mirror of git://gcc.gnu.org/git/gcc.git
682 lines
27 KiB
Java
682 lines
27 KiB
Java
/*
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* Written by Doug Lea with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/licenses/publicdomain
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*/
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package java.util.concurrent;
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import java.util.*;
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import java.util.concurrent.locks.*;
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import java.util.concurrent.atomic.*;
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/**
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* A counting semaphore. Conceptually, a semaphore maintains a set of
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* permits. Each {@link #acquire} blocks if necessary until a permit is
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* available, and then takes it. Each {@link #release} adds a permit,
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* potentially releasing a blocking acquirer.
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* However, no actual permit objects are used; the {@code Semaphore} just
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* keeps a count of the number available and acts accordingly.
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*
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* <p>Semaphores are often used to restrict the number of threads than can
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* access some (physical or logical) resource. For example, here is
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* a class that uses a semaphore to control access to a pool of items:
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* <pre>
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* class Pool {
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* private static final int MAX_AVAILABLE = 100;
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* private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
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*
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* public Object getItem() throws InterruptedException {
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* available.acquire();
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* return getNextAvailableItem();
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* }
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*
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* public void putItem(Object x) {
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* if (markAsUnused(x))
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* available.release();
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* }
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*
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* // Not a particularly efficient data structure; just for demo
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*
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* protected Object[] items = ... whatever kinds of items being managed
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* protected boolean[] used = new boolean[MAX_AVAILABLE];
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*
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* protected synchronized Object getNextAvailableItem() {
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* for (int i = 0; i < MAX_AVAILABLE; ++i) {
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* if (!used[i]) {
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* used[i] = true;
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* return items[i];
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* }
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* }
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* return null; // not reached
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* }
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*
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* protected synchronized boolean markAsUnused(Object item) {
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* for (int i = 0; i < MAX_AVAILABLE; ++i) {
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* if (item == items[i]) {
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* if (used[i]) {
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* used[i] = false;
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* return true;
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* } else
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* return false;
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* }
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* }
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* return false;
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* }
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*
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* }
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* </pre>
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*
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* <p>Before obtaining an item each thread must acquire a permit from
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* the semaphore, guaranteeing that an item is available for use. When
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* the thread has finished with the item it is returned back to the
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* pool and a permit is returned to the semaphore, allowing another
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* thread to acquire that item. Note that no synchronization lock is
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* held when {@link #acquire} is called as that would prevent an item
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* from being returned to the pool. The semaphore encapsulates the
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* synchronization needed to restrict access to the pool, separately
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* from any synchronization needed to maintain the consistency of the
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* pool itself.
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*
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* <p>A semaphore initialized to one, and which is used such that it
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* only has at most one permit available, can serve as a mutual
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* exclusion lock. This is more commonly known as a <em>binary
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* semaphore</em>, because it only has two states: one permit
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* available, or zero permits available. When used in this way, the
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* binary semaphore has the property (unlike many {@link Lock}
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* implementations), that the "lock" can be released by a
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* thread other than the owner (as semaphores have no notion of
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* ownership). This can be useful in some specialized contexts, such
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* as deadlock recovery.
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*
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* <p> The constructor for this class optionally accepts a
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* <em>fairness</em> parameter. When set false, this class makes no
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* guarantees about the order in which threads acquire permits. In
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* particular, <em>barging</em> is permitted, that is, a thread
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* invoking {@link #acquire} can be allocated a permit ahead of a
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* thread that has been waiting - logically the new thread places itself at
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* the head of the queue of waiting threads. When fairness is set true, the
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* semaphore guarantees that threads invoking any of the {@link
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* #acquire() acquire} methods are selected to obtain permits in the order in
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* which their invocation of those methods was processed
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* (first-in-first-out; FIFO). Note that FIFO ordering necessarily
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* applies to specific internal points of execution within these
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* methods. So, it is possible for one thread to invoke
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* {@code acquire} before another, but reach the ordering point after
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* the other, and similarly upon return from the method.
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* Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not
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* honor the fairness setting, but will take any permits that are
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* available.
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*
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* <p>Generally, semaphores used to control resource access should be
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* initialized as fair, to ensure that no thread is starved out from
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* accessing a resource. When using semaphores for other kinds of
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* synchronization control, the throughput advantages of non-fair
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* ordering often outweigh fairness considerations.
