Ajax异步查询用户名时报错空指针问题？ ps:第一次问问题，不大清楚格式，请勿纠~~~

java.lang.NullPointerException
at cn.kpp.shop.user.action.UserAction.findByName(UserAction.java:51)

UserAction:

public class UserAction extends ActionSupport implements ModelDriven {

private static final long serialVersionUID = 1L;
// 模型驱动使用的对象
private User user = new User();

public User getModel() {
    return user;
}

//注入service
private UserService userService;

public void setUserService(UserService userService) {
    this.userService = userService;
}

/*
 * 1.跳转到注册页面的方法
 */
public String registPage(){
    return "registPage";
}

/*
 * 2.Ajax进行异步校验用户名的方法
 */
public String findByName() throws IOException{
    ****User existUser = userService.findByUsername(user.getUsername());** 这行报错？**
    // 获得response对象,项页面输出:
            HttpServletResponse response = ServletActionContext.getResponse();
            response.setContentType("text/html;charset=UTF-8");
            // 判断
            if (existUser != null) {
                // 查询到该用户:用户名已经存在
                response.getWriter().println("<font color='red'>用户名已经存在</font>");
            } else {
                // 没查询到该用户:用户名可以使用
                response.getWriter().println("<font color='green'>用户名可以使用</font>");
            }
            return NONE;    
}

}

注册页面：regist.jsp

/*

• 前台JS校验：非空
  */

  function checkForm() {
  //校验用户名，获得文本框的id值
  var username = document.getElementById("username").value;
  if(username == null || username ==""){
  alert("用户名不能为空！");
  return false;
  }
  //校验密码，获得文本框的id值
  var password = document.getElementById("password").value;
  if(password == null || password ==""){
  alert("密码不能为空！");
  return false;
  }
  //校验确认密码，获得文本框的id值
  var repassword = document.getElementById("repassword").value;
  if(repassword != password){
  alert("两次密码输入不一致！");
  return false;
  }
  }

  /*

  • Ajax异步校验：用户名是否存在
    */

    function checkUsername() {
    var username = document.getElementById("username").value;
    //1.创建异步交互对象
    var xhr = createXmlHttp();
    //2.设置监听
    xhr.onreadystatechange = function(){
    if(readystate == 4){
    if(xhr.status == 200){
    document.getElementById("span1").innerHTML = xhr.responseText;
    }
    }
    }
    //3.打开连接
    xhr.open("GET","${pageContext.request.contextPath}/user_findByName.action?time="+new Date().getTime()+"&username="+username,true);
    //4.发送
    xhr.send(null);
    }

    function createXmlHttp(){
    var xmlHttp;
    try{ // Firefox, Opera 8.0+, Safari
    xmlHttp=new XMLHttpRequest();
    }
    catch (e){
    try{// Internet Explorer
    xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
    }
    catch (e){
    try{
    xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    catch (e){}
    }
    }
    return xmlHttp;
    }

                    <form id="registerForm"  method="post" novalidate="novalidate" onsubmit="return checkForm()">
                        <table>
                            <tbody><tr>
                                <th>
                                    <span class="requiredField">*</span>用户名:
                                </th>
                                <td>
                                    <input type="text" id="username" name="username" class="text" maxlength="20" onblur="checkUsername();">
                                    <span id="span1"></span>
                                </td>
                            </tr>……

http://www.poluoluo.com/jzxy/201402/262209.html