若有不正之處請多多諒解，並歡迎批評指正。

請尊重作者勞動成果，轉載請標明原文連結：

Semaphore是JUC包提供的一個共享鎖，一般稱之為訊號量。

Semaphore通過自定義的同步器維護了一個或多個共享資源，執行緒通過呼叫acquire獲取共享資源，通過呼叫release釋放。

原始碼：

/*
 * 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.*;
import java.util.concurrent.locks.*;
 
import java.util.concurrent.atomic.*;

/**
 * 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>
 * 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 Lock}
 * implementations), that the &quot;lock&quot; 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 &quot;barging&quot; 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
     * &quot;barging&quot; 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() + "]";
    }
}

View Code

下面我們來詳細分下下Semaphore的工作原理。

一、建構函式

    public Semaphore(int permits) {
        sync = new NonfairSync(permits);
    }

    public Semaphore(int permits, boolean fair) {
        sync = fair ? new FairSync(permits) : new NonfairSync(permits);
    }

初始化Semaphore時需要指定共享資源的個數。Semaphore提供了兩種模式：公平模式&非公平模式。如果不指定工作模式的話，預設工作在非公平模式下。後面我們將看到，兩種模式的區別在於獲取共享資源時的排序策略。Semaphore有三個內部類：Sync&NonfairSync&FairSync。後兩個繼承自Sync，Sync繼承自AQS。除了序列化版本號之外，Semaphore只有一個成員變數sync，公平模式下sync初始化為FairSync，非公平模式下sync初始化為NonfairSync。

二、acquire 響應中斷獲取資源

Semaphore提供了兩種獲取資源的方式：響應中斷&不響應中斷。我們先來看一下響應中斷的獲取。

    public void acquire() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

acquire方法由同步器sync呼叫上層AQS提供的acquireSharedInterruptibly方法獲取：

    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

acquireSharedInterruptibly方法先檢測中斷。然後呼叫tryAcquireShared方法試圖獲取共享資源。這時公平模式和非公平模式的程式碼執行路徑發生分叉，FairSync和NonfairSync各自重寫了tryAcquireShared方法。

我們先來看下非公平模式下的tryAcquireShared方法：

        protected int tryAcquireShared(int acquires) {
            return nonfairTryAcquireShared(acquires);
        }

它直接代用了父類Sync提供的nonfairTryAcquireShared方法：

        final int nonfairTryAcquireShared(int acquires) {
            for (;;) {
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }

注意，這裡是一個CAS自旋。因為Semaphore是一個共享鎖，可能有多個執行緒同時申請共享資源，因此CAS操作可能失敗。直到成功獲取返回剩餘資源數目，或者發現沒有剩餘資源返回負值代表申請失敗。有一個問題，為什麼我們不在CAS操作失敗後就直接返回失敗呢？因為這樣做雖然不會導致錯誤，但會降低效率：在還有剩餘資源的情況下，一個執行緒因為競爭導致CAS失敗後被放入等待序列尾，一定在佇列頭部有一個執行緒被喚醒去試圖獲取資源，這比直接自旋繼續獲取多了操作等待佇列的開銷。

這裡“非公平”的語義體現在：如果一個執行緒通過nonfairTryAcquireShared成功獲取了共享資源，對於此時正在等待佇列中的執行緒來說，可能是不公平的：佇列中執行緒先到，卻沒能先獲取資源。

如果tryAcquireShared沒能成功獲取，acquireSharedInterruptibly方法呼叫doAcquireSharedInterruptibly方法將當前執行緒放入等待佇列並開始自旋檢測獲取資源：

    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

我們注意到，doAcquireSharedInterruptibly中，當一個執行緒從parkAndCheckInterrupt方法中被中斷喚醒之後，直接丟擲了中斷異常。還記得我們分析AQS時的doAcquireShared方法嗎，它在這裡的處理方式是用一個區域性變數interrupted記錄下這個異常但不立即處理，而是等到成功獲取資源之後返回這個中斷標誌，並在上層呼叫selfInterrupt方法補上中斷。這正是兩個方法的關鍵區別：是否及時響應中斷。

我們再來看公平模式下的tryAcquireShared方法：

        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;
            }
        }

相比較非公平模式的nonfairTryAcquireShared方法，公平模式下的tryAcquireShared方法在試圖獲取之前做了一個判斷，如果發現等對佇列中有執行緒在等待獲取資源，就直接返回-1表示獲取失敗。當前執行緒會被上層的acquireSharedInterruptibly方法呼叫doAcquireShared方法放入等待佇列中。這正是“公平”模式的語義：如果有執行緒先於我進入等待佇列且正在等待，就直接進入等待佇列，效果便是各個執行緒按照申請的順序獲得共享資源，具有公平性。

