输出参数resultType

1.简单类型(八个基本类型+String)

/MyBatisProject3/src/org/myy/mapper/studentMapper.xml

<select id="queryStudentCount" resultType="int">
    select count(1) from student1
</select>

/MyBatisProject3/src/org/myy/mapper/StudentMapper.java

int queryStudentCount();

/MyBatisProject3/src/org/myy/test/Test.java

//Connection - SqlSession操作Mybatis
//conf.xml->reader
Reader reader = Resources.getResourceAsReader("conf.xml");
//reader->sqlSession
//可以通过build的第二参数 指定数据库环境
SqlSessionFactory sessionFactory=new SqlSessionFactoryBuilder().build(reader,"development");
SqlSession session = sessionFactory.openSession();
                            
                            
StudentMapper studentMapper = session.getMapper(StudentMapper.class);
int count=studentMapper.queryStudentCount();
System.out.println(count);
session.close();

2.输出参数为实体对象类型

/MyBatisProject3/src/org/myy/mapper/studentMapper.xml

<select id="queryStuByStuno" parameterType="int" resultType="student">
    select * from student1 where stuno=${value}
</select>

/MyBatisProject3/src/org/myy/mapper/StudentMapper.java

Student queryStuByStuno(int stuno);

/MyBatisProject3/src/org/myy/test/Test.java

//Connection - SqlSession操作Mybatis
//conf.xml->reader
Reader reader = Resources.getResourceAsReader("conf.xml");
//reader->sqlSession
//可以通过build的第二参数 指定数据库环境
SqlSessionFactory sessionFactory=new SqlSessionFactoryBuilder().build(reader,"development");
SqlSession session = sessionFactory.openSession();
                                
                                
StudentMapper studentMapper = session.getMapper(StudentMapper.class);
                        
Student student=studentMapper.queryStuByStuno(1);
System.out.println(student);
session.close();

3.输出参数为实体对象类型的集合:虽然输出类型为集合，但是resultType依然写集合的元素类型()

/MyBatisProject3/src/org/myy/mapper/studentMapper.xml

<select id="queryAllStudent" resultType="student">
    select * from student1
</select>

/MyBatisProject3/src/org/myy/mapper/StudentMapper.java

List<Student> queryAllStudent();

/MyBatisProject3/src/org/myy/test/Test.java

// Connection - SqlSession操作Mybatis
// conf.xml->reader
Reader reader = Resources.getResourceAsReader("conf.xml");
// reader->sqlSession
// 可以通过build的第二参数 指定数据库环境
SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(reader, "development");
SqlSession session = sessionFactory.openSession();

StudentMapper studentMapper = session.getMapper(StudentMapper.class);
List<Student> students = studentMapper.queryAllStudent();
System.out.println(students);
session.close();

4.输出参数类型为HashMap

Hashmap本身是一个集合,但是可以存放多个元素

　　但是根据提示发现，返回值为Hashmap时，查询的结果只能是一个学生（no，name）

->结论:一个HashMap对应一个学生的多个元素（多个属性）

/MyBatisProject3/src/org/myy/mapper/studentMapper.xml

<select id="queryAllStudentOutByHashMap" resultType="HashMap">
    select stuno "no",stuname "name" from student1
</select>

/MyBatisProject3/src/org/myy/mapper/StudentMapper.java

List<HashMap<String, Object>> queryAllStudentOutByHashMap();

/MyBatisProject3/src/org/myy/test/Test.java

// Connection - SqlSession操作Mybatis
// conf.xml->reader
Reader reader = Resources.getResourceAsReader("conf.xml");
// reader->sqlSession
// 可以通过build的第二参数 指定数据库环境
SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(reader, "development");
SqlSession session = sessionFactory.openSession();

StudentMapper studentMapper = session.getMapper(StudentMapper.class);

List<HashMap<String, Object>> studentMap = studentMapper.queryAllStudentOutByHashMap();
System.out.println(studentMap);
session.close();

resultType:一般情况下

resultMap:实体类的属性、数据表的字段:类型、名字不同时

当属性名和字段名不一致时，除了使用resultMap以外,还可以使用resultType＋HashMap

a.result

/MyBatisProject3/src/org/myy/mapper/studentMapper.xml

<select id="qureyStudentById"  parameterType="int" resultMap="qureyStudentByIdMap">
        select id,name from student1 where id=#{id}
</select>
<resultMap type="student" id="qureyStudentByIdMap">
    <!--指定类中的属性和表中的字段对应关系  -->
    <id property="stuNo" column="id"/>
    <result property="stuName" column="name"/>
</resultMap>

/MyBatisProject3/src/org/myy/mapper/StudentMapper.java

    Student qureyStudentById(int stuno);

/MyBatisProject3/src/org/myy/test/Test.java

// Connection - SqlSession操作Mybatis
// conf.xml->reader
Reader reader = Resources.getResourceAsReader("conf.xml");
// reader->sqlSession
// 可以通过build的第二参数 指定数据库环境
SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(reader, "development");
SqlSession session = sessionFactory.openSession();

StudentMapper studentMapper = session.getMapper(StudentMapper.class);
Student student = studentMapper.qureyStudentById(1);
System.out.println(student);
            session.close();

b.resultType＋HashMap

/MyBatisProject3/src/org/myy/mapper/studentMapper.xml

<select id="qureyStudentByIdWithHashMap"  parameterType="int" resultType="HashMap">
        select id "stuNo",name "stuName" from student1 where id=#{id}
</select>

/MyBatisProject3/src/org/myy/mapper/StudentMapper.java

Student qureyStudentByIdWithHashMap(int stuno);

/MyBatisProject3/src/org/myy/test/Test.java

// Connection - SqlSession操作Mybatis
// conf.xml->reader
Reader reader = Resources.getResourceAsReader("conf.xml");
// reader->sqlSession
// 可以通过build的第二参数 指定数据库环境
SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(reader, "development");
SqlSession session = sessionFactory.openSession();

StudentMapper studentMapper = session.getMapper(StudentMapper.class);

Student student = studentMapper.qureyStudentByIdWithHashMap(1);
System.out.println(student);
session.close();

注意：如果n个字段，发现其中某一个字段始终为默认值，则可能是表的字段和类的属性名字写错。