ExecutorService that executes tasks sequentially but takes threads from a pool

Question

I am trying to build an implementation of the ExecutorService, let\'s call it SequentialPooledExecutor, with the following properties.

Answer 1

private Map> sessionTasks = new HashMap<>();
private ExecutorService pool = Executors.newFixedThreadPool(200);

public void submit(int sessionId, Runnable task) {  
    if (sessionTasks.containsKey(sessionId)) {
        sessionTasks.compute(sessionId, (i, c) -> c.thenRunAsync(task, pool));
    } else {
        sessionTasks.put(sessionId, CompletableFuture.runAsync(task, pool));
    }
}

If a session has no task, a new task is created and run in the provided pool. If a session already has a tasks when a new task is added, the latter is chained (with thenRun) to the previous one, ensuring order.