Async Servlet

package com.example;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.WebServlet;
import java.io.*;
import java.util.concurrent.*;

// asyncSupported=true is REQUIRED to enable async processing
@WebServlet(urlPatterns = "/async-task", asyncSupported = true)
public class AsyncServlet extends HttpServlet {

    // Use a thread pool for async tasks
    private ExecutorService executor;

    @Override
    public void init() {
        executor = Executors.newFixedThreadPool(10);
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        response.setContentType("text/html;charset=UTF-8");

        // 1. Start async processing — releases the request thread
        AsyncContext asyncContext = request.startAsync();

        // 2. Set a timeout (milliseconds). Default is 30000ms.
        asyncContext.setTimeout(60000);

        // 3. Add a listener to handle completion/timeout/error
        asyncContext.addListener(new AsyncListener() {
            public void onComplete(AsyncEvent event) throws IOException {
                System.out.println("Async task completed.");
            }
            public void onTimeout(AsyncEvent event) throws IOException {
                HttpServletResponse resp = (HttpServletResponse) event.getSuppliedResponse();
                resp.sendError(HttpServletResponse.SC_GATEWAY_TIMEOUT, "Request timed out.");
                event.getAsyncContext().complete();
            }
            public void onError(AsyncEvent event) throws IOException {
                System.err.println("Async error: " + event.getThrowable());
                event.getAsyncContext().complete();
            }
            public void onStartAsync(AsyncEvent event) {}
        });

        // 4. Submit the long-running task to the thread pool
        executor.submit(() -> {
            try {
                // Simulate a long-running operation (e.g., DB query, API call)
                Thread.sleep(3000);

                // Write the response from the async thread
                PrintWriter out = asyncContext.getResponse().getWriter();
                out.println("<h2>Async Task Completed!</h2>");
                out.println("<p>Processed in background thread: "
                          + Thread.currentThread().getName() + "</p>");

            } catch (Exception e) {
                try {
                    asyncContext.getResponse().getWriter()
                                .println("Error: " + e.getMessage());
                } catch (IOException ignored) {}
            } finally {
                // 5. MUST call complete() to commit the response
                asyncContext.complete();
            }
        });

        // The request thread is now free to handle other requests
        System.out.println("Request thread released: " + Thread.currentThread().getName());
    }

    @Override
    public void destroy() {
        executor.shutdown();
    }
}

@WebServlet(urlPatterns = "/report", asyncSupported = true)
public class AsyncDispatchServlet extends HttpServlet {

    private ExecutorService executor = Executors.newCachedThreadPool();

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        AsyncContext asyncContext = request.startAsync(request, response);
        asyncContext.setTimeout(30000);

        executor.submit(() -> {
            try {
                // Simulate fetching report data
                Thread.sleep(2000);
                String reportData = generateReport();

                // Store result as request attribute
                asyncContext.getRequest().setAttribute("reportData", reportData);

                // Dispatch to JSP for rendering (runs in a new request thread)
                asyncContext.dispatch("/WEB-INF/views/report.jsp");

            } catch (Exception e) {
                asyncContext.dispatch("/WEB-INF/views/error.jsp");
            }
        });
    }

    private String generateReport() {
        return "Sales Report: $50,000 total revenue";
    }
}