java-design-patterns/thread-pool/src/main/java/com/iluwatar/threadpool/App.java

package com.iluwatar.threadpool;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * 
 * Thread Pool pattern is where a number of threads are created to perform a number of tasks, which
 * are usually organized in a queue. The results from the tasks being executed might also be placed
 * in a queue, or the tasks might return no result. Typically, there are many more tasks than
 * threads. As soon as a thread completes its task, it will request the next task from the queue
 * until all tasks have been completed. The thread can then terminate, or sleep until there are new
 * tasks available.
 * <p>
 * In this example we create a list of tasks presenting work to be done. Each task is then wrapped
 * into a {@link Worker} object that implements {@link Runnable}. We create an
 * {@link ExecutorService} with fixed number of threads (Thread Pool) and use them to execute the
 * {@link Worker}s.
 *
 */
public class App {

  /**
   * Program entry point
   * 
   * @param args command line args
   */
  public static void main(String[] args) {

    System.out.println("Program started");

    // Create a list of tasks to be executed
    List<Task> tasks = new ArrayList<>();
    tasks.add(new PotatoPeelingTask(3));
    tasks.add(new PotatoPeelingTask(6));
    tasks.add(new CoffeeMakingTask(2));
    tasks.add(new CoffeeMakingTask(6));
    tasks.add(new PotatoPeelingTask(4));
    tasks.add(new CoffeeMakingTask(2));
    tasks.add(new PotatoPeelingTask(4));
    tasks.add(new CoffeeMakingTask(9));
    tasks.add(new PotatoPeelingTask(3));
    tasks.add(new CoffeeMakingTask(2));
    tasks.add(new PotatoPeelingTask(4));
    tasks.add(new CoffeeMakingTask(2));
    tasks.add(new CoffeeMakingTask(7));
    tasks.add(new PotatoPeelingTask(4));
    tasks.add(new PotatoPeelingTask(5));

    // Creates a thread pool that reuses a fixed number of threads operating off a shared
    // unbounded queue. At any point, at most nThreads threads will be active processing
    // tasks. If additional tasks are submitted when all threads are active, they will wait
    // in the queue until a thread is available.
    ExecutorService executor = Executors.newFixedThreadPool(3);

    // Allocate new worker for each task
    // The worker is executed when a thread becomes
    // available in the thread pool
    for (int i = 0; i < tasks.size(); i++) {
      Runnable worker = new Worker(tasks.get(i));
      executor.execute(worker);
    }
    // All tasks were executed, now shutdown
    executor.shutdown();
    while (!executor.isTerminated()) {
      Thread.yield();
    }
    System.out.println("Program finished");
  }
}