Java多线程高并发高级篇(一)-信号量semaphore

多线程高并发篇终于到了高级篇。基础篇和进阶篇主要讲了线程的相关内容以及一些原理分析，最最重要的就是JMM和AQS（AbstractQueuedSynchronizer ），理解了这两个基础，高并发涉及的内容就差不多了。高级篇中涉及一些常用的并发工具类和框架。

信号量，用个很形象的例子来说，就比如我们的安检限行，一次允许通过几个人，当看见安检员的NO PASS的牌子时，就等着吧。

类似的，信号量也需要设置准入数。我们看下源码中信号量的构造方法。在信号量的构造方法中，有两个构造方法。

 

/**
     * 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);
    }

 从构造方法中我们可以看出，信号量的准入数是必传的（准入数规定了一次可执行的线程数），默认使用的是非公平的队列同步器（当然可以根据第二个去设置使用公平的队列同步器）。

 

1.常用的几个主要方法

我们直接看源码。

/**
     * 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);
    }

 在信号量类的方法中，实现原理就是AQS，因此请参考AQS相关帖子内容。

 

我们只介绍下功能。

①acquire()：该方法获取一个准入许可，若无法获得，线程等待，直到有线程释放一个许可或者当前线程被中断，这也表达了另外一层意思--该方法是可以响应中断的。

②acquireUninterruptibly()：该方法获取一个准入许可，若无法获得，线程等待，直到有线程释放一个许可，不同的是该方法不响应中断。

③tryAcquire()：尝试获取一个许可，如果成功，返回true，如果失败，返回false，并且会立即返回，不等待。

④tryAcquire(long timeout, TimeUnit unit)：在给定的时间内尝试获取一个许可，如果成功，返回true，如果失败，返回false。

⑤release()：释放当前线程占用的许可，供其他等待许可的线程使用。

 

举例：

public class SemaphoreDemo implements Runnable {
	final Semaphore semp = new Semaphore(5);
	@Override
	public void run() {
		try {
			semp.acquire();
			//模拟处理业务逻辑
			Thread.sleep(2000);
			System.out.println(Thread.currentThread().getId()+" done!");
			semp.release();
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

	public static void main(String[] args) {
		ExecutorService exce = Executors.newFixedThreadPool(20);
		final SemaphoreDemo demo = new SemaphoreDemo();
		for (int i = 0; i < 20; i++) {
			exce.submit(demo);
		}
		
	}

}

 从执行结果我们可以看到，系统以5个线程一组为单位，依次输出，也就是说同时只允许5个线程同时进入模拟业务逻辑处。这里要注意，如果发生信号泄露（就是在申请了准入许可后，没有释放），那么可用的准入许可越来越少，直至都被占用。