三、acquireUnInterruptibly 不響應中斷獲取資源

    public void acquireUninterruptibly() {
        sync.acquireShared(1);
    }

acquireUnInterruptibly方法呼叫AQS提供的acquireShared方法：

    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }

acquireShared方法首先試圖獲取資源，這與acquireSharedInterruptibly方法相比，沒有先檢測中斷的這一步。緊接著呼叫doAcquireShared方法，由於這個方法我在另一篇博文中已經詳細分析過，這裡我們只關注它與doAcquireSharedInterruptibly方法的區別：

    private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                
            
  
           

              
              
            
            
相關推薦
			   
            
            
            
 

    

    
    
Java多執行緒之JUC包：Semaphore原始碼學習筆記
     
 若有不正之處請多多諒解，並歡迎批評指正。
請尊重作者勞動成果，轉載請標明原文連結：


Semaphore是JUC包提供的一個共享鎖，一般稱之為訊號量。
Semaphore通過自定義的同步器維護了一個或多個共享資源，執行緒通過呼叫acquire獲取共享資源，通過呼叫release釋放。
原始碼：
 

  
 

    

    
    
Java多執行緒之JUC包：Condition原始碼學習筆記
     
 若有不正之處請多多諒解，並歡迎批評指正。
請尊重作者勞動成果，轉載請標明原文連結：


Condition在JUC框架下提供了傳統Java監視器風格的wait、notify和notifyAll相似的功能。
Condition必須被繫結到一個獨佔鎖上使用。ReentrantLock中獲取Conditi 

  
 

    

    
    
Java多執行緒之JUC包：CyclicBarrier原始碼學習筆記
     
 若有不正之處請多多諒解，並歡迎批評指正。
請尊重作者勞動成果，轉載請標明原文連結：


CyclicBarrier是java.util.concurrent包中提供的同步工具。通過這個工具我們可以實現n個執行緒相互等待。我們可以通過引數指定達到公共屏障點之後的行為。
先上原始碼：



   

  
 

    

    
    
Java多執行緒之JUC包：AbstractQueuedSynchronizer（AQS）原始碼學習筆記
     
 若有不正之處請多多諒解，並歡迎批評指正。
請尊重作者勞動成果，轉載請標明原文連結：


AbstractQueuedSynchronizer（AQS）是一個同步器框架，在實現鎖的時候，一般會實現一個繼承自AQS的內部類sync，作為我們的自定義同步器。AQS內部維護了一個state成員和一個佇列。其中 

  
 

    

    
    
Java多執行緒之Condition實現原理和原始碼分析（四）
     
 
							
							
							章節概覽、


1、概述
上面的幾個章節我們基於lock(),unlock()方法為入口，深入分析了獨佔鎖的獲取和釋放。這個章節我們在此基礎上，進一步分析AQS是如何實現await，signal功能。其功能上和synchronize的wait，notify一樣。 

  
 

    

    
    
Java多執行緒之ThreadPoolExecutor實現原理和原始碼分析（五）
     
 
							
							
							章節概覽、

1、概述
執行緒池的顧名思義，就是執行緒的一個集合。需要用到執行緒，從集合裡面取出即可。這樣設計主要的作用是優化執行緒的建立和銷燬而造成的資源浪費的情況。Java中的執行緒池的實現主要是JUC下面的ThreadPoolExecutor類完成的。下面 

  
 

    

    
    
JDK中多執行緒之JUC集合的JDK原始碼解讀配合大神的一起看，秒懂。
     
 
                一、    “JUC集合”01之框架

1)  概要

之前，在"Java 集合系列目錄(Category)"中，講解了Java集合包中的各個類。接下來，將展開對JUC包中的集合進行學習。在學習之前，先溫習一下"Java集合包"。本章內容包括：

Java集合包

JUC中的 

  
 

    

    
    
Java多執行緒之ReentrantLock實現原理和原始碼分析（二）
     
 
							
							
							章節概覽、


1、ReentrantLock概述
ReentrantLock字面含義是可重入的互斥鎖，實現了和synchronize關鍵字一樣的獨佔鎖功能。但是ReentrantLock使用的是自旋鎖，通過CAS硬體原語指令實現的輕量級的鎖，不會引起上下文切換 

  
 

    

    
    