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*
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* <p>This class also provides convenience methods to {@link
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* #acquire(int) acquire} and {@link #release(int) release} multiple
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* permits at a time. Beware of the increased risk of indefinite
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* postponement when these methods are used without fairness set true.
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*
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* <p>Memory consistency effects: Actions in a thread prior to calling
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* a "release" method such as {@code release()}
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
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* actions following a successful "acquire" method such as {@code acquire()}
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* in another thread.
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*
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* @since 1.5
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* @author Doug Lea
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*
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*/
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public class Semaphore implements java.io.Serializable {
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private static final long serialVersionUID = -3222578661600680210L;
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/** All mechanics via AbstractQueuedSynchronizer subclass */
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private final Sync sync;
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/**
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* Synchronization implementation for semaphore. Uses AQS state
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* to represent permits. Subclassed into fair and nonfair
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* versions.
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*/
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abstract static class Sync extends AbstractQueuedSynchronizer {
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private static final long serialVersionUID = 1192457210091910933L;
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Sync(int permits) {
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setState(permits);
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}
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final int getPermits() {
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return getState();
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}
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final int nonfairTryAcquireShared(int acquires) {
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for (;;) {
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int available = getState();
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int remaining = available - acquires;
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if (remaining < 0 ||
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compareAndSetState(available, remaining))
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return remaining;
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}
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}
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protected final boolean tryReleaseShared(int releases) {
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for (;;) {
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int p = getState();
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if (compareAndSetState(p, p + releases))
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return true;
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}
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}
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final void reducePermits(int reductions) {
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for (;;) {
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int current = getState();
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int next = current - reductions;
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if (compareAndSetState(current, next))
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return;
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}
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}
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final int drainPermits() {
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for (;;) {
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int current = getState();
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if (current == 0 || compareAndSetState(current, 0))
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return current;
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}
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}
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}
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/**
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* NonFair version
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*/
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final static class NonfairSync extends Sync {
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private static final long serialVersionUID = -2694183684443567898L;
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NonfairSync(int permits) {
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super(permits);
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}
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protected int tryAcquireShared(int acquires) {
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return nonfairTryAcquireShared(acquires);
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}
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}
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/**
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* Fair version
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*/
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final static class FairSync extends Sync {
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private static final long serialVersionUID = 2014338818796000944L;
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FairSync(int permits) {
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super(permits);
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}
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protected int tryAcquireShared(int acquires) {
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Thread current = Thread.currentThread();
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for (;;) {
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Thread first = getFirstQueuedThread();
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if (first != null && first != current)
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return -1;
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int available = getState();
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int remaining = available - acquires;
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if (remaining < 0 ||
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compareAndSetState(available, remaining))
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return remaining;
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}
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}
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}
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/**
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* Creates a {@code Semaphore} with the given number of
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* permits and nonfair fairness setting.
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*
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* @param permits the initial number of permits available.
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* This value may be negative, in which case releases
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* must occur before any acquires will be granted.
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*/
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public Semaphore(int permits) {
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sync = new NonfairSync(permits);
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}
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/**
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* Creates a {@code Semaphore} with the given number of
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* permits and the given fairness setting.
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*
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* @param permits the initial number of permits available.
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* This value may be negative, in which case releases
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* must occur before any acquires will be granted.
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* @param fair {@code true} if this semaphore will guarantee
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* first-in first-out granting of permits under contention,
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* else {@code false}
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*/
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public Semaphore(int permits, boolean fair) {
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sync = (fair)? new FairSync(permits) : new NonfairSync(permits);
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}
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/**
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* Acquires a permit from this semaphore, blocking until one is
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* available, or the thread is {@linkplain Thread#interrupt interrupted}.
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*
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* <p>Acquires a permit, if one is available and returns immediately,
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* reducing the number of available permits by one.
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*
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* <p>If no permit is available then the current thread becomes
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* disabled for thread scheduling purposes and lies dormant until
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* one of two things happens:
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* <ul>
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* <li>Some other thread invokes the {@link #release} method for this
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* semaphore and the current thread is next to be assigned a permit; or
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* <li>Some other thread {@linkplain Thread#interrupt interrupts}
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* the current thread.
