dwr支持如下3种模式的消息推送：
- Polling，浏览器每隔一段时间向服务器发出请求，查看是否有更新的内容；
- Comet，
- Piggyback，服务器等待浏览器下一次发出请求时，将更新的内容合并到响应一起返回。
默认使用Piggyback模式，使用Polling/Comet模式需要额外的配置。
DWR简单入门介绍了基于DWR构建项目。
使用Maven构建项目，在pom.xml中添加如下配置
<repositories>
<repository>
<!-- Please consider setting up your own on-site repository proxy such
as with Nexus and pointing the url element below at that instead -->
<id>oss-sonatype-snapshots</id>
<name>OSS Sonatype Snapshots Repository</name>
<url>http://oss.sonatype.org/content/repositories/snapshots</url>
<releases>
<enabled>false</enabled>
</releases>
<snapshots>
<enabled>true</enabled>
</snapshots>
</repository>
</repositories>
<dependencies>
...
<dependency>
<groupId>org.directwebremoting</groupId>
<artifactId>dwr</artifactId>
<version>3.0.0-SNAPSHOT</version>
</dependency>
<dependency>
<groupId>commons-logging</groupId>
<artifactId>commons-logging</artifactId>
<version>1.2</version>
</dependency>
<!-- 使用Tomcat需要额外添加的依赖包 -->
<dependency>
<groupId>org.apache.tomcat</groupId>
<artifactId>catalina</artifactId>
<version>6.0.44</version>
</dependency>
</dependencies>
使用的容器是Tomcat（与使用Jetty的配置不同），在web.xml中添加如下配置
<servlet>
<servlet-name>dwr-invoker</servlet-name>
<servlet-class>org.directwebremoting.server.tomcat.DwrCometProcessor</servlet-class>
<init-param>
<param-name>debug</param-name>
<param-value>true</param-value>
</init-param>
<init-param>
<!-- 2.0 RC3之前的参数名为pollAndCometEnabled -->
<param-name>activeReverseAjaxEnabled</param-name>
<param-value>true</param-value>
</init-param>
<init-param>
<param-name>maxWaitAfterWrite</param-name>
<param-value>1000</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>dwr-invoker</servlet-name>
<url-pattern>/dwr/*</url-pattern>
</servlet-mapping>
dwr.xml中的配置
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE dwr PUBLIC
"-//GetAhead Limited//DTD Direct Web Remoting 3.0//EN"
"http://getahead.org/dwr/dwr30.dtd">
<dwr>
<allow>
<create creator="new" javascript="messagePush">
<param name="class" value="me.util.MessagePushUtil" />
</create>
</allow>
</dwr>
采用的调试方式是，打开接收页面（receiver.jsp）和发送页面（sender.jsp），在发送页面将消息发送至服务端，由服务端将消息推送至接受页面。JSP文件中使用JQuery替代了dwr提供的<script type='text/javascript' src='/dwr/util.js'></script>。
接收页面（receiver.jsp）
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%@ page isELIgnored="false"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<link rel="stylesheet" type="text/css" href="/css/easyui.css">
<link rel="stylesheet" type="text/css" href="/css/demo.css">
<script type="text/javascript" src="/js/jquery.js"></script>
<script type="text/javascript" src="/js/jquery.easyui.min.js"></script>
<script type='text/javascript' src='/dwr/engine.js'></script>
<script type='text/javascript' src='/dwr/interface/messagePush.js'></script>
<title>消息推送</title>
</head>
<body>
<h2>接受消息推送</h2>
</body>
<script>
$(function() {
dwr.engine.setActiveReverseAjax(true);
dwr.engine.setNotifyServerOnPageUnload(true);
messagePush.onPageLoad('<%=session.getAttribute("uid")%>');
});
function showMessage(autoMessage) {
$.messager.show({
title : "消息推送",
msg : autoMessage,
showType : 'slide',
timeout : 5000
});
}
</script>
</html>
发送页面（sender.jsp）
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%@ page isELIgnored="false"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<link rel="stylesheet" type="text/css" href="./css/easyui.css">
<link rel="stylesheet" type="text/css" href="./css/demo.css">
<script type="text/javascript" src="./js/jquery.js"></script>
<script type="text/javascript" src="./js/jquery.easyui.min.js"></script>
<script type='text/javascript' src='/dwr/engine.js'></script>
<script type='text/javascript' src='/dwr/interface/messagePush.js'></script>
<title>消息推送</title>
</head>
<body>
<h2>发送消息推送</h2>
<div id="p" class="easyui-panel"
style="width: 400px; height: 100x; padding: 10px;">
<div>
ID：<input id="uid" style="width: 40%; height: 25px">消息：<input
id="msg" style="width: 40%; height: 25px"></br> </br> <a
href="javascript:void(0)" style="width: 30%; height: 25px"
onclick="sendMsg()">发送</a>
</div>
</div>
</body>
<script>
function sendMsg() {
messagePush.sendMessageAuto($('#uid').val(), $('#msg').val());
$('#uid').val('');
$('#msg').val('');
}
</script>
</html>
me/util/MessagePushUtil.java中的内容
package me.util;
import java.util.Collection;
import javax.servlet.ServletException;
import org.apache.log4j.Logger;
import org.directwebremoting.Browser;
import org.directwebremoting.ScriptBuffer;
import org.directwebremoting.ScriptSession;
import org.directwebremoting.ScriptSessionFilter;
import org.directwebremoting.WebContextFactory;
