Difference between Spring MVC's @Async, DeferredResult and Callable

Question

I\'ve a long-running task defined in a Spring service. It is started by a Spring MVC controller. I want to start the service and return back an HttpResponse to the

Answer 1

Your controller is eventually a function executed by the servlet container (I will assume it is Tomcat) worker thread. Your service flow start with Tomcat and ends with Tomcat. Tomcat gets the request from the client, holds the connection, and eventually returns a response to the client. Your code (controller or servlet) is somewhere in the middle.

Consider this flow:

1. Tomcat get client request.
2. Tomcat executes your controller.
3. Release Tomcat thread but keep the client connection (don't return response) and run heavy processing on different thread.
4. When your heavy processing complete, update Tomcat with its response and return it to the client (by Tomcat).

Because the servlet (your code) and the servlet container (Tomcat) are different entities, then to allow this flow (releasing tomcat thread but keep the client connection) we need to have this support in their contract, the package javax.servlet, which introduced in Servlet 3.0 . Now, getting back to your question, Spring MVC use the new Servlet 3.0 capability when the return value of the controller is DeferredResult or Callable, although they are two different things. Callable is an interface that is part of java.util, and it is an improvement for the Runnable interface (should be implemented by any class whose instances are intended to be executed by a thread). Callable allows to return a value, while Runnable does not. DeferredResult is a class designed by Spring to allow more options (that I will describe) for asynchronous request processing in Spring MVC, and this class just holds the result (as implied by its name) while your Callable implementation holds the async code. So it means you can use both in your controller, run your async code with Callable and set the result in DeferredResult, which will be the controller return value. So what do you get by using DeferredResult as the return value instead of Callable? DeferredResult has built-in callbacks like onError, onTimeout, and onCompletion. It makes error handling very easy.In addition, as it is just the result container, you can choose any thread (or thread pool) to run on your async code. With Callable, you don't have this choice.

Regarding @Async, it is much more simple – annotating a method of a bean with @Async will make it execute in a separate thread. By default (can be overridden), Spring uses a SimpleAsyncTaskExecutor to actually run these methods asynchronously.

In conclusion, if you want to release Tomcat thread and keep the connection with the client while you do heavy processing, then your controller should return Callable or DeferredResult. Otherwise, you can run the code on method annotated with @Async.