實現反向Ajax功能，除了WebSocket之外，還有Servlet3.0提供的非同步Servlet.

實驗透過Servlet3.0的非同步Servlet實現獲取伺服器端JVM資訊的功能。
http://blog.itpub.net/29254281/viewspace-1064838/

1.非同步Servlet程式設定asyncSupported為true，並返回組裝的JS程式碼。

import java.io.IOException;

import java.io.PrintWriter;

import java.util.Date;

import java.util.Queue;

import java.util.concurrent.ConcurrentLinkedQueue;

import java.util.concurrent.Executors;

import java.util.concurrent.ScheduledExecutorService;

import java.util.concurrent.TimeUnit;

import javax.servlet.AsyncContext;

import javax.servlet.AsyncEvent;

import javax.servlet.AsyncListener;

import javax.servlet.ServletException;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

@WebServlet(urlPatterns = { "/ac.do" }, asyncSupported = true)

public class InitServlet extends HttpServlet implements Runnable {

    Queue<AsyncContext> queue = new ConcurrentLinkedQueue<AsyncContext>();

    ScheduledExecutorService singleThread = Executors.newSingleThreadScheduledExecutor();

    {

        singleThread.scheduleWithFixedDelay(this, 3, 3, TimeUnit.SECONDS);

    }

    @Override

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        doGet(req, resp);

    }

    @Override

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        final AsyncContext ac = req.startAsync();

        ac.setTimeout(10 * 60 * 1000);

        ac.addListener(new AsyncListener() {

            public void onComplete(AsyncEvent event) throws IOException {

                queue.remove(ac);

            }

            public void onTimeout(AsyncEvent event) throws IOException {

                queue.remove(ac);

            }

            public void onError(AsyncEvent event) throws IOException {

                queue.remove(ac);

            }

            public void onStartAsync(AsyncEvent event) throws IOException {

            }

        });

        queue.add(ac);

    }

    private String getJVMInfo() {

        long useMemory = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();

        useMemory = useMemory / 1024;

        return "已用記憶體:" + String.valueOf(useMemory) + "K";

    }

    /**

     * 這個函式太重要了！返回JS程式碼執行.它呼叫父視窗的update方法

     */

    private String htmlEscape(String message) {

        return ";

    }

    @Override

    public void run() {

        System.out.println("傳送廣播通知");

        String data = "廣播通知\n" + new Date() + "\n" + getJVMInfo();

        for (AsyncContext ac : queue) {

            try {

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

                PrintWriter writer = ac.getResponse().getWriter();

                writer.println(htmlEscape(data));

                writer.flush();

            } catch (IOException e) {

                queue.remove(ac);

                e.printStackTrace();

            }

        }

    }

}

2.前臺透過隱藏的iframe傳輸資訊

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "">

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

<title>WebSocket</title>

<script type="text/javascript"

    src=""></script>

<script type="text/javascript">

    $(document).ready(function() {

        var url = './ac.do';

        $('#comet-frame')[0].src = url;

    });

    function update(data) {

        $('#content').html(data);

    }

</script>

</head>

<body>

    <div id="content"></div>

    <iframe id="comet-frame" style="display: none;"></iframe>

</body>

</html>

參考:
http://www.ibm.com/developerworks/cn/java/j-lo-comet/

反向Ajax之非同步Servlet

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