dwr的原理介绍：网视频
dwr文字介绍：http://blog.csdn.net/yuchen837295036/article/details/52680387
可能出现的异常：
java DWR——Allocate exception for servlet dwr org.xml.sax.SAXException: Failed to resolve: [arg0]= GetAhead Limited DTD Direct Web Remoting 3.0 EN [arg1]=http: getahead.org dwr dwr30.dtd
异常原因：dwr.xml文件中的导入的版本不对，参考链接： http://maijunjin.github.io/dwr/2014/02/14/java-dwr-allocate-exception-for-servlet-dwr-org.xml.sax.saxexception-failed-to-resolve-arg0=-getahead-limited-dtd-direct-web-remoting-3.0-en-arg1=http-getahead.org-dwr-dwr30.dtd.html
本文还有一个例子：dwr消息推送实例--网页聊天窗口