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* </ul>
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*
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* <p>If the current thread:
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* <ul>
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* <li>has its interrupted status set on entry to this method; or
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* <li>is {@linkplain Thread#interrupt interrupted} while waiting
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* for a permit,
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* </ul>
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* then {@link InterruptedException} is thrown and the current thread's
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* interrupted status is cleared.
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*
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* @throws InterruptedException if the current thread is interrupted
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*/
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public void acquire() throws InterruptedException {
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sync.acquireSharedInterruptibly(1);
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}
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/**
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* Acquires a permit from this semaphore, blocking until one is
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* available.
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*
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* <p>Acquires a permit, if one is available and returns immediately,
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* reducing the number of available permits by one.
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*
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* <p>If no permit is available then the current thread becomes
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* disabled for thread scheduling purposes and lies dormant until
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* some other thread invokes the {@link #release} method for this
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* semaphore and the current thread is next to be assigned a permit.
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*
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* <p>If the current thread is {@linkplain Thread#interrupt interrupted}
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* while waiting for a permit then it will continue to wait, but the
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* time at which the thread is assigned a permit may change compared to
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* the time it would have received the permit had no interruption
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* occurred. When the thread does return from this method its interrupt
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* status will be set.
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*/
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public void acquireUninterruptibly() {
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sync.acquireShared(1);
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}
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/**
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* Acquires a permit from this semaphore, only if one is available at the
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* time of invocation.
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*
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* <p>Acquires a permit, if one is available and returns immediately,
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* with the value {@code true},
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* reducing the number of available permits by one.
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*
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* <p>If no permit is available then this method will return
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* immediately with the value {@code false}.
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*
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* <p>Even when this semaphore has been set to use a
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* fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
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* immediately acquire a permit if one is available, whether or not
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* other threads are currently waiting.
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* This "barging" behavior can be useful in certain
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* circumstances, even though it breaks fairness. If you want to honor
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* the fairness setting, then use
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* {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
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* which is almost equivalent (it also detects interruption).
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*
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* @return {@code true} if a permit was acquired and {@code false}
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* otherwise
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*/
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public boolean tryAcquire() {
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return sync.nonfairTryAcquireShared(1) >= 0;
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}
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/**
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* Acquires a permit from this semaphore, if one becomes available
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* within the given waiting time and the current thread has not
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* been {@linkplain Thread#interrupt interrupted}.
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*
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* <p>Acquires a permit, if one is available and returns immediately,
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* with the value {@code true},
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* reducing the number of available permits by one.
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*
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* <p>If no permit is available then the current thread becomes
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* disabled for thread scheduling purposes and lies dormant until
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* one of three things happens:
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* <ul>
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* <li>Some other thread invokes the {@link #release} method for this
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* semaphore and the current thread is next to be assigned a permit; or
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* <li>Some other thread {@linkplain Thread#interrupt interrupts}
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* the current thread; or
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* <li>The specified waiting time elapses.
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* </ul>
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*
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* <p>If a permit is acquired then the value {@code true} is returned.
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*
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* <p>If the current thread:
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* <ul>
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* <li>has its interrupted status set on entry to this method; or
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* <li>is {@linkplain Thread#interrupt interrupted} while waiting
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* to acquire a permit,
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* </ul>
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* then {@link InterruptedException} is thrown and the current thread's
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* interrupted status is cleared.
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*
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* <p>If the specified waiting time elapses then the value {@code false}
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* is returned. If the time is less than or equal to zero, the method
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* will not wait at all.
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*
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* @param timeout the maximum time to wait for a permit
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* @param unit the time unit of the {@code timeout} argument
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* @return {@code true} if a permit was acquired and {@code false}
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* if the waiting time elapsed before a permit was acquired
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* @throws InterruptedException if the current thread is interrupted
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*/
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public boolean tryAcquire(long timeout, TimeUnit unit)
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throws InterruptedException {
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return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
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}
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/**
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* Releases a permit, returning it to the semaphore.
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*
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* <p>Releases a permit, increasing the number of available permits by
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* one. If any threads are trying to acquire a permit, then one is
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* selected and given the permit that was just released. That thread
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* is (re)enabled for thread scheduling purposes.
