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

package com.iluwatar;

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.
 *  
 *  In this example we create a list of tasks presenting work to be done. Each task is then 
 *  wrapped into a Worker object that implements Runnable. We create an ExecutorService with
 *  fixed number of threads (Thread Pool) and use them to execute the Workers.
 *
 */
public class App {
	
    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()) {
    	}
    	System.out.println("Program finished");
    }
}