ThreadPool in Java

Thread pools address two different problems.

1. They usually provide improved performance when executing large numbers of asynchronous tasks, due to reduced per-task invocation overhead.
2. They provide a means of bounding and managing the resources, including threads, consumed when executing a collection of tasks.

Lets look at simple program which gives better understanding of how ThreadPool works in Java.

class Workers implements Runnable {
  private final int value;
  Workers(int value) {
    this.value = value;
  }
  public void run() {
    System.out.println("Value = " + value);
  }
}

public class ThreadPoolExample {
  public static void main(String[] args) {
    // The Executors class provides factory methods for the executor services provided
    // by java.util package.
    // newFixThreadPool creates a thread pool that reuses a fixed set of threads
    // operating off a shared unbounded queue. If any thread terminates due to
    // a failure during execution prior to shutdown, a new one will take its place
    // if needed to execute subsequent tasks.
    ExecutorService executors = Executors.newFixedThreadPool(5);

    for (int i = 0; i < 10; i++) {
      // Submits a Runnable task for execution and returns a Future
      // representing that task.
      executors.submit(new Workers(i));
    }

    // An ExecutorService can be shut down, which will cause it to stop
    // accepting new tasks. After being shut down, the executor will
    // eventually terminate, at which point no tasks are actively executing,
    // no tasks are awaiting execution, and no new tasks can be submitted.
    executors.shutdown();
    try {
      // Blocks until all tasks have completed execution after a shutdown request,
      // or the timeout occurs, or the current thread is interrupted, whichever
      // happens first.
      executors.awaitTermination(1000, TimeUnit.SECONDS);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }
}

Result of the program might differ from this result.

Value = 0
Value = 2
Value = 3
Value = 4
Value = 5
Value = 7
Value = 8
Value = 9
Value = 1
Value = 6