DWR 工作原理:
是通过动态把 Java 类生成为 Javascript。它的代码就像 Ajax 一样，你感觉调用就像发生在浏览器端，但是实际上代码调用发生在服务器端，DWR 负责数据的传递和转换。这种从 Java 到JavaScript的远程调用功能的方式使DWR用起来有种非常像RMI或者SOAP的常规RPC机制，而且DWR的优点在于不需要任何的网页浏览器插件就能运行在网页上。
Java 从根本上讲是同步机制，然而 AJAX 却是异步的。所以你调用远程方法时，当数据已经从网络上返回的时候，你需要提供有反调 （callback） 功能的 DWR，即你的js代码调用最后一个参数是一个回调函数，由回调函数处理数据返回后想做的操作。
Comet
Comet方式通俗的说就是一种长连接机制(long lived http)。同样是由Browser端主动发起请求，但是Server端以一种似乎非常慢的响应方式给出回答。这样在这个期间内，服务器端可以使用同一个connection把要更新的数据主动发送给Browser。因此请求可能等待较长的时间，期间没有任何数据返回，但是一旦有了新的数据，它将立即被发送到客户机。Comet又有很多种实现方式，但是总的来说对Server端的负载都会有增加.虽然对于单位操作来说，每次只需要建议一次connection,但是由于connection是保持较长时间的,对于 server端的资源的占用要有所增加。
优点： 实时性好（消息延时小）；性能好（能支持大量用户）
缺点： 长期占用连接，丧失了无状态高并发的特点。
应用： 股票系统、实时通讯。
1.先配置web.xml
<!-- dwr start -->
<servlet>
<!-- dwr框架的核心控制器 -->
<servlet-name>dwr-invoker</servlet-name>
<servlet-class>org.directwebremoting.servlet.DwrServlet</servlet-class>
<!-- 在服务器端启用反向ajax -->
<init-param>
<param-name>activeReverseAjaxEnabled</param-name>
<param-value>true</param-value>
</init-param>
<!-- 为了得到更详细的调试信息 -->
<init-param>
<param-name>debug</param-name>
<param-value>true</param-value>
</init-param>
<init-param>
<param-name>crossDomainSessionSecurity</param-name>
<param-value>false</param-value>
</init-param>
<init-param>
<param-name>allowScriptTagRemoting</param-name>
<param-value>true</param-value>
</init-param>
<init-param>
<param-name>org.directwebremoting.extend.ScriptSessionManager </param-name>
<param-value>com.worklite.core.tools.dwr.DWRScriptSessionManager</param-value>
</init-param>
<!-- 在服务器启动阶段，该servlet实例被创建 -->
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>dwr-invoker</servlet-name>
<!-- 发送如下的请求，都会交给dwr的核心控制器处理 -->
<url-pattern>/dwr/*</url-pattern>
</servlet-mapping>
<!-- dwr end -->
2.新建dwr.xml
<?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="DwrPush">
<param name="class" value="com.worklite.core.tools.dwr.DwrPush"/>
</create>
<create creator="new" javascript="ClientInitUtil">
<param name="class" value="com.worklite.core.tools.dwr.ClientInitUtil"/>
</create>
<create creator="new" javascript="WebMultiplePushUtil">
<param name="class" value="com.worklite.core.tools.dwr.WebMultiplePushUtil"/>
</create>
<create creator="new" javascript="WebSinglePushUtil">
<param name="class" value="com.worklite.core.tools.dwr.WebSinglePushUtil"/>
</create>
<create creator="new" javascript="WebSingleGamePushUtil">
<param name="class" value="com.worklite.core.tools.dwr.WebSingleGamePushUtil"/>
</create>
</allow>
</dwr>
3.编写后台需要的类。
（1）DWRScriptSessionListener
public class DWRScriptSessionListener implements ScriptSessionListener {
//维护一个Map key为session的Id， value为ScriptSession对象
public static final Map<String, ScriptSession> scriptSessionMap = new MirrorMap<String, ScriptSession>();
/**
* ScriptSession创建事件
*/
@Override
public void sessionCreated(ScriptSessionEvent event) {
WebContext webContext = WebContextFactory. get();
HttpSession session = webContext.getSession();
ScriptSession scriptSession = event.getSession();
scriptSessionMap.put(session.getId(), scriptSession); //添加scriptSession
}
/**
* ScriptSession销毁事件
*/
@Override
public void sessionDestroyed(ScriptSessionEvent event) {
//WebContext webContext = WebContextFactory. get();
//HttpSession session = webContext.getSession();
//scriptSessionMap.remove(session.getId()); //移除scriptSession
}
/**
* 获取所有ScriptSession
*/
public static Collection<ScriptSession> getScriptSessions(){
return scriptSessionMap.values();
}
public static ScriptSession getScriptSession(String str){
Collection<ScriptSession> sessions = DWRScriptSessionListener.getScriptSessions();
//遍历每一个ScriptSession
for (ScriptSession scriptSession : sessions){
String tag = (String)scriptSession.getAttribute(str);
if ( tag!=null&&str.equals(tag)) {
return scriptSession;
}
}
return null;
}
public static int Count(String wId){
Collection<ScriptSession> sessions = DWRScriptSessionListener.getScriptSessions();
//遍历每一个ScriptSession
int i=0;
for (ScriptSession scriptSession : sessions){
String tag = (String)scriptSession.getAttribute(wId);
if ( tag!=null&&wId.equals(tag)) {
i++;
}
}
return i;
}
public static boolean getCustomList(String wId,int customId){
Collection<ScriptSession> sessions = DWRScriptSessionListener.getScriptSessions();
//遍历每一个ScriptSession
for (ScriptSession scriptSession : sessions){
String tagId=customId+wId;
String tag = (String)scriptSession.getAttribute(tagId);
if(tag!=null&&tagId.equals(tag)){
return true;
}
}
return false;
}
public static List<String> getCustomList(String wId){
List<String> customList=new ArrayList<String>();
Collection<ScriptSession> sessions = DWRScriptSessionListener.getScriptSessions();
//遍历每一个ScriptSession
for (ScriptSession scriptSession : sessions){
