Introduction to Java semaphores Semaphore

Introduction to Java semaphores Semaphore

The semaphore class also appears in java1.5, which is located in the java. util. concurrent package.

First, let's take a look at his document explanation:

A count semaphore. In terms of concept, semaphores maintain a license set. If necessary, eachacquire()And then obtain the license. Eachrelease()Add a license to release a blocked recipient. However, do not use the actual license object,SemaphoreIt only counts available license numbers and takes corresponding actions. There are two very important methods: acquire and release.

Now let's take a look at the introduction of these two methods.

 

public void acquire()             throws InterruptedException

From this semaphore, a license is obtained, and the thread is blocked until a license is provided. Otherwise, the thread is interrupted. Get a license (if one is provided) and return immediately, reduce the number of available licenses by 1.

 

If no license is available, you are prohibited from using the current thread for Thread Scheduling purposes and putting it in sleep state before one of the following two cases:

• Some other threads call this semaphorerelease()Method, and the current thread is the next thread to be allocated with a license; or some other threads interrupt the current thread.

  If the current thread:

  • This method sets its interrupted status to on, or isInterrupted.

    ThrowInterruptedExceptionAnd clear the interrupted status of the current thread.

    Throw:InterruptedException-If the current thread is interrupted

    public void release()

    Release a license and return it to the semaphore. Release a license and increase the number of available licenses by 1. If any thread tries to obtain the license, select a thread and give it the license just released. Then enable (or re-enable) the thread for the purpose scheduled by the thread.

     

    Threads that do not require the release of the license must callacquire()To obtain the license. Establish the correct usage of semaphores through programming conventions in applications.

    Now let's write an example to verify it.

     

    In the preceding example, only five threads are allowed to access the code between acquire () and release () at the same time.

     

    Package cn.kge.com. thread; import java. util. concurrent. executorService; import java. util. concurrent. executors; import java. util. concurrent. semaphore; public class ThreadSemaphoreTest {public static void main (String [] args) {// thread pool ExecutorService exec = Executors. newCachedThreadPool (); // only five threads can simultaneously access final Semaphore semp = new Semaphore (5); // simulate 20 clients to access for (int index = 0; index <20; index ++) {final int NO = index; Runnable run = new Runnable () {public void run () {try {// obtain the license semp. acquire (); System. out. println ("Thread" + Thread. currentThread (). getName () + "Enter, current"); Thread. sleep (long) (Math. random () * 10000); // release after access. If the following statements are blocked, only five records can be printed on the console, and then the thread will be congested. out. println ("Thread" + Thread. currentThread (). getName () + "coming soon"); semp. release () ;}catch (InterruptedException e) {}}; exec.exe cute (run) ;}// exit the thread pool exec. shutdown ();}}

    
    Result:

     

    Thread pool-1-thread-1 entry, which currently has
    Thread pool-1-thread-2 entry, the current has
    Thread pool-1-thread-4 entry, the current has
    Thread pool-1-thread-6 entry, the current has
    Thread pool-1-thread-5 entry, the current has
    Thread pool-1-thread-6 is about to exit
    Thread pool-1-thread-8 entry, the current has
    Thread pool-1-thread-4 is about to exit
    Thread pool-1-thread-9 entry, the current has
    Thread pool-1-thread-8 is about to exit
    Thread pool-1-thread-10 entry, the current has
    Thread pool-1-thread-5 is about to exit
    Thread pool-1-thread-3 entry, which currently has
    Thread pool-1-thread-1 is about to exit
    Thread pool-1-thread-7 entry, which currently has
    Thread pool-1-thread-2 is about to exit
    Thread pool-1-thread-13 entry, the current has
    Thread pool-1-thread-13 is about to exit
    Thread pool-1-thread-12 entry, the current has
    Thread pool-1-thread-10 is about to exit
    Thread pool-1-thread-11 entry, the current has
    Thread pool-1-thread-3 is about to exit
    Thread pool-1-thread-14 entry, the current has
    Thread pool-1-thread-12 is about to exit
    Thread pool-1-thread-16 entry, the current has
    Thread pool-1-thread-7 is about to exit
    Thread pool-1-thread-17 enters, and the current
    Thread pool-1-thread-9 is about to exit
    Thread pool-1-thread-18, which is already in use
    Thread pool-1-thread-16 is about to exit
    Thread pool-1-thread-15 entry, the current has
    Thread pool-1-thread-14 is about to exit
    Thread pool-1-thread-19 to enter, the current already exists
    Thread pool-1-thread-18 is about to exit
    Thread pool-1-thread-20, which is currently in use
    Thread pool-1-thread-11 is about to exit
    Thread pool-1-thread-17 is about to exit
    Thread pool-1-thread-15 is about to exit
    Thread pool-1-thread-19 is about to exit
    Thread pool-1-thread-20 is about to exit