java - Concept of semaphores and similarities and differences between locks and semaphores

• signal

Purpose: You can use a semaphore to limit the number of threads accessing a shared resource

Meaning: Semaphores refer to objects that control access to common resources. Before accessing the resource, the thread must obtain a permission from the semaphore. After accessing the resource, the thread must return the permission to the semaphore. Once permission is granted, the total number of available permissions in the semaphore is decremented by 1. Once the permission is released, the number of permissions in the semaphore is increased by 1.

# Only one permitted semaphore can be used to simulate a mutually exclusive lock.

Example:

                       

Similarities and differences between locks and semaphores Where?

(The lock here refers to the mutex lock)

Difference:

Scope

1. Semaphore: inter-process or inter-thread
   Mutex lock: inter-thread

Execution when locking

1. Semaphore: If the value of the semaphore > 0; then other threads can perform tasks. And after successful execution, value--; if value=0, then thread sem_wait causes the thread to block until value is released after sem_post is released, and other threads can execute based on value.
   Mutex lock: Only one object is locked (locked), then no other thread can access the locked object.

One is synchronization and the other is mutual exclusion

1. Semaphore: used in multi-threading Multitasking is synchronized. When one thread completes an action, it tells other threads through a semaphore that the other threads are performing certain actions.
   Mutex lock: used for mutual exclusion in multi-threads and multi-tasks. After a thread occupies a certain resource, other threads cannot access it. Only when this thread is unlocked can other threads use it.

In other words, the semaphore is not necessarily about locking a certain resource, but a process concept. For example: there are two threads A and B, and thread B has to wait for thread A to complete. After a certain task, you can perform the following steps. This task does not necessarily involve locking a certain resource, but can also perform some calculations or data processing. The thread mutex is the concept of "locking a certain resource". During the lock period, other threads cannot operate on the protected data. In some cases the two are interchangeable.

Related articles:

Examples of mutex semaphores and multi-thread waiting mechanisms in Java

Detailed explanation of basic usage examples of PHP semaphores

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