making sure one task completes before another starts in java

making sure one task completes before another starts in java

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making sure one task completes before another starts in java
Tag : java , By : PaulPlum
Date : November 24 2020, 05:47 AM

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Java executors: how to be notified, without blocking, when a task completes?

Tag : java , By : Dré
Date : March 29 2020, 07:55 AM
This might help you Define a callback interface to receive whatever parameters you want to pass along in the completion notification. Then invoke it at the end of the task.
You could even write a general wrapper for Runnable tasks, and submit these to ExecutorService. Or, see below for a mechanism built into Java 8.
class CallbackTask implements Runnable {

  private final Runnable task;

  private final Callback callback;

  CallbackTask(Runnable task, Callback callback) {
    this.task = task;
    this.callback = callback;

  public void run() {

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;

public class GetTaskNotificationWithoutBlocking {

  public static void main(String... argv) throws Exception {
    ExampleService svc = new ExampleService();
    GetTaskNotificationWithoutBlocking listener = new GetTaskNotificationWithoutBlocking();
    CompletableFuture<String> f = CompletableFuture.supplyAsync(svc::work);
    System.out.println("Exiting main()");

  void notify(String msg) {
    System.out.println("Received message: " + msg);


class ExampleService {

  String work() {
    sleep(7000, TimeUnit.MILLISECONDS); /* Pretend to be busy... */
    char[] str = new char[5];
    ThreadLocalRandom current = ThreadLocalRandom.current();
    for (int idx = 0; idx < str.length; ++idx)
      str[idx] = (char) ('A' + current.nextInt(26));
    String msg = new String(str);
    System.out.println("Generated message: " + msg);
    return msg;

  public static void sleep(long average, TimeUnit unit) {
    String name = Thread.currentThread().getName();
    long timeout = Math.min(exponential(average), Math.multiplyExact(10, average));
    System.out.printf("%s sleeping %d %s...%n", name, timeout, unit);
    try {
      System.out.println(name + " awoke.");
    } catch (InterruptedException abort) {
      System.out.println(name + " interrupted.");

  public static long exponential(long avg) {
    return (long) (avg * -Math.log(1 - ThreadLocalRandom.current().nextDouble()));


Starts several tasks at once and then start another task when one completes

Tag : chash , By : KS9
Date : March 29 2020, 07:55 AM
I hope this helps you . I have a code that runs in a asp.net web api which download image files from another sites. , You can use SemaphoreSlim to throttle asynchronous work:
private readonly SemaphoreSlim _mutex = new SemaphoreSlim(10);


var downloads = images.Select(file => DownloadAsync(file));
await Task.WhenAll(downloads);


private async Task DownloadAsync(string file)
  await _mutex.WaitAsync();
    using (WebClient client = new WebClient())
      await client.DownloadFileTaskAsync(file, path + Path.GetFileName(file));                    

Java Timer is waiting for Task to finish before next Task starts

Tag : java , By : Joe Sweeney
Date : March 29 2020, 07:55 AM
wish helps you Quoting the Javadoc:

Java application hungs up for long time even script on remote server completes the task

Tag : java , By : Sonal
Date : March 29 2020, 07:55 AM

With Spark Java RDDs, how can I process individual task results immediately as each one completes without waiting for co

Tag : java , By : jonagh
Date : March 29 2020, 07:55 AM
like below fixes the issue Either you push your logic to be distributed as well (e.g. using a udf) or you can create a similar process in spark streaming and work with small batches
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