dwr comet

dwr的Reverse Ajax，新瓶装老酒，其实就是comet技术,即服务端向客户端push js代码，典型的应用场景就是目前比较火的比赛图文直播，还有聊天室等，消息订阅模式的一种实现。

优点：控制权反转，打开一个网页（或进行一次操作）后，订阅了一个消息，一个客户端服务端连接将维持，服务端检测到数据更新，将自动push数据到客户端，这样服务端对于程序调动的灵活性更加大，避免了客户端大量的循环请求数据。

缺点：连接维持，访问数量一大连接数将很快饱和。

关于comet优点比较权威的阐述：Ajax improves single-user responsiveness. Comet improves application responsiveness for collaborative, multi-user applications and does it without the performance headaches associated with intermittent polling.

看一下dwr官方示例，很简单的clock实现：

html代码：

<input type="button" value="Start / Stop" onclick="Clock.toggle();"/> <h2 id="clockDisplay"></h2>

<input type="button" value="Start / Stop" onclick="Clock.toggle();"/> <h2 id="clockDisplay"></h2>

js代码就一行，打开相应属性：
dwr.engine.setActiveReverseAjax(true);
服务端clock类：

public class Clock implements Runnable {
    public Clock() {
        ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(1);
        executor.scheduleAtFixedRate(this, 1, 1, TimeUnit.SECONDS);
    }

    public void run() {
        if (active) {
            setClockDisplay(new Date().toString());
        }
    }

    /**
     * Called from the client to turn the clock on/off
     */
    public synchronized void toggle() {
        active = !active;

        if (active) {
            setClockDisplay("Started");
        }
        else {
            setClockDisplay("Stopped");
        }
    }

    /**
     * Actually alter the clients.
     * @param output The string to display.
     */
    public void setClockDisplay(final String output) {
        String page = ServerContextFactory.get().getContextPath() + "/reverseajax/clock.html";
        Browser.withPage(page, new Runnable() {
            public void run() {
                Util.setValue("clockDisplay", output);
            }
        });
    }

    protected transient boolean active = false;
}

public class Clock implements Runnable {
    public Clock() {
        ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(1);
        executor.scheduleAtFixedRate(this, 1, 1, TimeUnit.SECONDS);
    }

    public void run() {
        if (active) {
            setClockDisplay(new Date().toString());
        }
    }

    /**
     * Called from the client to turn the clock on/off
     */
    public synchronized void toggle() {
        active = !active;

        if (active) {
            setClockDisplay("Started");
        }
        else {
            setClockDisplay("Stopped");
        }
    }

    /**
     * Actually alter the clients.
     * @param output The string to display.
     */
    public void setClockDisplay(final String output) {
        String page = ServerContextFactory.get().getContextPath() + "/reverseajax/clock.html";
        Browser.withPage(page, new Runnable() {
            public void run() {
                Util.setValue("clockDisplay", output);
            }
        });
    }

    protected transient boolean active = false;
}

个人设想：这个特性可以用来远程实时监控服务器运行状态，服务端自定义调用接口，定义一套xml协议来实时传送数据，也可以利用Web Service。

评论

3 楼 fantasyshe 2010-10-19  

没有代码下载，没真相

2 楼 high_java 2010-08-24  

sorry, 点错了

1 楼 high_java 2010-08-24