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Semaphore.java
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707 lines (671 loc) · 27.5 KB
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/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
/*
*
*
*
*
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
import java.util.Collection;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
/**
* A counting semaphore. Conceptually, a semaphore maintains a set of
* permits. Each {@link #acquire} blocks if necessary until a permit is
* available, and then takes it. Each {@link #release} adds a permit,
* potentially releasing a blocking acquirer.
* However, no actual permit objects are used; the {@code Semaphore} just
* keeps a count of the number available and acts accordingly.
*
* <p>Semaphores are often used to restrict the number of threads than can
* access some (physical or logical) resource. For example, here is
* a class that uses a semaphore to control access to a pool of items:
* <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>Before obtaining an item each thread must acquire a permit from
* the semaphore, guaranteeing that an item is available for use. When
* the thread has finished with the item it is returned back to the
* pool and a permit is returned to the semaphore, allowing another
* thread to acquire that item. Note that no synchronization lock is
* held when {@link #acquire} is called as that would prevent an item
* from being returned to the pool. The semaphore encapsulates the
* synchronization needed to restrict access to the pool, separately
* from any synchronization needed to maintain the consistency of the
* pool itself.
*
* <p>A semaphore initialized to one, and which is used such that it
* only has at most one permit available, can serve as a mutual
* exclusion lock. This is more commonly known as a <em>binary
* semaphore</em>, because it only has two states: one permit
* available, or zero permits available. When used in this way, the
* binary semaphore has the property (unlike many {@link java.util.concurrent.locks.Lock}
* implementations), that the "lock" can be released by a
* thread other than the owner (as semaphores have no notion of
* ownership). This can be useful in some specialized contexts, such
* as deadlock recovery.
*
* <p> The constructor for this class optionally accepts a
* <em>fairness</em> parameter. When set false, this class makes no
* guarantees about the order in which threads acquire permits. In
* particular, <em>barging</em> is permitted, that is, a thread
* invoking {@link #acquire} can be allocated a permit ahead of a
* thread that has been waiting - logically the new thread places itself at
* the head of the queue of waiting threads. When fairness is set true, the
* semaphore guarantees that threads invoking any of the {@link
* #acquire() acquire} methods are selected to obtain permits in the order in
* which their invocation of those methods was processed
* (first-in-first-out; FIFO). Note that FIFO ordering necessarily
* applies to specific internal points of execution within these
* methods. So, it is possible for one thread to invoke
* {@code acquire} before another, but reach the ordering point after
* the other, and similarly upon return from the method.
* Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not
* honor the fairness setting, but will take any permits that are
* available.
*
* <p>Generally, semaphores used to control resource access should be
* initialized as fair, to ensure that no thread is starved out from
* accessing a resource. When using semaphores for other kinds of
* synchronization control, the throughput advantages of non-fair
* ordering often outweigh fairness considerations.
*
* <p>This class also provides convenience methods to {@link
* #acquire(int) acquire} and {@link #release(int) release} multiple
* permits at a time. Beware of the increased risk of indefinite
* postponement when these methods are used without fairness set true.
*
* <p>Memory consistency effects: Actions in a thread prior to calling
* a "release" method such as {@code release()}
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
* actions following a successful "acquire" method such as {@code acquire()}
* in another thread.
*
* @since 1.5
* @author Doug Lea
*/
public class Semaphore implements java.io.Serializable {
private static final long serialVersionUID = -3222578661600680210L;
/** All mechanics via AbstractQueuedSynchronizer subclass */
private final Sync sync;
/**
* Synchronization implementation for semaphore. Uses AQS state
* to represent permits. Subclassed into fair and nonfair
* versions.
*/
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;
}
}
final int drainPermits() {
for (;;) {
int current = getState();
if (current == 0 || compareAndSetState(current, 0))
return current;
}
}
}
/**
* 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);
}
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;
}
}
}
/**
* 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 Semaphore(int permits) {
sync = new NonfairSync(permits);
}
/**
* Creates a {@code Semaphore} with the given number of
* permits and the given 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} 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);
}
/**
* Acquires a permit from this semaphore, blocking until one is
* available, or the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires a permit, if one is available and returns immediately,
* 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 two 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.
* </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.
*
* @throws InterruptedException if the current thread is interrupted
*/
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
/**
* Acquires a permit from this semaphore, blocking until one is
* available.
*
* <p>Acquires a permit, if one is available and returns immediately,
* 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
* some other thread invokes the {@link #release} method for this
* semaphore and the current thread is next to be assigned a permit.
*
* <p>If the current thread is {@linkplain Thread#interrupt interrupted}
* while waiting for a permit then it will continue to wait, but the
* time at which the thread is assigned a permit may change compared to
* the time it would have received the permit had no interruption
* occurred. When the thread does return from this method its interrupt
* status will be set.
*/
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
/**
* Acquires a permit from this semaphore, only if one is available at the
* time of invocation.
*
* <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 this method will return
* immediately with the value {@code false}.
*
* <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(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
* @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));
}
/**
* Releases a permit, returning it to the semaphore.
*
* <p>Releases a permit, increasing the number of available permits by
* one. If any threads are trying to acquire a permit, then one is
* selected and given the permit that was just released. That thread
* is (re)enabled for thread scheduling purposes.
*
* <p>There is no requirement that a thread that releases a permit must
* have acquired that permit by calling {@link #acquire}.
* Correct usage of a semaphore is established by programming convention
* in the application.
*/
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);
}
/**
* Returns the current number of permits available in this semaphore.
*
* <p>This method is typically used for debugging and testing purposes.
*
* @return the number of permits available in this semaphore
*/
public int availablePermits() {
return sync.getPermits();
}
/**
* Acquires and returns all permits that are immediately available.
*
* @return the number of permits acquired
*/
public int drainPermits() {
return sync.drainPermits();
}
/**
* Shrinks the number of available permits by the indicated
* reduction. This method can be useful in subclasses that use
* semaphores to track resources that become unavailable. This
* method differs from {@code acquire} in that it does not block
* waiting for permits to become available.
*
* @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);
}
/**
* Returns {@code true} if this semaphore has fairness set true.
*
* @return {@code true} if this semaphore has fairness set true
*/
public boolean isFair() {
return sync instanceof FairSync;
}
/**
* Queries whether any threads are waiting to acquire. Note that
* because cancellations may occur at any time, a {@code true}
* return does not guarantee that any other thread will ever
* acquire. This method is designed primarily for use in
* monitoring of the system state.
*
* @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() + "]";
}
}