import me.message.MessagePushServlet;
public class MessagePushUtil {
private static final Logger LOGGER = Logger.getLogger(MessagePushUtil.class);
public void onPageLoad(String userID) {
ScriptSession scriptSession = WebContextFactory.get().getScriptSession();
// 工厂方法get()返回WebContext实例，通过WebContext获取servlet参数
// ScriptSession与HttpSession类似
scriptSession.setAttribute("uid", userID);
MessagePushServlet mpServlet = new MessagePushServlet();
try {
mpServlet.init();
LOGGER.info(String.format("消息推送初始化成功，uid：%s", userID));
} catch (ServletException e) {
LOGGER.error(String.format("消息推送初始化错误，uid：%s", userID));
}
}
/**
* 根据userID向指定用户推送消息
*
* @param userID
* @param autoMessage
*/
public void sendMessageAuto(String userID, String autoMessage) {
final String uid = userID;
final String message = autoMessage;
Browser.withAllSessionsFiltered(new ScriptSessionFilter() {
// 实现过滤器中的match()方法
public boolean match(ScriptSession session) {
if (session.getAttribute("uid") == null)
return false;
else
return (session.getAttribute("uid")).equals(uid);
}
}, new Runnable() {
private ScriptBuffer script = new ScriptBuffer();
public void run() {
// 调用JSP中定义的showMessage()方法，实现消息的前端显示
script.appendCall("showMessage", message);
Collection<ScriptSession> sessions = Browser.getTargetSessions();
for (ScriptSession scriptSession : sessions) {
scriptSession.addScript(script);
}
LOGGER.info(String.format("向用户推送消息，uid：%s，message：%s", uid, message));
}
});
}
}
me/message/MessagePushServlet.java中的内容，覆盖DwrServlet中的init()方法实现ScriptSession监听器，在页面中加入engine.js时，ScriptSession创建，默认由org.directwebremoting.impl.DefaultScriptSessionManager管理，当发生unload事件时，DefaultScriptSessionManager会被通知销毁ScriptSession。
package me.message;
import javax.servlet.ServletException;
import javax.servlet.http.HttpSession;
import org.apache.log4j.Logger;
import org.directwebremoting.Container;
import org.directwebremoting.ServerContextFactory;
import org.directwebremoting.WebContextFactory;
import org.directwebremoting.event.ScriptSessionEvent;
import org.directwebremoting.event.ScriptSessionListener;
import org.directwebremoting.extend.ScriptSessionManager;
import org.directwebremoting.servlet.DwrServlet;
public class MessagePushServlet extends DwrServlet {
private static final long serialVersionUID = 4298890285665323894L;
private static final Logger LOGGER = Logger.getLogger(MessagePushServlet.class);
@Override
public void init() throws ServletException {
Container container = ServerContextFactory.get().getContainer();
// 工厂方法get()返回ServerContext实例
ScriptSessionManager manager = container.getBean(ScriptSessionManager.class);
ScriptSessionListener listener = new ScriptSessionListener() {
public void sessionCreated(ScriptSessionEvent ev) {
HttpSession session = WebContextFactory.get().getSession();
String userID = (String) session.getAttribute("uid");
LOGGER.info("a ScriptSession is created");
ev.getSession().setAttribute("uid", userID);
}
public void sessionDestroyed(ScriptSessionEvent ev) {
LOGGER.info("a ScriptSession is distroyed");
}
};
manager.addScriptSessionListener(listener);
}
}
运行前还要修改Tomcat下的server.xml中的配置
<!--
<Connector connectionTimeout="20000" port="8080"
protocol="HTTP/1.1" redirectPort="8443"/>
-->
<Connector URIEncoding="UTF-8" connectionTimeout="20000" port="8080"
protocol="org.apache.coyote.http11.Http11NioProtocol" redirectPort="8443"/>
运行项目，打开http://localhost:8080/receiver.jsp，页面加载时，调用
messagePush.onPageLoad('<%=session.getAttribute("uid")%>');
调用服务端的me.util.MessagePushUtil.onPageLoad(String userID)方法，初始化成功（可以使用字符串，如"123456789",替换'<%=session.getAttribute("uid")%>'）。打开http://localhost:8080/sender.jsp，填写刚才的uid（123456789）和消息内容，点击发送，调用
messagePush.sendMessageAuto($('#uid').val(), $('#msg').val());
调用服务端的me.util.MessagePushUtil.sendMessageAuto(String userID, String autoMessage)方法，调用receiver.jsp的showMessage(autoMessage)方法，可以在接收页面看到来自服务端推送的消息。
页面预览如下，
发送页面
接收页面
除此之外，在log4j.properties中添加类似的配置
log4j.rootCategory=INFO, console
log4j.category.org.directwebremoting.log=INFO, dwr, console
log4j.appender.console=org.apache.log4j.ConsoleAppender
log4j.appender.console.layout=org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern=[%5p] %d{yyyy-MM-dd HH:mm:ss} %l - %m%n
log4j.appender.dwr=org.apache.log4j.RollingFileAppender
log4j.appender.dwr.File=../logs/dwr.log
log4j.appender.dwr.MaxFileSize=100KB
log4j.appender.dwr.MaxBackupIndex=1
log4j.appender.dwr.layout=org.apache.log4j.PatternLayout
log4j.appender.dwr.layout.ConversionPattern=[%5p] %d{yyyy-MM-dd HH:mm:ss} %l - %m%n
参考资料
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