Create a Job Queue or Task Controller and dynamically add task to it in Java

Question

I want to create a job queue to execute multiple tasks. but, my requirement is I should be able to add tasks to that job queue at any time and all those tasks should be executed sequentially. I searched for some solutions on the internet and found these two links:

1. Java Thread Pool Executor Example

2. Java Executor Framework Tutorial and Best Practices

But I can't use both of these solutions. Because after starting the ExecutorService I can't add a new task to the service. Because we know that It may throw InterruptedException or ConcurrentModificationException.

Answer 1

You can use a BlockingQueue to keep waiting in a separate thread until one or more Runnable show up.

public class Mainer {
    private static final BlockingQueue<Runnable> queue = new ArrayBlockingQueue<>(15);

    public static void main(String[] args) {
        Thread t = new Thread(() -> {
            while (true) {
                try {
                    queue.take().run();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

            }
        });
        t.start();

        for (int i = 0; i < 10; i++) {
            queue.add(() -> {
                System.out.println("Hello");
            });
        }
    }

}

Answer 2

I think that you should be using an ExecutorService.

• It maintains a queue of tasks to be run. You can provide your own queue if you need to.
• New tasks can be added at any time using the submit method.
• Futures may be obtained when submitting tasks.
• Tasks may be run one at a time, or in parallel using a pool of worker threads.
• If you use multiple threads, the various executor service implementations provide different pool management strategies.
• There are operations for draining and shutting down the service.

If you want an ExecutorService that will always run the tasks one at a time, make sure that you specify a thread pool with a maximum size of one thread.

you please give some example links

The javadocs have examples.

---

The objections that you raised are all based on incorrect information.

1. But I can't use both of these solution. Because after starting ExecutorService I can't add new task to the service.

Yes you can. That is what ExecutorService.submit(...) does.

2. Because we know that it may throw InterruptedException ...

It will only throw InterruptedException if your application does something to cause that to happen; e.g. shutting down the ExecutorService using shutdownNow(). (If you don't want that, you can use shutdown() instead, or simply not shut down the executor at all. Refer to the javadocs for more details.)

Anyway, your code can deal with the exception, as the code in the other answer does ... albeit that it handles it in a way that you shouldn't do in production code. (Printing a stacktrace to standard output is a bad idea. And ignoring an InterruptedException could be wrong thing, in some contexts.)

3. ... or ConcurrentModificationException.

AFAIK, the only scenario where an ExecutorService can throw ConcurrentModificationException is if you are using a custom Queue class that isn't implemented properly.

Answer 3

Adding implementation to Stephen answer

public class MyController {
    ExecutorService executor = Executors.newSingleThreadExecutor();
    Logger logger = Logger.getLogger(MyController.class.getName());

    //POST
    public void addTask(String taskId){
        Runnable task = createTask(taskId);
        executor.submit(task);
    }

    public void shutdown(){
        executor.shutdown();
    }

    private Runnable createTask(String taskId) {
        return new Runnable() {
            @Override
            public void run() {
                try {
                    taskId.toString();
                    //your task
                }catch (Exception e){
                    logger.log(Level.SEVERE,"error executing task"+taskId);
                }
            }
        };
    }
}