Introduction

Asynchronous servlets let you release the original container thread while work continues in the background. The two key control methods are dispatch() and complete(), and they mean different things: dispatch() sends processing back through the servlet container, while complete() tells the container the async request is finished and the response can be closed out.

Starting Async Processing

A servlet enters async mode by calling startAsync() on a request that supports async processing:

1import java.io.IOException;
2import jakarta.servlet.AsyncContext;
3import jakarta.servlet.ServletException;
4import jakarta.servlet.annotation.WebServlet;
5import jakarta.servlet.http.HttpServlet;
6import jakarta.servlet.http.HttpServletRequest;
7import jakarta.servlet.http.HttpServletResponse;
8
9@WebServlet(value = "/work", asyncSupported = true)
10public class AsyncServlet extends HttpServlet {
11    @Override
12    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
13        AsyncContext asyncContext = req.startAsync();
14
15        asyncContext.start(() -> {
16            try {
17                Thread.sleep(500);
18                resp.getWriter().write("done");
19                asyncContext.complete();
20            } catch (Exception e) {
21                throw new RuntimeException(e);
22            }
23        });
24    }
25}

Once async mode starts, the original request thread can return to the container instead of blocking until the background work completes.

What complete() Does

complete() means: “the asynchronous cycle is over.” After you write the response and call complete(), the container can finalize the response and release request resources.

Use complete() when your async code has fully generated the response and no further servlet or filter processing is needed.

That makes it the terminal action for a fully self-contained async workflow.

What dispatch() Does

dispatch() means: “resume processing through the container.” Instead of finishing the request immediately, it forwards control to another servlet, JSP, or mapped path.

Example:

1AsyncContext asyncContext = req.startAsync();
2
3asyncContext.start(() -> {
4    req.setAttribute("result", "background work finished");
5    asyncContext.dispatch("/result");
6});

In this case, the container will route the request to "/result", where normal servlet processing continues with the request attributes preserved.

This is useful when the background task prepares data but a standard servlet or JSP should produce the final response.

dispatch() Is Not the Same as complete()

This is the key behavioral distinction:

• 'complete() ends async processing and finalizes the response'
• 'dispatch() hands the request back to the container for another processing stage'

After dispatch(), the request is not “done” in the same way. The target resource gets a chance to process it, and that resource can write the response or even start async processing again if needed.

So dispatch() continues the lifecycle. complete() ends it.

Do Not Call Them Carelessly Together

A common source of confusion is whether you should call complete() after dispatch(). In normal usage, no. Once you dispatch, the target resource or the container-managed flow is responsible for what happens next.

Calling complete() immediately after dispatch() from the original async code can interfere with the intended lifecycle because you are trying to end the request while also asking the container to continue processing it.

The safe mental model is:

• if this async block generates the final response, call complete()
• if this async block hands off to another resource, call dispatch()

Choose one based on the control flow you need.

Timeouts and Error Handling Still Matter

Async servlets can time out. They can also fail on background threads where normal synchronous error flow does not behave quite the same way. That means you should set timeouts deliberately and register listeners when the workflow matters.

Even though the API is asynchronous, the lifecycle is still owned by the container. Ignoring timeout and completion behavior usually leads to hanging requests or incomplete responses.

Common Pitfalls

The biggest mistake is thinking dispatch() finishes the request. It does not; it routes the request back into the container for more processing.

Another issue is calling complete() after dispatch() without a clear lifecycle reason. In most cases, the two represent different endings to the async block, not steps that should always be chained.

Developers also sometimes forget to mark the servlet as asyncSupported = true, which prevents async processing from working at all.

Finally, background threads still need careful exception handling. If async work fails silently and neither dispatches nor completes properly, the request can stall.

Summary

• 'startAsync() moves request handling into asynchronous mode.'
• 'complete() ends the async lifecycle and finalizes the response.'
• 'dispatch() hands the request back to the container for more processing.'
• Use complete() when your async code finishes the response itself.
• Use dispatch() when another servlet or view should continue request processing.