Implementing Asynchronous Requests in Spring Boot: Callable, WebAsyncTask, and DeferredResult

Introduction – Before Servlet 3.0 each HTTP request was handled by a single thread from start to finish. Servlet 3.0 introduced asynchronous processing, allowing the container thread to be released early, reducing load and increasing throughput.

In a Spring Boot application there are four ways to implement async endpoints: ResponseBodyEmitter , SseEmitter , StreamingResponseBody (not covered here), and the three methods described below: AsyncContext (native servlet, rarely used), Callable , WebAsyncTask , and DeferredResult .

Using Callable – Returning a java.util.concurrent.Callable from a controller method marks the endpoint as asynchronous. Spring MVC starts async processing, submits the Callable to an AsyncTaskExecutor , and releases the servlet thread while the result is computed in another thread.

@GetMapping("/testCallAble")
public Callable
testCallAble() {
    return () -> {
        Thread.sleep(40000);
        return "hello";
    };
}

The client still receives a plain String response, identical to a synchronous method.

Using WebAsyncTask – WebAsyncTask is a wrapper around Callable that adds timeout, error, and completion callbacks.

@GetMapping("/webAsyncTask")
public WebAsyncTask
webAsyncTask() {
    WebAsyncTask
result = new WebAsyncTask<>(30003, () -> {
        return "success";
    });
    result.onTimeout(() -> {
        log.info("timeout callback");
        return "timeout callback";
    });
    result.onCompletion(() -> log.info("finish callback"));
    return result;
}

The timeout set on the WebAsyncTask overrides any global timeout configuration.

Using DeferredResult – DeferredResult allows the controller to return immediately while another thread later sets the actual result.

private Map
> deferredResultMap = new ConcurrentHashMap<>();

@GetMapping("/testDeferredResult")
public DeferredResult
testDeferredResult() {
    DeferredResult
deferredResult = new DeferredResult<>();
    deferredResultMap.put("test", deferredResult);
    return deferredResult;
}

@GetMapping("/testSetDeferredResult")
public String testSetDeferredResult() throws InterruptedException {
    DeferredResult
deferredResult = deferredResultMap.get("test");
    boolean flag = deferredResult.setResult("testSetDeferredResult");
    if (!flag) {
        log.info("Result already processed, operation ignored");
    }
    return "ok";
}

When another thread calls deferredResult.setResult(...) , the pending request completes with that value. It is also possible to configure a per‑request timeout that takes precedence over the global setting.

Providing a Thread Pool – Asynchronous handling should use a dedicated thread pool instead of the default executor.

@Bean("mvcAsyncTaskExecutor")
public AsyncTaskExecutor asyncTaskExecutor() {
    ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
    executor.setCorePoolSize(5);
    executor.setMaxPoolSize(10);
    executor.setQueueCapacity(10);
    executor.setThreadNamePrefix("fyk-mvcAsyncTask-Thread-");
    asyncServiceExecutor.setBeanName("TaskId" + taskId);
    executor.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy());
    executor.setWaitForTasksCompleteOnShutdown(true);
    executor.setAwaitTerminationSeconds(30);
    executor.initialize();
    return executor;
}

@Configuration
public class FykWebMvcConfigurer implements WebMvcConfigurer {
    @Autowired
    @Qualifier("mvcAsyncTaskExecutor")
    private AsyncTaskExecutor asyncTaskExecutor;

    @Override
    public void configureAsyncSupport(AsyncSupportConfigurer configurer) {
        // 0 or negative means never timeout
        configurer.setDefaultTimeout(60001);
        configurer.setTaskExecutor(asyncTaskExecutor);
    }
}

When to Use Asynchronous Requests – Async processing improves throughput only when the request spends a lot of time waiting (e.g., calling external services). If the request is CPU‑bound, moving work to another thread does not help and may even add overhead.

Conclusion – The article ends with a promotional note encouraging readers to follow the author’s public account for PDF collections of Spring Cloud, Spring Boot, and MyBatis advanced tutorials.