register.jsp

<body>
    <form>A
      用户名:<input id="usernameId" type="text" name="username">
    </form>
    <span id="tipID"></span>
    <script type="text/javascript">
      document.getElementById("usernameId").οnblur=function(){
        //获取input标签的值
        var username = this.value;
      var ajax = new XMLHttpRequest();
      var method = "GET";
      var url = "${pageContext.request.contextPath}/AjaxRegisterServlet?username="+username;
      ajax.open(method,url);
      ajax.send(null);
      //-------------------数据已发出
      ajax.onreadystatechange = function(){
        if(ajax.readyState == 4){
          if(ajax.status == 200){
            var tip = ajax.responseText;
            document.getElementById("tipID").innerHTML = tip;
          }
        }
      }   
      }
    </script>
  </body>

AjaxRegisterServlet.java

public class AjaxRegisterServlet extends HttpServlet {
  public void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {

    //以流的方式将结果响应到ajax异步对象中
    response.setContentType("text/html;charset=UTF-8");
    String username = request.getParameter("username");
    //解决乱码问题
    byte[] bytes = username.getBytes("ISO8859-1");
    username = new String(bytes,"UTF-8");
    String tip = "";
    if("杰克".equals(username)){
      tip = "用户名已经存在";
    }else{
      tip = "可以注册";
    }
    PrintWriter pw = response.getWriter();
    pw.write(tip);
    pw.flush();
    pw.close();
  }
}

用ajax验证用户名是否可以注册 GET方式_用户名