i have created an async Task-Thread which creates inside with “Executors.newFixedThreadPool(8)” 8-Worker-Threads for long running jobs. Lets say i have 8 long running jobs, all works fine. All 8 Worker-Jobs complete in for example 10seconds. The Problem is now, if i set the FixedThreadPoolSize to 7 (One Thread less than my amount of Jobs) and i get again my 8 Jobs. It took not round about 10 seconds, it took nearly 70seconds. But why is this so? It looks like there are no more threads available from the pool. But the 7 Threads before are done. And for the last Thread he took 60seconds.

My TaskThread looks like this:

 public TaskThread(ExecutorService executorService, List<String> workerJobs) {
    this.workerJobs = workerJobs;
    this.executorService = Executors.newFixedThreadPool(7, Thread.ofVirtual().name("Thread-", 1).factory());
  }


  @Override
  public void run() {
    List<CompletableFuture<String>> jobs = workerJobs.stream().map(job ->
      CompletableFuture.supplyAsync(new WorkerJob(),executorService)
    ).toList();

    List<String> taskResultModels = jobs
      .stream()
      .map(CompletableFuture::join)
      .toList();
    
    //Save result to DB
  }
}

And my WorkerThread like this:

public class WorkerJob implements Supplier<String> {

  @Override
  public String get() {
    try{
      return "Long Running Job";
    }catch(Exception e){
      return "Error";
    }
  }
}

I start the TaskThread also with an ExecutorService:

Executors.newSingleThreadExecutor(new TaskThread()).submit()