Java Semaphore examples
By：Roy.LiuLast updated：2019-08-11
In Java, we can use Semaphore to limit the number of threads to access a certain resource.
1. What is Semaphore?
In short, a semaphore maintains a set of permits (tickets), each acquire() will take a permit (ticket) from semaphore, each release() will return back the permit (ticket) back to the semaphore. If permits (tickets) are not available, acquire() will block until one is available.
// 5 tickets Semaphore semaphore = new Semaphore(5); // take 1 ticket semaphore.acquire(); // 4 semaphore.availablePermits(); // return back ticket semaphore.release(); // 5 semaphore.availablePermits();
2. Java Semaphore
A Java Semaphore example to limit the number of tasks running in ExecutorService. In this example, 5 Callable tasks are submitted to ExecutorService, but only 2 tasks are running concurrently.
TaskLimitSemaphore.java
package com.mkyong.concurrency.synchronizer.semaphore;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.*;
// Throttle task submission
public class TaskLimitSemaphore {
private final ExecutorService executor;
private final Semaphore semaphore;
public TaskLimitSemaphore(ExecutorService executor, int limit) {
this.executor = executor;
this.semaphore = new Semaphore(limit);
public <T> Future<T> submit(final Callable<T> task) throws InterruptedException {
semaphore.acquire();
System.out.println("semaphore.acquire()...");
return executor.submit(() -> {
try {
return task.call();
} finally {
semaphore.release();
System.out.println("semaphore.release()...");
});
private static final DateFormat sdf = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
public static void main(String[] args) throws InterruptedException {
ExecutorService executor = Executors.newCachedThreadPool();
// Only 2 tasks are able to run concurrently
TaskLimitSemaphore obj = new TaskLimitSemaphore(executor, 2);
obj.submit(() -> {
System.out.println(getCurrentDateTime() + " : task1 is running!");
Thread.sleep(2000);
System.out.println(getCurrentDateTime() + " : task1 is done!");
return 1;
});
obj.submit(() -> {
System.out.println(getCurrentDateTime() + " : task2 is running!");
Thread.sleep(2000);
System.out.println(getCurrentDateTime() + " task2 is done!");
return 2;
});
obj.submit(() -> {
System.out.println(getCurrentDateTime() + " task3 is running!");
Thread.sleep(2000);
System.out.println(getCurrentDateTime() + " task3 is done!");
return 3;
});
obj.submit(() -> {
System.out.println(getCurrentDateTime() + " task4 is running!");
Thread.sleep(2000);
System.out.println(getCurrentDateTime() + " task4 is done!");
return 4;
});
obj.submit(() -> {
System.out.println(getCurrentDateTime() + " task5 is running!");
Thread.sleep(2000);
System.out.println(getCurrentDateTime() + " task5 is done!");
return 5;
});
executor.shutdown();
private static String getCurrentDateTime() {
return sdf.format(new Date());
Output
semaphore.acquire()... semaphore.acquire()... 2018/12/06 18:45:22 : task1 is running! 2018/12/06 18:45:22 : task2 is running! 2018/12/06 18:45:24 : task1 is done! 2018/12/06 18:45:24 task2 is done! semaphore.release()... semaphore.acquire()... semaphore.release()... semaphore.acquire()... 2018/12/06 18:45:24 task3 is running! 2018/12/06 18:45:24 task4 is running! 2018/12/06 18:45:26 task4 is done! 2018/12/06 18:45:26 task3 is done! semaphore.acquire()... semaphore.release()... semaphore.release()... 2018/12/06 18:45:26 task5 is running! 2018/12/06 18:45:28 task5 is done! semaphore.release()...
3. Mutex
In short, always new Semaphore(1), only one thread is allowed to access a certain resource.
PrintSequenceCallable.java
package com.mkyong.concurrency.synchronizer.semaphore;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
public class SshLoginSemaphore {
private final Semaphore mutex;
// only 1 user is allow
public SshLoginSemaphore() {
this.mutex = new Semaphore(1);
private void ssh(String user) throws InterruptedException {
mutex.acquire();
System.out.println(getCurrentDateTime() + " : " + user + " mutex.acquire()");
Thread.sleep(2000);
mutex.release();
System.out.println(getCurrentDateTime() + " : " + user + " mutex.release()");
private static final DateFormat sdf = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
public static void main(String[] args) {
ExecutorService executor = Executors.newFixedThreadPool(5);
SshLoginSemaphore task = new SshLoginSemaphore();
// submit 3 tasks
executor.submit(() -> {
try {
task.ssh("mkyong");
} catch (InterruptedException e) {
e.printStackTrace();
});
executor.submit(() -> {
try {
task.ssh("yflow");
} catch (InterruptedException e) {
e.printStackTrace();
});
executor.submit(() -> {
try {
task.ssh("zilap");
} catch (InterruptedException e) {
e.printStackTrace();
});
executor.shutdown();
private static String getCurrentDateTime() {
return sdf.format(new Date());
Output
Review the time (seconds), only one thread is allowed, one acquire() and one release()
2018/12/06 18:54:25 : mkyong mutex.acquire() 2018/12/06 18:54:27 : yflow mutex.acquire() 2018/12/06 18:54:27 : mkyong mutex.release() 2018/12/06 18:54:29 : zilap mutex.acquire() 2018/12/06 18:54:29 : yflow mutex.release() 2018/12/06 18:54:31 : zilap mutex.release()
References
From：一号门
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