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*
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* <p>There is no requirement that a thread that releases a permit must
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* have acquired that permit by calling {@link #acquire}.
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* Correct usage of a semaphore is established by programming convention
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* in the application.
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*/
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public void release() {
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sync.releaseShared(1);
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}
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/**
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* Acquires the given number of permits from this semaphore,
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* blocking until all are available,
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* or the thread is {@linkplain Thread#interrupt interrupted}.
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*
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* <p>Acquires the given number of permits, if they are available,
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* and returns immediately, reducing the number of available permits
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* by the given amount.
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*
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* <p>If insufficient permits are available then the current thread becomes
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* disabled for thread scheduling purposes and lies dormant until
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* one of two things happens:
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* <ul>
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* <li>Some other thread invokes one of the {@link #release() release}
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* methods for this semaphore, the current thread is next to be assigned
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* permits and the number of available permits satisfies this request; or
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* <li>Some other thread {@linkplain Thread#interrupt interrupts}
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* the current thread.
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* </ul>
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*
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* <p>If the current thread:
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* <ul>
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* <li>has its interrupted status set on entry to this method; or
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* <li>is {@linkplain Thread#interrupt interrupted} while waiting
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* for a permit,
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* </ul>
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* then {@link InterruptedException} is thrown and the current thread's
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* interrupted status is cleared.
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* Any permits that were to be assigned to this thread are instead
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* assigned to other threads trying to acquire permits, as if
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* permits had been made available by a call to {@link #release()}.
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*
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* @param permits the number of permits to acquire
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* @throws InterruptedException if the current thread is interrupted
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* @throws IllegalArgumentException if {@code permits} is negative
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*/
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public void acquire(int permits) throws InterruptedException {
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if (permits < 0) throw new IllegalArgumentException();
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sync.acquireSharedInterruptibly(permits);
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}
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/**
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* Acquires the given number of permits from this semaphore,
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* blocking until all are available.
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*
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* <p>Acquires the given number of permits, if they are available,
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* and returns immediately, reducing the number of available permits
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* by the given amount.
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*
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* <p>If insufficient permits are available then the current thread becomes
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* disabled for thread scheduling purposes and lies dormant until
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* some other thread invokes one of the {@link #release() release}
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* methods for this semaphore, the current thread is next to be assigned
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* permits and the number of available permits satisfies this request.
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*
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* <p>If the current thread is {@linkplain Thread#interrupt interrupted}
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* while waiting for permits then it will continue to wait and its
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* position in the queue is not affected. When the thread does return
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* from this method its interrupt status will be set.
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*
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* @param permits the number of permits to acquire
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* @throws IllegalArgumentException if {@code permits} is negative
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*
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*/
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public void acquireUninterruptibly(int permits) {
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if (permits < 0) throw new IllegalArgumentException();
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sync.acquireShared(permits);
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}
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/**
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* Acquires the given number of permits from this semaphore, only
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* if all are available at the time of invocation.
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*
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* <p>Acquires the given number of permits, if they are available, and
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* returns immediately, with the value {@code true},
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* reducing the number of available permits by the given amount.
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*
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* <p>If insufficient permits are available then this method will return
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* immediately with the value {@code false} and the number of available
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* permits is unchanged.
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*
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* <p>Even when this semaphore has been set to use a fair ordering
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* policy, a call to {@code tryAcquire} <em>will</em>
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* immediately acquire a permit if one is available, whether or
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* not other threads are currently waiting. This
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* "barging" behavior can be useful in certain
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* circumstances, even though it breaks fairness. If you want to
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* honor the fairness setting, then use {@link #tryAcquire(int,
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* long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }
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* which is almost equivalent (it also detects interruption).
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*
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* @param permits the number of permits to acquire
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* @return {@code true} if the permits were acquired and
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* {@code false} otherwise
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* @throws IllegalArgumentException if {@code permits} is negative
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*/
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public boolean tryAcquire(int permits) {
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if (permits < 0) throw new IllegalArgumentException();
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return sync.nonfairTryAcquireShared(permits) >= 0;
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}
|
|
|
|
/**
|
|
* 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() + "]";
|
|
}
|
|
}
|