Ajax判断用户名是否存在并

//javaScript部分：

var xmlHttpRequest;
function createXmlHttpRetuest() {
	if (window.ActiveXObject) {
		try {
			xmlHttpRequest = new ActiveXObject("Microsoft.XMLHTTP");
		} catch (e) {
			xmlHttpRequest = new ActiveXObject("Msxml2.XMLHTTP");
		}
		return xmlHttpRequest;
	} else if (window.XMLHttpRequest) {
		return new XMLHttpRequest();
	}
}
function validate(field) {
	if (trim(field.value).length != 0) {
		createXmlHttpRetuest();
		url = "validate!validateIsUserNameExist.action?user.userId ="
				+ field.value;
		xmlHttpRequest.open('post', url, true);
		xmlHttpRequest.setRequestHeader("if-Modified-Since", "0");
		xmlHttpRequest.onreadystatechange = function() {
			var responseStr = trim(xmlHttpRequest.responseText);
			if (xmlHttpRequest.readyState == 4 && xmlHttpRequest.status == 200) {
				if (responseStr != '') {
					document.getElementById("isExitUserId").innerHTML = "<font color='red'>"
							+ responseStr + "</font>";
				} else {
					document.getElementById("isExitUserId").innerHTML = "";
				}
			} else {
				alert('响应失败，错误码 ：' + xmlHttpRequest.status);
			}
		};
		xmlHttpRequest.send(null);
	} else {
		document.getElementById("isExitUserId").innerHTML = "";
	}
}

html部分：

当焦点离开输入域的时候触发事件：

<td width="78%">
							<input name="user.userId" type="text" class="text1" id="userId"
								size="10" maxlength="10" onblur="validate(this)">
							<span id="isExitUserId"></span>
						</td>

Action部分：

/**
	 * 利用Ajax判断用户名是否存在
	 * 
	 * @author Jeelon
	 * @return SUCCESS
	 * @throws IOException
	 */
	public String validateIsUserNameExist() throws IOException {
		if ((user = tUserBiz.findById(user.getUserId())) != null) {
			HttpServletResponse response = ServletActionContext.getResponse();
			response.setContentType("text/plain");// 设置输出为文字流
			response.setCharacterEncoding("UTF-8");
			PrintWriter out = response.getWriter();
			out.print("*该用户已存在");
			out.flush();
			out.close();
		}
		return null;
	}