Java 實現 Comet 長連接技術：Tomcat、Jetty 與 Servlet 3.0 異步處理對比

Comet 反向 Ajax 技術在 Java 容器中的三種實現範式

Comet（Server-Side Push）讓服務器主動推送數據到瀏覽器，打破 HTTP 請求-響應模型。本文對比 Tomcat 原生 API、Jetty Continuation、Servlet 3.0 標準化異步三種實現路徑。

Comet 三大變體對比

Tomcat 實現：CometProcessor 接口

// server.xml 配置 NIO 連接器
<Connector port="8080" protocol="org.apache.coyote.http11.Http11NioProtocol" />

// Servlet 實現
public class WeatherServlet extends HttpServlet implements CometProcessor {
    private MessageSender sender = new MessageSender();
    
    public void event(CometEvent event) throws IOException, ServletException {
        HttpServletRequest req = event.getHttpServletRequest();
        HttpServletResponse resp = event.getHttpServletResponse();
        
        switch (event.getEventType()) {
            case BEGIN:
                req.setAttribute("org.apache.tomcat.comet.timeout", 60_000);
                sender.setConnection(resp);
                new Thread(new DataProducer(sender)).start();
                break;
            case END: case ERROR:
                event.close();
                break;
            case READ:
                throw new UnsupportedOperationException();
        }
    }
}

特點：Tomcat 專有 API，事件模型清晰 (BEGIN/READ/END/ERROR)，需配置 NIO 連接器。

Jetty 實現：Continuation 機制

public class WeatherServlet extends HttpServlet {
    private MessageSender sender = new MessageSender();
    
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) 
            throws IOException, ServletException {
        Continuation cont = ContinuationSupport.getContinuation(req);
        if (cont.isPending()) {
            // 恢復階段：發送數據
            resp.getWriter().write((String) cont.getAttribute("data"));
            cont.complete();
        } else {
            // 掛起階段：釋放線程等待數據
            cont.suspend(5_000); // 5秒超時
            new Thread(new DataProducer(cont)).start();
        }
    }
}

特點：Jetty 專有 Continuation API，語義為「掛起-恢復」，線程釋放更直觀。

Servlet 3.0 標準化：AsyncContext

@WebServlet(urlPatterns = "/weather", asyncSupported = true, asyncTimeout = 5000)
public class WeatherServlet extends HttpServlet {
    private MessageSender sender = new MessageSender();
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) 
            throws IOException, ServletException {
        AsyncContext async = req.startAsync(req, resp);
        sender.setConnection(async); // 傳遞 AsyncContext 而非 Response
        async.start(new DataProducer(sender)); // 容器管理線程池
    }
}

// 發送端
class DataProducer implements Runnable {
    private AsyncContext async;
    public void run() {
        try {
            async.getResponse().getWriter().write("data...");
            async.complete(); // 關鍵：標記完成
        } catch (IOException e) { /* handle */ }
    }
}

優勢：標準化、可移植、容器管理線程池、支援過濾器鏈異步、超時自動處理。

架構選型建議

• 新項目：強制 Servlet 3.0+ (Tomcat 7+/Jetty 8+)，代碼可移植
• 舊版容器：Tomcat 6 用 CometProcessor，Jetty 6/7 用 Continuation
• 高併發場景：結合 Netty/Undertow 等非阻塞框架，或升級 Servlet 3.1 非阻塞 IO
• 生產環境：考慮 WebSocket (JSR 356) 替代 Comet，雙向通訊、更低開銷

客戶端通用代碼 (長輪詢)

function poll() {
    fetch('/weather')
        .then(r => r.text())
        .then(data => {
            document.getElementById('forecast').innerHTML = data;
            poll(); // 立即發起下一輪
        })
        .catch(() => setTimeout(poll, 5000)); // 錯誤重試
}
poll();