edu.emory.mathcs.backport.java.util.concurrent.Semaphore Maven / Gradle / Ivy
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/*
* 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/licenses/publicdomain
*/
package edu.emory.mathcs.backport.java.util.concurrent;
import java.util.Collection;
import edu.emory.mathcs.backport.java.util.concurrent.helpers.WaitQueue.*;
import edu.emory.mathcs.backport.java.util.concurrent.helpers.*;
/**
* 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.
*
* 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:
*
* 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;
* }
*
* }
*
*
* 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.
*
*
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 binary
* semaphore, 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 edu.emory.mathcs.backport.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.
*
*
The constructor for this class optionally accepts a
* fairness parameter. When set false, this class makes no
* guarantees about the order in which threads acquire permits. In
* particular, barging 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.
*
*
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.
*
*
This class also provides convenience methods to {@link
* #acquire(int) acquire} and {@link #release(int) release} multiple
* permits at a time. BACKPORT NOTE: currently, these methods are only
* supported for FAIR semaphores.
*
*
Memory consistency effects: Actions in a thread prior to calling
* a "release" method such as {@code release()}
* happen-before
* 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;
private final Sync sync;
/**
* Synchronization implementation for semaphore.
* Subclassed into fair and nonfair versions.
*/
static abstract class Sync implements java.io.Serializable {
private static final long serialVersionUID = 1192457210091910933L;
/** current number of available permits **/
int permits_;
protected Sync(int permits) {
this.permits_ = permits;
}
abstract void acquireUninterruptibly(int n);
abstract void acquire(int n) throws InterruptedException;
public boolean attempt(int n) {
synchronized (this) {
if (permits_ >= n) {
permits_ -= n;
return true;
}
else {
return false;
}
}
}
abstract boolean attempt(int n, long nanos) throws InterruptedException;
abstract void release(int n);
public synchronized int getPermits() {
return permits_;
}
public synchronized int drain() {
int acquired = permits_;
permits_ = 0;
return acquired;
}
public synchronized void reduce(int reduction) {
permits_ -= reduction;
}
abstract boolean hasQueuedThreads();
abstract int getQueueLength();
abstract Collection getQueuedThreads();
}
/**
* Nonfair version
*/
final static class NonfairSync extends Sync {
private static final long serialVersionUID = -2694183684443567898L;
protected NonfairSync(int initialPermits) {
super(initialPermits);
}
private static void checkAgainstMultiacquire(int n) {
if (n != 1) {
throw new UnsupportedOperationException(
"Atomic multi-acquire supported only in FAIR semaphores");
}
}
public void acquireUninterruptibly(int n) {
if (n == 0) return;
checkAgainstMultiacquire(n);
synchronized (this) {
if (permits_ > 0) {
--permits_;
return;
}
// else must wait
boolean wasInterrupted = Thread.interrupted();
try {
while (true) {
try {
wait();
}
catch (InterruptedException e) {
wasInterrupted = true;
// no need to notify; if we were signalled, we will
// act as signalled (interruption is ignored anyway)
}
if (permits_ > 0) {
--permits_;
return;
}
}
}
finally {
if (wasInterrupted) Thread.currentThread().interrupt();
}
}
}
public void acquire(int n) throws InterruptedException {
if (Thread.interrupted()) throw new InterruptedException();
if (n == 0) return;
checkAgainstMultiacquire(n);
synchronized (this) {
while (permits_ <= 0) {
try {
wait();
}
catch (InterruptedException ex) {
notify();
throw ex;
}
}
--permits_;
}
}
public boolean attempt(int n, long nanos) throws InterruptedException {
if (Thread.interrupted()) throw new InterruptedException();
if (n == 0) return true;
checkAgainstMultiacquire(n);
synchronized (this) {
if (permits_ > 0) {
--permits_;
return true;
}
else if (nanos <= 0)
return false;
else {
try {
long deadline = Utils.nanoTime() + nanos;
for (; ; ) {
TimeUnit.NANOSECONDS.timedWait(this, nanos);
if (permits_ > 0) {
--permits_;
return true;
}
else {
nanos = deadline - Utils.nanoTime();
if (nanos <= 0)
return false;
}
}
}
catch (InterruptedException ex) {
notify();
throw ex;
}
}
}
}
public synchronized void release(int n) {
if (n < 0) throw new IllegalArgumentException("Negative argument");
permits_ += n;
for (int i = 0; i < n; ++i) notify();
}
public boolean hasQueuedThreads() {
throw new UnsupportedOperationException("Use FAIR version");
}
public int getQueueLength() {
throw new UnsupportedOperationException("Use FAIR version");
}
public Collection getQueuedThreads() {
throw new UnsupportedOperationException("Use FAIR version");
}
}
/**
* Fair version
*/
final static class FairSync extends Sync implements QueuedSync {
private static final long serialVersionUID = 2014338818796000944L;
private transient WaitQueue wq_ = new FIFOWaitQueue();
FairSync(int initialPermits) {
super(initialPermits);
}
public void acquireUninterruptibly(int n) {
if (precheck(n)) return;
WaitQueue.WaitNode w = new Node(n);
w.doWaitUninterruptibly(this);
}
public void acquire(int n) throws InterruptedException {
if (Thread.interrupted()) throw new InterruptedException();
if (precheck(n)) return;
WaitQueue.WaitNode w = new Node(n);
w.doWait(this);
}
public boolean attempt(int n, long nanos) throws InterruptedException {
if (Thread.interrupted()) throw new InterruptedException();
if (precheck(n)) return true;
if (nanos <= 0) return false;
WaitQueue.WaitNode w = new Node(n);
return w.doTimedWait(this, nanos);
}
protected synchronized boolean precheck(int n) {
boolean pass = (permits_ >= n);
if (pass) permits_ -= n;
return pass;
}
public synchronized boolean recheck(WaitQueue.WaitNode w) {
Node node = (Node)w;
boolean pass = (permits_ >= node.requests);
if (pass) permits_ -= node.requests;
else wq_.insert(w);
return pass;
}
public void takeOver(WaitQueue.WaitNode n) {}
protected synchronized Node getSignallee(int n) {
Node w = (Node)wq_.extract();
permits_ += n;
if (w == null) {
return null;
}
else if (w.requests > permits_) {
// not enough permits released for the "next in line"
wq_.putBack(w);
return null;
}
else {
permits_ -= w.requests;
return w;
}
}
public void release(int n) {
if (n < 0) throw new IllegalArgumentException("Negative argument");
for (;;) {
Node w = getSignallee(n);
