JSP页面内容：
<td align="right" class="bg_table02" >
	<font color="red">*</font>手机：
</td>
<td class="bg_table02">
	<html:text property="subMobile" size="20" styleClass="input" />
<input class=button01 name="captchaBtn" type="button"
				value="获取验证码" onclick="getCaptcha()" />
</td>
JSP页面中的javascript代码：
var xmlhttp;   
	function  operurl(){
	    if (window.XMLHttpRequest) {
	          xmlhttp = new XMLHttpRequest();                 
	    } else {
	          xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
	    }
	}

function getCaptcha(){
if(isNumber(document.forms(0).subMobile.value)){
operurl();			 xmlhttp.open("get","<%=request.getContextPath()%>/qualityOpinionCas.cas?functionCode=getCaptcha&subMobile="+document.forms(0).subMobile.value);
				 xmlhttp.onreadystatechange= function (){
				  if(xmlhttp.readyState==4){
					if(xmlhttp.status==200){
						var res=xmlhttp.responseText;
						if(res!=""){
						   document.forms(0).rcaptcha.value=res;
						   //按钮灰10秒
						   document.forms(0).captchaBtn.disabled=true;
						   setTimeout("document.forms(0).captchaBtn.disabled=false;",10000);  
						   return;
						}
					}
				  }
				 };
			     xmlhttp.send();
		}
 }

Action中代码：
/**
	 * 获取验证码
	 */
	private ActionForward getCaptcha(BActionContext context)
			throws BsiException {
		HttpServletRequest request = context.getRequest();
		HttpServletResponse response = context.getResponse();
		String subMobile = request.getParameter("subMobile");
		String captcha = get5Radom();//获取随机生成5位随机数
		String content = "";
		if (null != subMobile) {
			String message = "您好，欢迎使用Ajax技术，验证码为:"+ captcha + "。您有任何疑问，欢迎拨打热线进行咨询";
			try {
				CpfzUtil.SMSExceues(subMobile, message); //调用一个接口，用于发送短信
				content = captcha;
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
		try {
			request.setCharacterEncoding("gbk");
			response.setContentType("text/xml;charset=gbk");
		} catch (UnsupportedEncodingException el) {
			el.printStackTrace();
		}
		PrintWriter out;
		try {
			out = response.getWriter();
			out.print(content);
			out.flush();
			out.close();

		} catch (IOException e) {
			e.printStackTrace();
		}

		return null;
	}
	
	// 随机生成5位随机数
	private String get5Radom() {
		String newString = null;
		// 得到0.0到1.0之间的数字,并扩大100000倍
		double doubleP = Math.random() * 100000;
		// 如果数据等于100000,则减少1
		if (doubleP >= 100000) {     
			doubleP = 99999;
		}
		// 然后把这个数字转化为不包含小数点的整数
		int tempString = (int) Math.ceil(doubleP);
		// 转化为字符串
		newString = "" + tempString;
		// 把得到的数增加为固定长度,为5位
		while (newString.length() < 5) {
			newString = "0" + newString;
		}
		return newString;
	}