When to use Spring @Async vs Callable controller (async controller, servlet 3)

Question

I would like to know the general use case of using @Async and Servlet 3 asynchronous request implementation in Spring using Callable.

As I understand, @Async is for making any method (specifically, any service method) execute asynchronously.

@Async
void doSomething(String s) {
// this will be executed asynchronously
}

and any controller which returns Callable

  @RequestMapping("/view")
public Callable<String> callableWithView(final Model model) {
    return new Callable<String>() {
        @Override
        public String call() throws Exception {
            Thread.sleep(2000);
            model.addAttribute("foo", "bar");
            model.addAttribute("fruit", "apple");
            return "views/html";
        }
    };
}

I am confused on whento use what. What will be the effect of using Asynchronous servlet/controller and with spring @Async together?

Solution

This post has explanation for what you are looking for

Excerpt:

In some cases you can return to the client immediately while a background job completes processing. For example sending an email, kicking off a database job, and others represent fire-and-forget scenarios that can be handled with Spring's @Async support or by posting an event to a Spring Integration channel and then returning a confirmation id the client can use to query for the results.

Callable return type makes a controller method asynchronous. This is usually used in situations like long polling. Read this post by the same author for more information.

Also callable is an alternative for Runnable, in the sense, It can return results and throw checked exceptions.

Say you have a method

public String aMethod(){

}

This can be made asynchronous by simply returning a Callable interface.

public Callable<String>  aMethod(){

}