证任何其他线程会获得。
* 该方法主要用于监控系统状态。
*
* @return {@code true} if there may be other threads waiting to
* acquire the lock
*/
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
/**
* 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 of the
* system state, not for synchronization control.
*
* @return the estimated number of threads waiting for this lock
*/
public final int getQueueLength() {
return sync.getQueueLength();
}
/**
* 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
*/
protected Collection<Thread> getQueuedThreads() {
return sync.getQueuedThreads();
}
/**
* Returns a string identifying this semaphore, as well as its state.
* The state, in brackets, includes the String {@code "Permits ="}
* followed by the number of permits.
*
* @return a string identifying this semaphore, as well as its state
*/
public String toString() {
return super.toString() + "[Permits = " + sync.getPermits() + "]";
}