TaskLimitSemaphore.java

package com.codecafe.concurrency.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 support 2 tasks
    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());
  }
}