建表sql:

   CREATE TABLE employee    (    id INT(8) PRIMARY KEY AUTO_INCREMENT ,    lastName VARCHAR(25),    email VARCHAR(64),    gender INT(1),    department_id INT,    birthday DATE,    CONSTRAINT FK_department_id FOREIGN KEY(department_id)  REFERENCES department(id) ON DELETE NO ACTION ON UPDATE NO ACTION     )ENGINE=INNODB DEFAULT CHARSET =utf8;            CREATE TABLE department    (    id INT PRIMARY KEY,    departmentName VARCHAR(64)    )ENGINE=INNODB DEFAULT CHARSET=utf8;

on null context object 异常

做查询所有员工功能时，一直报这个错误，网上找了半天，也没搞定。

这个错误根本原因还是上下文对象为null导致的。

Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1011E: Method call: Attempted to call method getId() on null context object
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2020-04-10 18:21:59.581 ERROR 30800 --- [nio-8088-exec-9] s.e.ErrorMvcAutoConfiguration$StaticView : Cannot render error page for request [/wang/table] and exception [Exception evaluating SpringEL expression: "emp.getId()" (template: "basic-table" - line 51, col 29)] as the response has already been committed. As a result, the response may have the wrong status code.

                         <tr th:each="emp : ${employeeList}">                             <td th:text="${emp.getId()}" ></td>                             <td th:text="${emp.getLastName()}"></td>                             <td th:text="${emp.getEmail()}">12 May 2017</td> <!--                            thymeleaf的三元表达式-->                             <td th:text="${emp.getGender()==0?'女':(emp.getGender()==1 ? '男':null)}">12 May 2017</td>                             <td th:text="${emp.getDepartment()!=null?emp.getDepartment().getDepartmentName():null}">12 May 2017</td> <!--   日期格式转换：  #dates是thymeleaf提供的一个工具，它能帮我们展示想要的日期格式-->                             <td th:text="${#dates.format(emp.getBirthday(),'yyyy-MM-dd HH:mm:ss')}">Pending</label></td>                              <td > <!--  在修改的时候,我们需要把员工id传递给后台,后台得到员工的详细信息,之后进入修改页面,这里使用的restful风格. -->                                  <a th:href="@{/employee/toUpdatePage/} + ${emp.getId()}"  class="btn btn-sm btn-primary" th:text="修改" ></a>                                     <a th:href="@{/employee/delete/} + ${emp.getId()}" class="btn btn-sm btn-danger" th:text="删除"></a>                                 </td>                             </tr>

前端代码：之前已经验证过，没有问题，那么根本原因还是在后端，后端没有正确传递数据。

最后果然，是因为我的mapper中的sql写的不对。

错误版本：

由于一开始，我的对象和table的字段一致，我以为就不要写column 和result对应了。其实还是太年轻。。。。

    <select id="queryAllEmployee" resultMap="employeeMap">         select e.id as eId,d.id as dId ,e.email,e.birthday,e.gender,e.lastName,d.departmentName          from employee e ,department d          where e.department_id = d.id     </select>      <resultMap id="employeeMap" type="employee">                  <association property="department" javaType="department">                     </association>     </resultMap>

正确版本：

    <select id="queryAllEmployee" resultMap="employeeMap">         select e.id as eId,d.id as dId ,e.email,e.birthday,e.gender,e.lastName,d.departmentName          from employee e ,department d          where e.department_id = d.id     </select>      <resultMap id="employeeMap" type="employee">         <result column="eId" property="id"></result>         <result column="email" property="email"></result>         <result column="lastName" property="lastName"></result>         <result column="birthday" property="birthday"></result>         <result column="gender" property="gender"></result>         <association property="department" javaType="department">             <result column="dId" property="id"></result>             <result column="departmentName" property="departmentName"></result>         </association>     </resultMap>