[JSP] jsp+ajax发送GET请求的方法

<form action="servlet/LoginServlet" method="post">
  <table>
    <tr>
      <td>用户账号：</td>
      <td><input type="text" name="username" /></td>
    </tr>
    <tr>
      <td>用户密码：</td>
      <td><input type="password" name="password"/></td>
    </tr>
    <tr>
      <td><input type="submit" value="注册"/></td>
      <td><input type="reset" value="重置"></td>
    </tr>
  </table>
</form>

<servlet>
<servlet-name>LoginServlet</servlet-name>
<servlet-class>login.LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>LoginServlet</servlet-name>
<url-pattern>/servlet/LoginServlet</url-pattern>
</servlet-mapping>

public void doGet(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
    response.setContentType("text/html;charaet=UTF-8");
    PrintWriter out = response.getWriter();
    String name=request.getParameter("username");
    System.out.println(name);
    if(name.equals("admin"))
      out.print(false);
    else
      out.print(true);
    out.flush();
    out.close();
}

<script type="text/javascript">
  //创建XMLHttpRequest
  function createXmlHttpRequest(){
    if(window.XMLHttpRequest){
      return new XMLHttpRequest();
    }else{
      return new ActiveXObject("Microsoft.XMLHTTP");
    }
  }
  //当用户账号输入框失去焦点时调用该方法
  function checkUser(obj){
    //获取输入框输入的值
    var user = obj.value;
    //如果输入框中的值为空，那么弹窗提示，并且让该输入框获得焦点
    if(!user){
      alert("用户名不能为空！");
      obj.focus();
      return;
    }
    //不为空时，使用Ajax请求向后台发送信息，验证该用户名是否可用
    //get请求字符串
    var url="servlet/LoginServlet?username="+user;
    //调用方法创建XMLHttpRequest对象
    XmlHttpRequest = createXmlHttpRequest();
    //设置回调函数
    XmlHttpRequest.onreadystatechange=finish;
    //初始化xmlhttprequest
    XmlHttpRequest.open("GET",url,true);
    //发送请求
    XmlHttpRequest.send(null);
  }
  //回调函数
  function finish(){
    if(XmlHttpRequest.readyState == 4&& XmlHttpRequest.status == 200){
      var result = XmlHttpRequest.responseText;
      if(result =="true"){
        alert("用户名可用！");
      }else{
        alert("用户名不可用！");
      }
    }
  }
</script>