光标移动，触发ajax时间，然后到action里面判断是否存在，然后返回提示文字。

<td align="center">注册名</td>
<td><input type="text" name="user.userName" id="zhuceming"
style="width: 160px;" onblur="checkOnlyUser(this.value);">
</td>

<tr class="wrong_msg nodisplay">
???????<td colspan="2" align="center"><span style="color: red">&nbsp;${onlyUserWord}</span>
???????</td>
??????</tr>

?

	function checkOnlyUser(username){
		if(username == ""){
			ymPrompt.alert("不能为空",null,null,'用户信息提示框')
			return false;
		}
		$.ajax({
			type : "post",
			url : "doOnlyUser",
			beforeSend: function(XMLHttpRequest){
		            },
			data : {
				username : username ,
				random : Math.random()
			},
			success : function(data) {
				$(".wrong_msg span").html(data);
				$(".wrong_msg").show();
			}
		});
	}

?

进入struts.xml

		<action name="doOnlyUser" class="showUserAction" method="onlyUser">
			<result name="success">/popedom/userManage.jsp</result>
			<result name="nonono">/popedom/userManage.jsp</result>
		</action>

?

?

进入action方法

	/**
	 * 验证唯一
	 */
	public String onlyUser() {
		try {
			HttpServletResponse response = ServletActionContext.getResponse();
			response.setContentType("text/html;charset=utf-8");
			PrintWriter out = response.getWriter();
			user = this.userService.getOnlyUser(username);
			
			if (user != null) {
				onlyUserWord = "用户名重复";
			} else {
				onlyUserWord = "恭喜你,可以注册";
			}
			out.println(onlyUserWord);
			out.flush();
			out.close();
			return "success";
		} catch (IOException e) {
			e.printStackTrace();
			return "nonono";
		}
	}

?

	public User getOnlyUser(String userName) {
		Map<String, Object> paramMap = new HashMap<String, Object>();
		paramMap.put("userName", userName);
		return this.userMapper.getOnlyUser(paramMap);
	}

?

	<select id="getOnlyUser" resultMap="userResultMap" parameterType="java.util.HashMap"> 
		    select u.* from ts_user_info u where u.USER_NAME=#{userName} 
	</select>

?

总结：然后又回到页面，显示出来那个字符串关键字，就这么简单。