Iterator<String> iter = scriptSession.getAttributeNames();
while(iter.hasNext()){
String str=iter.next();
if(str!=null&&str.contains(String.valueOf(","+wId))){
String customId = str.split(",")[0];
if (!customList.contains(customId))
customList.add(customId);
}
}
}
return customList;
}
public void init() throws ServletException {
Container container = ServerContextFactory.get().getContainer();
ScriptSessionManager manager = container.getBean(ScriptSessionManager.class);
ScriptSessionListener listener = new ScriptSessionListener() {
public void sessionCreated(ScriptSessionEvent ev) {
WebContext webContext = WebContextFactory. get();
HttpSession session = webContext.getSession();
ScriptSession scriptSession = ev.getSession();
scriptSessionMap.put(session.getId(), scriptSession); //添加scriptSession
}
public void sessionDestroyed(ScriptSessionEvent ev) {
WebContext webContext = WebContextFactory. get();
HttpSession session = webContext.getSession();
ScriptSession scriptSession = scriptSessionMap.remove(session.getId()); //移除scriptSession
}
};
manager.addScriptSessionListener(listener);
}
}
（2）DWRScriptSessionManager
public class DWRScriptSessionManager extends DefaultScriptSessionManager {
public DWRScriptSessionManager(){
//绑定一个ScriptSession增加销毁事件的监听器
this.addScriptSessionListener( new DWRScriptSessionListener());
Log.debug( "bind DWRScriptSessionListener");
}
}
（3）DwrPush注册scriptSession
public class DwrPush {
public void onPageLoad(final String tag){
//获取当前的ScriptSession
ScriptSession scriptSession = WebContextFactory.get().getScriptSession();
scriptSession.setAttribute( tag, tag);
}
}
（4）WebMultiplePushUtil 用于群发
public class WebMultiplePushUtil {
public void push(final String wId,final String content,final String command){
//过滤器
ScriptSessionFilter filter = new ScriptSessionFilter() {
public boolean match(ScriptSession scriptSession) {
String tag = (String)scriptSession.getAttribute(wId);
if ( tag== null) {
return false;
} else {
return wId.equals(tag);
}
}
};
Runnable run = new Runnable(){
private ScriptBuffer script = new ScriptBuffer();
public void run() {
//设置要调用的 js及参数
script.appendCall( "showMessage","{"wId":""+wId+"","content":""+content+"","command":""+command+""}");
//得到所有ScriptSession
Collection<ScriptSession> sessions = Browser.getTargetSessions();
//遍历每一个ScriptSession
for (ScriptSession scriptSession : sessions){
scriptSession.addScript( script);
}
}
};
//执行推送
Browser. withAllSessionsFiltered(filter, run); //注意这里调用了有filter功能的方法
}
}
（5）WebSingleGamePushUtil 用于单发游戏
public class WebSingleGamePushUtil {
public void push(final String customId,final String wId,final Webcast_game_push push){
Runnable run = new Runnable(){
private ScriptBuffer script = new ScriptBuffer();
public void run() {
//设置要调用的 js及参数
script.appendCall( "show",JSONObject.fromObject(push).toString());
ScriptSession scriptSession =DWRScriptSessionListener.getScriptSession(customId+","+wId);
if(null!=scriptSession){
scriptSession.addScript( script);
}
}
};
//执行推送
Browser.withAllSessions(run);
}
}
（6）WebSinglePushUtil 用于单发通知
public class WebSinglePushUtil {
public void push(final String customId,final String wId,final String content,final String command){
//过滤器
ScriptSessionFilter filter = new ScriptSessionFilter() {
public boolean match(ScriptSession scriptSession) {
String tagId=customId+","+wId;
String tag = (String)scriptSession.getAttribute(tagId);
if ( tag== null) {
return false;
} else {
return tagId.equals(tag);
}
}
};
Runnable run = new Runnable(){
private ScriptBuffer script = new ScriptBuffer();
public void run() {
//设置要调用的 js及参数
script.appendCall( "show","{"customId":""+customId+"","wId":""+wId+"","content":""+content+"","command":""+command+""}");
//得到所有ScriptSession
Collection<ScriptSession> sessions = Browser.getTargetSessions();
//遍历每一个ScriptSession
for (ScriptSession scriptSession : sessions){
scriptSession.addScript( script);
}
}
};
//执行推送
Browser. withAllSessionsFiltered(filter, run); //注意这里调用了有filter功能的方法
}
}
4.在jsp页面引入相应的js
<script type="text/javascript" src="/worklite/dwr/engine.js"></script>
<script type="text/javascript" src="/worklite/dwr/util.js"></script>
<script type="text/javascript" src="/worklite/dwr/interface/DwrPush.js"></script>
dwr.engine.setActiveReverseAjax(true);
dwr.engine.setAsync(true);
dwr.engine._errorHandler = function(message, ex) {dwr.engine._debug("Error: " + ex.name + ", " + ex.message, true);};
<body onload="dwr.engine.setActiveReverseAjax(true);dwr.engine.setNotifyServerOnPageUnload(true);onPageLoad();">
在页面上要先掉用下DwrPush.onPageLoad（），加入scriptSession
function onPageLoad(){
DwrPush.onPageLoad(wId);
DwrPush.onPageLoad(""+customId+","+wId);
}
然后在代码中调用相应的push方法即可完成推送。