标签：对象 r文件 方法 str 注解 tty value 讲解 代理
mybatis框架让我们能在编程中只需要编写一个接口，然后再编写mapper映射文件，无需编写接口的实现类。其实这是mybatis通过动态代理，把mapper映射文件的内容转化为真正的执行部分。因此我们在编程中，需要特别关注接口和映射文件的编写。本节主要讲解接口的返回值类型在mapper文件中的编写方式。
我们思考下，返回值类型一般分为
由于每次建立工程比较复杂，可以参考第一节：mybatis入门来搭建一个简单的工程，然后来测试本节内容。
注意本节内容关注点是mapper接口的返回值类型和mapper文件的resultType的编写。
1、mapper接口，我们简单查询所有记录，返回Long类型的值。
public interface PersonMapper { Long getTotalNumberOfPerson(); }
2、mapper映射文件
<select id="getTotalNumberOfPerson" resultType="long"> select count(*) from person </select>
注意在mapper文件中，只需要 resultType 为 long 类型即可。
3、测试
public class Main { public static void main(String[] args) throws IOException { InputStream resourceAsStream = Resources.getResourceAsStream("mybatis-config.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream); SqlSession sqlSession = sqlSessionFactory.openSession(); PersonMapper mapper = sqlSession.getMapper(PersonMapper.class); Long totalNumberOfPerson= mapper.getTotalNumberOfPerson(); System.out.println(totalNumberOfPerson); //10 sqlSession.close(); } }
这种查询直接返回和数据库表对应的相关实体，并且只有一条记录，一般都是按照id去查询，也就是第一节：mybatis入门中演示的样例，我们再次说明下。
1、mapper接口
public interface PersonMapper { Person getPerson(Integer id); }
2、mapper映射文件
<select id="getPerson" resultType="com.yefengyu.mybatis.entity.Person"> select id, first_name firstName, last_name lastName, age, email, address from person where id = #{id} </select>
3、测试
public class Main { public static void main(String[] args) throws IOException { InputStream resourceAsStream = Resources.getResourceAsStream("mybatis-config.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream); SqlSession sqlSession = sqlSessionFactory.openSession(); PersonMapper mapper = sqlSession.getMapper(PersonMapper.class); Person person = mapper.getPerson(1); System.out.println(person); sqlSession.close(); } }
总结：在查询单个对象，返回的是实体类型的时候，只需要将resultType设置为全类名即可。
1、mapper接口，根据address查询多个Person对象
public interface PersonMapper { List<Person> getPersons(String address); }
2、mapper映射文件，注意对于mapper接口中返回List类型的，mapper映射文件的resultType只需要设置List所包含的对象的类型即可。
<select id="getPersons" resultType="com.yefengyu.mybatis.entity.Person"> select id, first_name firstName, last_name lastName, age, email, address from person where address = #{address} </select>
3、测试
public class Main { public static void main(String[] args) throws IOException { InputStream resourceAsStream = Resources.getResourceAsStream("mybatis-config.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream); SqlSession sqlSession = sqlSessionFactory.openSession(); PersonMapper mapper = sqlSession.getMapper(PersonMapper.class); List<Person> persons = mapper.getPersons("beijing"); for (int i = 0; i < persons.size(); i++) { System.out.println(persons.get(i)); } sqlSession.close(); } }
4、结果：
Person{id=1, firstName=‘Schmitt‘, lastName=‘Carine‘, age=25, email=‘null‘, address=‘beijing‘}
Person{id=2, firstName=‘King‘, lastName=‘Jean‘, age=36, email=‘Jean@163.com‘, address=‘beijing‘}
Person{id=8, firstName=‘Gao‘, lastName=‘Diego‘, age=45, email=‘66666@qq.com‘, address=‘beijing‘}
Person{id=9, firstName=‘Piestrzeniewicz‘, lastName=‘Schmitt‘, age=36, email=‘44444@qq.com‘, address=‘beijing‘}
Person{id=10, firstName=‘Frdrique‘, lastName=‘Juri‘, age=25, email=‘99999@qq.com‘, address=‘beijing‘}
1、mapper接口
public interface PersonMapper { Map<String, Object> getPersonMap(Integer id); }
2、mapper映射文件,此时需要把resultType设置为map
<select id="getPersonMap" resultType="map"> select id, first_name firstName, last_name lastName, age, email, address from person where id = #{id} </select>
3、测试
public class Main { public static void main(String[] args) throws IOException { InputStream resourceAsStream = Resources.getResourceAsStream("mybatis-config.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream); SqlSession sqlSession = sqlSessionFactory.openSession(); PersonMapper mapper = sqlSession.getMapper(PersonMapper.class); Map<String, Object> personMap = mapper.getPersonMap(1); System.out.println(personMap); sqlSession.close(); } }
4、结果如下，注意在使用map接收返回值的时候，如果某个字段为null，那么就不会封装到map中，比如下面的结果中就没有Email字段的结果。
{firstName=Schmitt, lastName=Carine, address=beijing, id=1, age=25}
1、mapper接口
public interface PersonMapper { @MapKey("id") Map<Integer, Person> getPersonsMap(String address); }
2、mapper映射文件
<select id="getPersonsMap" resultType="map"> select id, first_name firstName, last_name lastName, age, email, address from person where address = #{address} </select>
3、测试
public class Main { public static void main(String[] args) throws IOException { InputStream resourceAsStream = Resources.getResourceAsStream("mybatis-config.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream); SqlSession sqlSession = sqlSessionFactory.openSession(); PersonMapper mapper = sqlSession.getMapper(PersonMapper.class); Map<Integer, Person> personMap = mapper.getPersonsMap("beijing"); for (Map.Entry<Integer, Person> personEntry : personMap.entrySet()) { System.out.println(personEntry.getKey() + " : " + personEntry.getValue()); } sqlSession.close(); } }
4、结果
1 : {firstName=Schmitt, lastName=Carine, address=beijing, id=1, age=25}
2 : {firstName=King, lastName=Jean, address=beijing, id=2, age=36, email=Jean@163.com}
8 : {firstName=Gao, lastName=Diego, address=beijing, id=8, age=45, email=66666@qq.com}
9 : {firstName=Piestrzeniewicz, lastName=Schmitt, address=beijing, id=9, age=36, email=44444@qq.com}
10 : {firstName=Frdrique, lastName=Juri, address=beijing, id=10, age=25, email=99999@qq.com}注意：mapper接口需要使用MapKey注解指定将某个属性的值作为map的key。本例中使用person表的主键id对应的实体的属性id作为key。除此之外，mapper映射文件中resultType设置为map。此外如果某个字段为null，那么就不插入到map中。
标签：对象 r文件 方法 str 注解 tty value 讲解 代理
原文地址：https://www.cnblogs.com/ye-feng-yu/p/10987587.html
