[Mybatis]Mapped Statements collection does not contain value for selectOne

学习使用 Mybatis 时，碰到 Mapped Statements collection does not contain value for selectOne 异常。

代码如下：

public class TestEmp {

    private SqlSession session;

    @Before
    public void before() throws IOException {
        SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
        InputStream is = Resources.getResourceAsStream("sqlMapperConfig.xml");
        SqlSessionFactory factory = builder.build(is);
        session = factory.openSession();
    }

    @After
    public void after() {
        session.close();
    }
    
    @Test
    public void testSelectOne() {
        Emp emp = session.selectOne("selectOne");
        System.out.println(emp);
    }

}

调用 selectOne 方法时报错，原因就是 Mybatis 没有读取到对应的方法 id，再具体一点就是在 sqlMapperConfig 文件中没有声明读取对应的 XML 映射文件。

找到 configurations 标签下的 mappers 子标签，配置 mapper 标签，设置 resource 属性为对应的实体类的 XML 映射文件即可，这样就不会报错了。

<configuration>
    <properties resource="jdbc.properties"/>
    <settings>
        <setting name="logImpl" value="LOG4J"/>
    </settings>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>
    <!--加载mapper映射文件-->
    <mappers>
        <mapper resource="mapper/EmpMapper.xml"/>
    </mappers>
</configuration>