Ajax的基本用法

客户端js：

<script type="text/javascript">
function testBack(){
//创建对象
var req;
if(window.XMLHTTPRequest){
req = new XMLHttpRequest();
}else if(window.ActiveXObject){
req = new ActiveXObject("Msxml2.XMLHTTP");
}
//创建请求 请求方式 请求路径  同步/异步
req.open("post","HelloBack",true);
//监听响应信息
req.onreadystatechange = function(){
if(req.readyState==4){
if(req.status==200){
//var result = req.responseText;
//eval("var user = "+result);
//alert(user.uname);
//eval("var users = "+result);
//alert(users[2].realname);
var result  = req.responseXML;
alert(result.getElementsByTagName("uname")[0].firstChild.data);
}
}
}
//发送请求
req.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
req.send(null);
}
</script>

servlet：

public class HelloBack extends HttpServlet {


@Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
resp.setCharacterEncoding("utf-8");
//第一种普通文本
//resp.setContentType("text/html;charset=utf-8");
//resp.getWriter().print("普通文本");
//Json格式数据
//resp.setContentType("text/html;charset=utf-8");
//resp.getWriter().print("[{uname:'zhangsan',realname:'张三三1'},{uname:'zhangsan',realname:'张三三2'},{uname:'zhangsan',realname:'张三三3'}]");
//XML格式的数据 返回数据的格式必须要正确
resp.setContentType("text/xml;charset=utf-8");
resp.getWriter().print("<user><uname>zhangsan</uname></user>");
}
}