我已经创建了可视化，这应该有助于理解这个想法。信号量控制多线程环境中对公共资源的访问。

ExecutorService executor = Executors.newFixedThreadPool(7);

Semaphore semaphore = new Semaphore(4);

Runnable longRunningTask = () -> {
    boolean permit = false;
    try {
        permit = semaphore.tryAcquire(1, TimeUnit.SECONDS);
        if (permit) {
            System.out.println("Semaphore acquired");
            Thread.sleep(5);
        } else {
            System.out.println("Could not acquire semaphore");
        }
    } catch (InterruptedException e) {
        throw new IllegalStateException(e);
    } finally {
        if (permit) {
            semaphore.release();
        }
    }
};

// execute tasks
for (int j = 0; j < 10; j++) {
    executor.submit(longRunningTask);
}
executor.shutdown();

输出

Semaphore acquired
Semaphore acquired
Semaphore acquired
Semaphore acquired
Could not acquire semaphore
Could not acquire semaphore
Could not acquire semaphore

本文中的示例代码

假设一辆出租车可以容纳3人(后面)+2人(前面)，包括司机。因此，一个信号量一次只允许5个人在一辆车里。 互斥只允许一个人坐在汽车的一个座位上。

因此，互斥量是允许对资源的独占访问(如操作系统线程)，而信号量是允许在同一时间访问n个资源。

硬件或软件标志。在多任务系统中，信号量是一个变量，其值表示公共资源的状态。需要资源的进程检查信号量以确定资源状态，然后决定如何继续。

信号量是一个包含自然数(即大于或等于零的整数)的对象，在自然数上定义了两个修改操作。一个运算V，给自然数加1。另一个操作，P，将自然数减少1。这两个活动都是原子的(即没有其他操作可以与V或P同时执行)。

因为自然数0不能减少，所以在包含0的信号量上调用P将阻塞调用进程(/thread)的执行，直到该数字不再为0,P可以成功(原子地)执行为止。

正如在其他回答中提到的，信号量可用于将对某个资源的访问限制为最大(但可变的)进程数。

信号量也可以用作…信号量。 例如，如果有多个进程将数据排队到队列中，而只有一个任务使用队列中的数据。如果您不希望您的消费任务不断地轮询队列以获取可用数据，您可以使用信号量。

在这里，信号量不是用作排除机制，而是用作信号机制。 消费任务正在等待信号量 生产任务正在发送信号量。

这样，当且仅当有数据要退出队列时，消费任务才会运行

Michael Barr的文章《互斥量和信号量揭秘》很好地介绍了互斥量和信号量的不同之处，以及什么时候应该和不应该使用它们。我在这里摘录了几个关键段落。

关键在于应该使用互斥来保护共享资源，而应该使用信号量来发送信号。通常不应该使用信号量来保护共享资源，也不应该使用互斥量来发送信号。例如，在使用信号量来保护共享资源方面，使用bouncer类比是有问题的——您可以这样使用它们，但这可能会导致难以诊断错误。

While mutexes and semaphores have some similarities in their implementation, they should always be used differently. The most common (but nonetheless incorrect) answer to the question posed at the top is that mutexes and semaphores are very similar, with the only significant difference being that semaphores can count higher than one. Nearly all engineers seem to properly understand that a mutex is a binary flag used to protect a shared resource by ensuring mutual exclusion inside critical sections of code. But when asked to expand on how to use a "counting semaphore," most engineers—varying only in their degree of confidence—express some flavor of the textbook opinion that these are used to protect several equivalent resources.

...

在这一点上，一个有趣的类比是使用浴室钥匙的想法来保护共享资源-浴室。如果一家商店只有一间浴室，那么一把钥匙就足以保护这一资源，防止多人同时使用。

如果有多个浴室，人们可能会试图用相同的键来设置多个键——这类似于误用信号量。一旦你有了一个键，你实际上不知道哪个浴室是可用的，如果你沿着这条路走下去，你可能最终会使用互斥锁来提供该信息，并确保你没有使用已经被占用的浴室。

A semaphore is the wrong tool to protect several of the essentially same resource, but this is how many people think of it and use it. The bouncer analogy is distinctly different - there aren't several of the same type of resource, instead there is one resource which can accept multiple simultaneous users. I suppose a semaphore can be used in such situations, but rarely are there real-world situations where the analogy actually holds - it's more often that there are several of the same type, but still individual resources, like the bathrooms, which cannot be used this way.

...

The correct use of a semaphore is for signaling from one task to another. A mutex is meant to be taken and released, always in that order, by each task that uses the shared resource it protects. By contrast, tasks that use semaphores either signal or wait—not both. For example, Task 1 may contain code to post (i.e., signal or increment) a particular semaphore when the "power" button is pressed and Task 2, which wakes the display, pends on that same semaphore. In this scenario, one task is the producer of the event signal; the other the consumer.

...

Here an important point is made that mutexes interfere with real time operating systems in a bad way, causing priority inversion where a less important task may be executed before a more important task because of resource sharing. In short, this happens when a lower priority task uses a mutex to grab a resource, A, then tries to grab B, but is paused because B is unavailable. While it's waiting, a higher priority task comes along and needs A, but it's already tied up, and by a process that isn't even running because it's waiting for B. There are many ways to resolve this, but it most often is fixed by altering the mutex and task manager. The mutex is much more complex in these cases than a binary semaphore, and using a semaphore in such an instance will cause priority inversions because the task manager is unaware of the priority inversion and cannot act to correct it.

...

The cause of the widespread modern confusion between mutexes and semaphores is historical, as it dates all the way back to the 1974 invention of the Semaphore (capital "S", in this article) by Djikstra. Prior to that date, none of the interrupt-safe task synchronization and signaling mechanisms known to computer scientists was efficiently scalable for use by more than two tasks. Dijkstra's revolutionary, safe-and-scalable Semaphore was applied in both critical section protection and signaling. And thus the confusion began. However, it later became obvious to operating system developers, after the appearance of the priority-based preemptive RTOS (e.g., VRTX, ca. 1980), publication of academic papers establishing RMA and the problems caused by priority inversion, and a paper on priority inheritance protocols in 1990, 3 it became apparent that mutexes must be more than just semaphores with a binary counter.

互斥:资源共享

信号:信号

在没有仔细考虑副作用的情况下，不要用一种药物来替代另一种药物。