AJAX，或者说是异步JavaScript和XML，描述了一种使用混合了HTML(或XHTML)和层叠样式表作为表达信息,来创建交互式的Web应用的开发技术;文档对象模型（DOM），JavaScript,动态地显示和与表达信息进行交互;并且XMLHttpRequest对象与Web服务器异步地交换和处理数据。
DWR(直接Web远程控制)项目是在Apache许可下的一个开源的解决方案，它供给那些想要以一种简单的方式使用AJAX和 XMLHttpRequest的开发者。它具有一套Javascript功能集，它们把从HTML页面调用应用服务器上的Java对象的方法简化了。它操控不同类型的参数，并同时保持了HTML代码的可读性，它可以与spring结合在一起。
举个非常实用的小例子，检测是否有重复的用户名
1。web。xml的配置（包括spring和dwr）
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app
PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
"http://java.sun.com/j2ee/dtds/web-app_2_3.dtd">
<web-app>
<context-param>
<param-name>log4jConfigLocation</param-name>
<param-value>/WEB-INF/log4j.properties</param-value>
</context-param>
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/applicationContext.xml,/WEB-INF/main-servlet.xml</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.util.Log4jConfigListener</listener-class>
</listener>
<servlet>
<servlet-name>context</servlet-name>
<servlet-class>org.springframework.web.context.ContextLoaderServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet>
<servlet-name>dwr</servlet-name>
<display-name>DWR Servlet</display-name>
<description>Direct Web Remoter Servlet</description>
<servlet-class>uk.ltd.getahead.dwr.DWRServlet</servlet-class>
<init-param>
<param-name>debug</param-name>
<param-value>true</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>dwr</servlet-name>
<url-pattern>/dwr/*</url-pattern>
</servlet-mapping>
</web-app>
2。dwr。xml的配置
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE dwr PUBLIC "-//GetAhead Limited//DTD Direct Web Remoting 1.0//EN" "http://www.getahead.ltd.uk/dwr/dwr10.dtd">
<dwr>
<allow>
<create
creator="spring"
javascript="UserManager">
<param name="beanName" value="UserManager"/>
</create>
</allow>
</dwr>
3。applicationContext.xml文件
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
<beans>
<bean id="propertyConfigurer" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="location">
<value>/WEB-INF/jdbc.properties</value>
</property>
</bean>
<!-- Local DataSource that works in any environment -->
<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
<property name="driverClassName">
<value>${jdbc.driverClassName}</value>
</property>
<property name="url">
<value>${jdbc.url}</value>
</property>
<property name="username">
<value>${jdbc.username}</value>
</property>
<property name="password">
<value>${jdbc.password}</value>
</property>
</bean>
<!--OrderManager-->
<bean id="UserManager" class="dwr.manager.UserManager">
<property name="dataSource">
<ref local="dataSource"/>
</property>
</bean>
</beans>
4。。spring的bean
package dwr.manager;
import org.springframework.jdbc.core.support.JdbcDaoSupport;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
public class UserManager extends JdbcDaoSupport {
private final Log log = LogFactory.getLog(getClass().getName());
public boolean getUser(String username)
{
String sql="select count(*) from user where userName=? ";
int i=getJdbcTemplate().queryForInt(sql,new Object[]{username});
if(i>0)
return true;
else
return false;
}
}
5。jsp的代码
<%@ page contentType="text/html;charset=GBK" language="java" %>
<html>
<head><title>Simple jsp page</title></head>
<script type='text/javascript' src='<%=request.getContextPath()%>/dwr/interface/UserManager.js'>
</script>
<script type='text/javascript' src='<%=request.getContextPath()%>/dwr/engine.js'>
</script>
<script type='text/javascript'
src='<%=request.getContextPath()%>/dwr/util.js'></script>
<script type="text/javascript">
function showResult(b) {
if (b) {
alert("已经有重复的登录名了！");
} else {
alert("登录名尚未注册过！");
}
}function checkRepeat(s) {
var o = getElement(s);
if (o) {
UserManager.getUser(o.value, showResult);
}
}
function getElement(name) {
var result = null;
var objs = document.getElementsByName(name);
if (objs) {
var o = objs[0];
result = o;
}
return result;
}
</script>
<body>
<input type="text" name="user" >
<input type="button" onclick="checkRepeat('user')" value="检查是否重复">
</body>
</html>
6 classpath加入dwr和spring的jar文件，注意必须有hibernate3.jar和bsf.jar，在IE和FireFox下，就可以用了。
