Concurrency control Artifact Semaphore Java Concurrency Tool class

Semaphore (semaphore) is used to control the number of threads that are notified of access to a particular resource by coordinating individual threads to ensure reasonable use of public resources.

We can understand so semaphore, such as a toilet only 6 pits, while only to meet 6 people to the toilet (abnormal), others want to squat, only waiting in line, if someone from the toilet out, the back of a person can go in. In this example, the person is a thread, the squat pit indicates that the thread is executing, leaving indicates that the thread has finished executing, and the number of pits represents the semaphore number.

I. Application scenarios for Semaphore

Semaphore can be used for traffic control, especially for applications with limited public resources, such as database connections. If there is a need to read tens of thousands of files of data, because all are IO-intensive tasks, we can start dozens of threads to read concurrently, but if the memory is read, but also need to store in the database, and the number of database connections only 10, At this point we have to control only 10 threads to get the database connection to save the data, otherwise the error will not get the database connection. This time, you can use semaphore to do flow control. The simple implementation is as follows:

　　

 Public classSemaphoretest {Private Static Final intThread_count = 30; Private StaticExecutorservice ThreadPool =Executors.newfixedthreadpool (Thread_count); //only 10 database links, creating 10 semaphores here    Private StaticSemaphore Semaphore =NewSemaphore (10);  Public Static voidMain (string[] args) {BooleanShutdownthreadpool =false;  for(intindex = 0; Index < Thread_count; index++) {Threadpool.execute (NewRunnable () {@Override Public voidrun () {Try {                        //Get a signalSemaphore.acquire (); //Perform ActionsSystem.out.println ("Wait for write data ..."); Thread.Sleep (1000); //Release Signalsemaphore.release (); } Catch(interruptedexception e) {e.printstacktrace ();            }                }            }); if(Index = = thread_count-1) {Shutdownthreadpool=true; }        }         while(!Shutdownthreadpool)        {Threadpool.shutdown (); }    }}

Although 30 threads are created in the code above, there can be only one thread executing concurrently. The semaphore method Semaphore (int permits) accepts an integer number that represents the number of licenses available. Semaphore (10) indicates that 10 threads are allowed to obtain a license, that is, the maximum number of concurrent is 10. Semaphore is also very simple to use, first the thread uses the semaphore acquire () method to obtain a license, and then call the release () method to return the license. You can also use the Tryacquire () method to try to obtain a license.

Two. Other methods of semaphore

　　int availablepermits (): Returns the number of licenses currently available

　　int getqueuelength (): Gets the number of threads that are waiting to acquire a license

　　boolean hasqueuedthreads (): Gets whether there are still threads waiting to get a license

　　void reducepermits (int reduction): reduction of reduction licenses

Concurrency control Artifact Semaphore Java Concurrency Tool class