A semaphore is a signaling mechanism used to coordinate between threads. Example: One thread is downloading files from the internet and another thread is analyzing the files. This is a classic producer/consumer scenario. The producer calls signal() on the semaphore when a file is downloaded. The consumer calls wait() on the same semaphore in order to be blocked until the signal indicates a file is ready. If the semaphore is already signaled when the consumer calls wait, the call does not block. Multiple threads can wait on a semaphore, but each signal will only unblock a single thread.

计数信号量跟踪信号的数量。例如，如果生产者连续发出三次信号，wait()可以被调用三次而不阻塞。二进制信号量不算数，只有“等待”和“有信号”两种状态。

互斥锁(互斥锁)是由单个线程拥有的锁。只有获得锁的线程才能再次释放它。其他试图获取锁的线程将被阻塞，直到当前所有者线程释放它。互斥锁本身并不锁任何东西——它实际上只是一个标志。但是代码可以检查互斥锁的所有权，以确保一次只有一个线程可以访问某个对象或资源。

A monitor is a higher-level construct which uses an underlying mutex lock to ensure thread-safe access to some object. Unfortunately the word "monitor" is used in a few different meanings depending on context and platform and context, but in Java for example, a monitor is a mutex lock which is implicitly associated with an object, and which can be invoked with the synchronized keyword. The synchronized keyword can be applied to a class, method or block and ensures only one thread can execute the code at a time.

Monitor是设计用于从多个线程访问的对象。监视对象的成员函数或方法将强制互斥，因此在给定时间只有一个线程可以对对象执行任何操作。如果一个线程正在执行该对象的成员函数，那么任何试图调用该对象的成员函数的其他线程都必须等待，直到第一个线程完成。

信号量是较低级别的对象。您可以使用信号量来实现监视器。信号量本质上就是一个计数器。当计数器为正数时，如果线程试图获取信号量，则允许获取信号量，计数器则递减。当一个线程完成时，它释放信号量，并增加计数器。

如果当一个线程试图获取信号量时计数器已经为零，那么它必须等待另一个线程释放信号量。如果一个线程释放一个信号量时有多个线程在等待，那么其中一个线程会得到它。释放信号量的线程不一定是获取信号量的线程。

显示器就像一个公共厕所。每次只能有一个人进入。他们锁上门防止其他人进来，做他们的事情，然后在他们离开的时候打开门。

信号量就像一个自行车租赁场所。他们有一定数量的自行车。如果你想租一辆自行车，他们有一辆免费的，那么你可以骑，否则你必须等待。当有人归还自行车时，其他人就可以拿走它。如果你有一辆自行车，那么你可以把它交给别人归还——自行车租赁公司不在乎谁来归还，只要他们能把自行车拿回来就行。

信号量:

在并发系统中使用计数器或标志来控制对某些共享资源的访问，意味着使用信号量。

例子:

一个只允许50名乘客获得任何剧院/公共汽车/火车/游乐设施/教室的50个座位(共享资源)的柜台。只有在有人空出座位的情况下，才允许新乘客进入。 一个二进制标志，指示任何浴室的空闲/占用状态。 交通灯是旗帜的一个很好的例子。他们通过调节道路上车辆的通行来控制流量(共享资源)

标志只显示资源的当前状态，没有计数或资源上正在等待或运行的对象的任何其他信息。

监控:

Monitor通过与对对象感兴趣的线程通信来同步对对象的访问，要求它们获得访问权或等待某些条件变为真。

例子:

父亲可能会充当女儿的监工，只允许她一次和一个男人约会。 学校老师使用警棍，只允许一个孩子在课堂上发言。 最后一个技术问题是，同步Account对象上的事务(通过线程)以保持完整性。

一句话回答:

监视器:一次只能控制一个线程在监视器中执行。(需要获取锁才能执行单线程)

信号量:保护共享资源的锁。(需要获取锁才能访问资源)

A semaphore is a signaling mechanism used to coordinate between threads. Example: One thread is downloading files from the internet and another thread is analyzing the files. This is a classic producer/consumer scenario. The producer calls signal() on the semaphore when a file is downloaded. The consumer calls wait() on the same semaphore in order to be blocked until the signal indicates a file is ready. If the semaphore is already signaled when the consumer calls wait, the call does not block. Multiple threads can wait on a semaphore, but each signal will only unblock a single thread.

计数信号量跟踪信号的数量。例如，如果生产者连续发出三次信号，wait()可以被调用三次而不阻塞。二进制信号量不算数，只有“等待”和“有信号”两种状态。

互斥锁(互斥锁)是由单个线程拥有的锁。只有获得锁的线程才能再次释放它。其他试图获取锁的线程将被阻塞，直到当前所有者线程释放它。互斥锁本身并不锁任何东西——它实际上只是一个标志。但是代码可以检查互斥锁的所有权，以确保一次只有一个线程可以访问某个对象或资源。

A monitor is a higher-level construct which uses an underlying mutex lock to ensure thread-safe access to some object. Unfortunately the word "monitor" is used in a few different meanings depending on context and platform and context, but in Java for example, a monitor is a mutex lock which is implicitly associated with an object, and which can be invoked with the synchronized keyword. The synchronized keyword can be applied to a class, method or block and ensures only one thread can execute the code at a time.

下面的解释实际上解释了monitor的wait()和signal()与semaphore的P和V的区别。

监控器中对条件变量的wait()和signal()操作类似于对计数信号量的P和V操作。

A wait statement can block a process's execution, while a signal statement can cause another process to be unblocked. However, there are some differences between them. When a process executes a P operation, it does not necessarily block that process because the counting semaphore may be greater than zero. In contrast, when a wait statement is executed, it always blocks the process. When a task executes a V operation on a semaphore, it either unblocks a task waiting on that semaphore or increments the semaphore counter if there is no task to unlock. On the other hand, if a process executes a signal statement when there is no other process to unblock, there is no effect on the condition variable. Another difference between semaphores and monitors is that users awaken by a V operation can resume execution without delay. Contrarily, users awaken by a signal operation are restarted only when the monitor is unlocked. In addition, a monitor solution is more structured than the one with semaphores because the data and procedures are encapsulated in a single module and that the mutual exclusion is provided automatically by the implementation.

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