Java多執行緒系列--“JUC原子類”03之 AtomicLong原子類
     
 轉自：https://www.cnblogs.com/skywang12345/p/3514593.html（含部分修改） 
概要 
AtomicInteger, AtomicLong和AtomicBoolean這3個基本型別的原子類的原理和用法相似。本章以AtomicLong對基本型別的原子類進行介紹。內容 

  
 

    

    
    
Java多執行緒系列---“JUC原子類”04之 AtomicLongArray原子類
     
 轉自：https://www.cnblogs.com/skywang12345/p/3514604.html（含部分修改） 
概要 
AtomicIntegerArray, AtomicLongArray, AtomicReferenceArray這3個數組型別的原子類的原理和用法相似。本章以AtomicLo 

  
 

    

    
    
Java多執行緒系列---“JUC原子類”05之 AtomicReference原子類
     
 轉自：http://www.cnblogs.com/skywang12345/p/3514623.html（部分修改） 
概要 
本章對AtomicReference引用型別的原子類進行介紹。內容包括： 
 
 AtomicReference介紹和函式列表 
 AtomicReference原始碼分析(基於J 

  
 

    

    
    
Java多執行緒系列---“JUC原子類”06之 AtomicLongFieldUpdater原子類
     
 轉自：http://www.cnblogs.com/skywang12345/p/3514635.html （含部分修改） 
概要 
AtomicIntegerFieldUpdater, AtomicLongFieldUpdater和AtomicReferenceFieldUpdater這3個修改類的成員的原 

  
 

    

    
    
Java多執行緒系列---“JUC原子類”01之 原子類的實現（CAS演算法）
     
 轉自：https://blog.csdn.net/ls5718/article/details/52563959  & https://blog.csdn.net/mmoren/article/details/79185862（含部分修改） 
  
在JDK 5之前Java語言是靠 

  
 

    

    
    
Java多執行緒系列---“JUC原子類”02之 框架
     
 轉自：http://www.cnblogs.com/skywang12345/p/3514589.html 
  
根據修改的資料型別，可以將JUC包中的原子操作類可以分為4類。 
1. 基本型別: AtomicInteger, AtomicLong, AtomicBoolean ;2.& 

  
 

    

    
    
Java多執行緒系列---“JUC鎖”01之 框架
     
 轉自：http://www.cnblogs.com/skywang12345/p/3496098.html（含部分修改） 
  
本章，我們介紹鎖的架構；後面的章節將會對它們逐個進行分析介紹。目錄如下： 
 
 01. Java多執行緒系列--“JUC鎖”01之 框架 
 02.  

  
 

    

    
    
Java多執行緒系列---“JUC鎖”02之 ReentrantLock
     
  
 轉自：https://www.jianshu.com/p/96c89e6e7e90 & https://blog.csdn.net/Somhu/article/details/78874634 （含部分修改） 
 一.ReentrantLock鎖 
 1.Lock介面 
 Lock，鎖 

  
 

    

    
    
Java多執行緒系列---“JUC鎖”06之 公平鎖（下）
     
 轉自：http://www.cnblogs.com/skywang12345/p/3496609.html 
釋放公平鎖(基於JDK1.7.0_40) 
1. unlock() 
unlock()在ReentrantLock.java中實現的，原始碼如下： 
 
 public void unlock() { 

  
 

    

    
    
Java多執行緒系列--“JUC執行緒池”01之 執行緒池架構
     
 
							
							
							概要



前面分別介紹了”Java多執行緒基礎”、”JUC原子類”和”JUC鎖”。本章介紹JUC的最後一部分的內容——執行緒池。內容包括：


執行緒池架構圖
執行緒池示例




執行緒池架構圖



執行緒池的架構圖如下： 




1、Executor
 

  
 

    

    
    
Java多執行緒系列--“JUC執行緒池”05之 執行緒池原理(四)
     
 
							
							
							概要

本章介紹執行緒池的拒絕策略。內容包括：

拒絕策略介紹
拒絕策略對比和示例

拒絕策略介紹

執行緒池的拒絕策略，是指當任務新增到執行緒池中被拒絕，而採取的處理措施。
當任務新增到執行緒池中之所以被拒絕，可能是由於：第一，執行緒池異常關閉。第二，任務數量 

  
 

    

    
    
讀書筆記：java多執行緒之執行緒同步
     
 
                閱讀的書籍：《java瘋狂講義》

關鍵詞：執行緒安全問題，同步程式碼塊，同步方法，釋放同步監視器的鎖定，同步鎖，死鎖


執行緒安全問題：當使用多個執行緒來訪問同一個資料時，會導致一些錯誤情況的發生


到底什麼是執行緒安全問題呢，先看一個經典的案例：銀行取錢的問題