用户名重复时抛出null异常

相关截图：

问题描述：

controller中判断用户名密码为空这些信息都能正确返回。但是用户名重复时就会抛出null异常，异常截图如上；
这个null异常是怎么回事？

相关代码：

package com.he.mall.controller;

import com.he.mall.common.ApiRestResponse;
import com.he.mall.exception.HeMallException;
import com.he.mall.exception.HeMallExceptionEnum;
import com.he.mall.model.pojo.User;
import com.he.mall.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.util.StringUtils;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseBody;

/**
 * 用户控制器
 */
@Controller
public class UserController {
    @Autowired
    UserService userService;

    @GetMapping("/test")
    @ResponseBody
    public User personalPage() {
        return userService.getUser();
    }
    @PostMapping("/register")
    @ResponseBody
    public ApiRestResponse register(@RequestParam("userName") String userName, @RequestParam("password") String password) throws HeMallException {
        //userName判断是否为空
        if (StringUtils.isEmpty(userName)) {
            return ApiRestResponse.error(HeMallExceptionEnum.NEED_USERNAME);
        }
        //password判断是否为空
        if (StringUtils.isEmpty(password)) {
            return ApiRestResponse.error(HeMallExceptionEnum.NEED_PASSWORD);
        } else if (password.length() < 8) {
            //密码长度不能少于8位
            return ApiRestResponse.error(HeMallExceptionEnum.PASSWORD_TOO_SHORT);
        }
        //保存至数据库
        userService.register(userName, password);
        return ApiRestResponse.success();
    }

}

相关代码：

package com.he.mall.exception;

public class HeMallException extends Exception {
    private final Integer status;
    private final String msg;

    public HeMallException(Integer status, String msg) {
        this.status = status;
        this.msg = msg;
    }

    public HeMallException(HeMallExceptionEnum exceptionEnum) {
        this(exceptionEnum.getStatus(), exceptionEnum.getMsg());
    }

    public Integer getStatus() {
        return status;
    }

    public String getMsg() {
        return msg;
    }

}

相关代码：

package com.he.mall.exception;

/**
 *      异常枚举
 */
public enum HeMallExceptionEnum {
    NEED_USERNAME(10001,"用户名不能为空"),
    NEED_PASSWORD(10002,"密码不能为空"),
    PASSWORD_TOO_SHORT(10003,"密码长度不能少于8位"),
    NAME_EXISTED(10004,"用户已存在，注册失败"),
    INSERT_FAILDED(10005,"系统错误，注册失败");

    Integer status;//异常状态代码
    String msg;//异常信息

    HeMallExceptionEnum(Integer status, String msg) {
        this.status = status;
        this.msg = msg;
    }

    public Integer getStatus() {
        return status;
    }

    public void setStatus(Integer status) {
        this.status = status;
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }
}

相关代码：

​package com.he.mall.service.impl;

import com.he.mall.exception.HeMallException;
import com.he.mall.exception.HeMallExceptionEnum;
import com.he.mall.model.dao.UserMapper;
import com.he.mall.model.pojo.User;
import com.he.mall.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

/**
 *      UserService实现类
 */
@Service
public class UserServiceImpl implements UserService {
    @Autowired
    UserMapper userMapper;
    @Override
    public User getUser() {
        return userMapper.selectByPrimaryKey(1);
    }

    @Override
    public void register(String userName, String password) throws HeMallException {
        //查询是否存在相同userName
        User userResult = userMapper.selectByUserName(userName);
        if (userResult != null) {
            throw new HeMallException(HeMallExceptionEnum.NAME_EXISTED);
        }
        //保存至数据库
        User user = new User();
        user.setUsername(userName);
        user.setPassword(password);
        int count = userMapper.insertSelective(user);
        //保存失败时处理
        if (count == 0) {
            throw new HeMallException(HeMallExceptionEnum.INSERT_FAILDED);
        }

    }
}