if (w == null) return; // no one to signal, or not enough permits
if (w.signal(this)) return; // notify if still waiting, else skip
n = w.requests;
}
}
public synchronized boolean hasQueuedThreads() {
return wq_.hasNodes();
}
public synchronized int getQueueLength() {
return wq_.getLength();
}
public synchronized Collection getQueuedThreads() {
return wq_.getWaitingThreads();
}
private void readObject(java.io.ObjectInputStream in)
throws java.io.IOException, ClassNotFoundException {
in.defaultReadObject();
synchronized (this) {
wq_ = new FIFOWaitQueue();
}
}
final static class Node extends WaitQueue.WaitNode {
final int requests;
Node(int requests) { this.requests = requests; }
}
}
/**
* 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)? (Sync)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}.
*
*
Acquires a permit, if one is available and returns immediately,
* reducing the number of available permits by one.
*
*
If no permit is available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of two things happens:
*
* - Some other thread invokes the {@link #release} method for this
* semaphore and the current thread is next to be assigned a permit; or
*
- Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
*
*
* If the current thread:
*
* - has its interrupted status set on entry to this method; or
*
- is {@linkplain Thread#interrupt interrupted} while waiting
* for a permit,
*
* 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.acquire(1);
}
/**
* Acquires a permit from this semaphore, blocking until one is
* available.
*
* Acquires a permit, if one is available and returns immediately,
* reducing the number of available permits by one.
*
*
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.
*
*
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.acquireUninterruptibly(1);
}
/**
* Acquires a permit from this semaphore, only if one is available at the
* time of invocation.
*
*
Acquires a permit, if one is available and returns immediately,
* with the value {@code true},
* reducing the number of available permits by one.
*
*
If no permit is available then this method will return
* immediately with the value {@code false}.
*
*
Even when this semaphore has been set to use a
* fair ordering policy, a call to {@code tryAcquire()} will
* 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.attempt(1);
}
/**
* 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}.
*
*
Acquires a permit, if one is available and returns immediately,
* with the value {@code true},
* reducing the number of available permits by one.
*
*
If no permit is available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of three things happens:
*
* - Some other thread invokes the {@link #release} method for this
* semaphore and the current thread is next to be assigned a permit; or
*
- Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
- The specified waiting time elapses.
*
*
* If a permit is acquired then the value {@code true} is returned.
*
*
If the current thread:
*
* - has its interrupted status set on entry to this method; or
*
- is {@linkplain Thread#interrupt interrupted} while waiting
* to acquire a permit,
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* 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.attempt(1, unit.toNanos(timeout));
}
/**
* Releases a permit, returning it to the semaphore.
*
*
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.
*
*
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.release(1);
}
/**
* Acquires the given number of permits from this semaphore,
* blocking until all are available,
* or the thread is {@linkplain Thread#interrupt interrupted}.
*
*
Acquires the given number of permits, if they are available,
* and returns immediately, reducing the number of available permits
* by the given amount.
*
*
If insufficient permits are available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of two things happens:
*
* - 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
*
- Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
*
*
* If the current thread:
*
* - has its interrupted status set on entry to this method; or
*
- is {@linkplain Thread#interrupt interrupted} while waiting
* for a permit,
*
* 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.acquire(permits);
}
/**
* Acquires the given number of permits from this semaphore,
* blocking until all are available.
*
* Acquires the given number of permits, if they are available,
* and returns immediately, reducing the number of available permits
* by the given amount.
*
*
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.
*
*
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) {
sync.acquireUninterruptibly(permits);
}
/**
* Acquires the given number of permits from this semaphore, only
* if all are available at the time of invocation.
*
*
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.
*
*
If insufficient permits are available then this method will return
* immediately with the value {@code false} and the number of available
* permits is unchanged.
*
*
Even when this semaphore has been set to use a fair ordering
* policy, a call to {@code tryAcquire} will
* 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.attempt(permits);
}
/**
* 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}.
*
*
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.
*
*
If insufficient permits are available then
* the current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happens:
*
* - 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
*
- Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
- The specified waiting time elapses.
*
*
* If the permits are acquired then the value {@code true} is returned.
*
*
If the current thread:
*
* - has its interrupted status set on entry to this method; or
*
- is {@linkplain Thread#interrupt interrupted} while waiting
* to acquire the permits,
*
* 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()}.
*
* 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.attempt(permits, unit.toNanos(timeout));
}
/**
* Releases the given number of permits, returning them to the semaphore.
*
*
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.
*
*
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.release(permits);
}
/**
* Returns the current number of permits available in this semaphore.
*
*
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.drain();
}
/**
* 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.reduce(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 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() + "]";
}
}