Executing Java callback on a new thread

Question

On this project, a Manager performs event queuing, and to return the result of the event a callback is used (the callback does not extend Runnable). The manager runs on a separate thread, dispatching the events. Once the events terminate, this same thread calls the callbacks. This means that the next event will not be dispatched before the callback of the previous event terminates. In order to avoid this, I though about having the manager create a new thread for each callback, and executing the callbacks there. How good is this solution in terms of design practices, and is there a better way to achieve this?

Answer 1

A simple Callback code:

import java.util.concurrent.*;
import java.util.*;

public class CallBackDemo{
    public CallBackDemo(){
        System.out.println("creating service");
        ExecutorService service = Executors.newFixedThreadPool(10);

        try{
            for ( int i=0; i<10; i++){
                Callback callback = new Callback(i+1);
                MyCallable myCallable = new MyCallable((long)i+1,callback);
                Future<Long> future = service.submit(myCallable);
                //System.out.println("future status:"+future.get()+":"+future.isDone());
            }
        }catch(Exception err){
            err.printStackTrace();
        }
        service.shutdown();
    }
    public static void main(String args[]){
        CallBackDemo demo = new CallBackDemo();
    }
}
class MyCallable implements Callable<Long>{
    Long id = 0L;
    Callback callback;
    public MyCallable(Long val,Callback obj){
        this.id = val;
        this.callback = obj;
    }
    public Long call(){
        //Add your business logic
        System.out.println("Callable:"+id+":"+Thread.currentThread().getName());
        callback.callbackMethod();
        return id;
    }
}
class Callback {
    private int i;
    public Callback(int i){
        this.i = i;
    }
    public void callbackMethod(){
        System.out.println("Call back:"+i);
        // Add your business logic
    }
}

output:

creating service
Callable:1:pool-1-thread-1
Call back:1
Callable:2:pool-1-thread-2
Call back:2
Callable:8:pool-1-thread-8
Call back:8
Callable:3:pool-1-thread-3
Call back:3
Callable:10:pool-1-thread-10
Callable:4:pool-1-thread-4
Call back:10
Callable:7:pool-1-thread-7
Call back:7
Callable:6:pool-1-thread-6
Call back:6
Callable:9:pool-1-thread-9
Callable:5:pool-1-thread-5
Call back:9
Call back:4
Call back:5

Summary:

1. Replace Manager with ExecutorService of your preferred choice.
2. Either your can pass Callaback object to Callable/Runnable object Or you can create Callback object inside Callable/Runnable. In my example, I have explicitly passed Callback object to Callable.

3. Before returning the result, Callable object invokes Callback method. If you want to block on proceeding further unless you get response from current event, just uncomment below line.

   System.out.println("future status:"+future.get()+":"+future.isDone());
   

I think you are going to avoid it and hence keep above line commented. You don't have to create new thread for Callback method invocation. If you want to process Callback event asynchronously, you can create one more ExecutorService and submit the event.

Answer 2

I would have the thread which executes the task, also execute the call back. Instead of creating a Thread each time, I suggest you use an ExecutorService.

public static <T> void submit(ExecutorService service, 
                              Callable<T> callable,
                              Consumer<T> callback) {
    service.submit(() -> {
         try {
             callback.accept(callable.call());
         } catch (Throwable t) {
             // log the Throwable
         }
     });
 }