Java Persistence with MyBatis 3(中文版) 第四章使用注解配置SQL映射器

时间：2022-03-10 17:48

在上一章，我们看到了我们是怎样在映射器Mapper XML配置文件中配置映射语句的。MyBatis也支持使用注解来配置映射语句。当我们使用基于注解的映射器接口时，我们不再需要在XML配置文件中配置了。如果你愿意，你也可以同时使用基于XML和基于注解的映射语句。

本章将涵盖以下话题：

l 在映射器Mapper接口上使用注解

l 映射语句

           @Insert，@Update，@Delete,@SeelctStatements

l 结果映射

           一对一映射

           一对多映射

l 动态SQL

           @SelectProvider

           @InsertProvider

           @UpdateProvider

           @DeleteProvider

package com.mybatis3.mappers; public interface StudentMapper { @Insert("INSERT INTO STUDENTS(STUD_ID,NAME,EMAIL,ADDR_ID, PHONE) VALUES(#{studId},#{name},#{email},#{address.addrId},#{phone})") int insertStudent(Student student); } 使用了@Insert注解的insertMethod()方法将返回insert语句执行后影响的行数。

@Insert("INSERT INTO STUDENTS(NAME,EMAIL,ADDR_ID, PHONE) VALUES(#{name},#{email},#{address.addrId},#{phone})") @Options(useGeneratedKeys = true, keyProperty = "studId") int insertStudent(Student student);
这里STUD_ID列值将会通过MySQL数据库自动生成。并且生成的值将会被设置到student对象的studId属性中。

StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
mapper.insertStudent(student);
int studentId = student.getStudId();

有一些数据库如Oracle，并不支持AUTO_INCREMENT列属性，它使用序列（SEQUENCE）来产生主键的值。

我们可以使用@SelectKey注解来为任意SQL语句来指定主键值，作为主键列的值。

假设我们有一个名为STUD_ID_SEQ的序列来生成STUD_ID主键值。

@Insert("INSERT INTO STUDENTS(STUD_ID,NAME,EMAIL,ADDR_ID, PHONE) 
VALUES(#{studId},#{name},#{email},#{address.addrId},#{phone})")
@SelectKey(statement="SELECT STUD_ID_SEQ.NEXTVAL FROM DUAL", 
keyProperty="studId", resultType=int.class, before=true)
int insertStudent(Student student);

这里我们使用了@SelectKey来生成主键值，并且存储到了student对象的studId属性上。由于我们设置了before=true,该语句将会在执行INSERT语句之前执行。

如果你使用序列作为触发器来设置主键值，我们可以在INSERT语句执行后，从sequence_name.currval获取数据库产生的主键值。

@Insert("INSERT INTO STUDENTS(NAME,EMAIL,ADDR_ID, PHONE) 
VALUES(#{name},#{email},#{address.addrId},#{phone})")
@SelectKey(statement="SELECT STUD_ID_SEQ.CURRVAL FROM DUAL", 
keyProperty="studId", resultType=int.class, before=false)
int insertStudent(Student student);

@Update("UPDATE STUDENTS SET NAME=#{name}, EMAIL=#{email}, PHONE=#{phone} WHERE STUD_ID=#{studId}") int updateStudent(Student student);

使用了@Update的updateStudent()方法将会返回执行了update语句后影响的行数。

StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
int noOfRowsUpdated = mapper.updateStudent(student);

@Delete("DELETE FROM STUDENTS WHERE STUD_ID=#{studId}") int deleteStudent(int studId); 使用了@Delete的deleteStudent()方法将会返回执行了update语句后影响的行数。

package com.mybatis3.mappers; public interface StudentMapper { @Select("SELECT STUD_ID AS STUDID, NAME, EMAIL, PHONE FROM STUDENTS WHERE STUD_ID=#{studId}") Student findStudentById(Integer studId); }

为了将列名和Studentbean属性名匹配，我们为stud_id起了一个studId的别名。如果返回了多行结果，将抛出 TooManyResultsException异常。

package com.mybatis3.mappers; public interface StudentMapper { @Select("SELECT * FROM STUDENTS") @Results( { @Result(id = true, column = "stud_id", property = "studId"), @Result(column = "name", property = "name"), @Result(column = "email", property = "email"), @Result(column = "addr_id", property = "address.addrId") }) List<Student> findAllStudents(); } gxlsystem.com,布布扣

例如，看下面的findStudentById()和findAllStudents()方法：

@Select("SELECT * FROM STUDENTS WHERE STUD_ID=#{studId}")
@Results(
{
    @Result(id = true, column = "stud_id", property = "studId"),
    @Result(column = "name", property = "name"),
    @Result(column = "email", property = "email"),
    @Result(column = "addr_id", property = "address.addrId")
})
Student findStudentById(int studId);
@Select("SELECT * FROM STUDENTS")
@Results(
{
    @Result(id = true, column = "stud_id", property = "studId"),
    @Result(column = "name", property = "name"),
    @Result(column = "email", property = "email"),
    @Result(column = "addr_id", property = "address.addrId")
})
List<Student> findAllStudents();

这里两个语句的@Results配置完全相同，但是我必须得重复它。这里有一个解决方法。我们可以创建一个映射器Mapper配置文件，然后配置<resultMap>元素，然后使用@ResultMap注解引用此<resultMap>。

在StudentMapper.xml中定义一个ID为StudentResult的<resultMap>。

<mapper namespace="com.mybatis3.mappers.StudentMapper">
  <resultMap type="Student" id="StudentResult">
    <id property="studId" column="stud_id" />
    <result property="name" column="name" />
    <result property="email" column="email" />
    <result property="phone" column="phone" />
  </resultMap>
</mapper>

在StudentMapper.java中，使用@ResultMap引用名为StudentResult的resultMap。

public interface StudentMapper
{
    @Select("SELECT * FROM STUDENTS WHERE STUD_ID=#{studId}")
    @ResultMap("com.mybatis3.mappers.StudentMapper.StudentResult")
    Student findStudentById(int studId);
    @Select("SELECT * FROM STUDENTS")
    @ResultMap("com.mybatis3.mappers.StudentMapper.StudentResult")
    List<Student> findAllStudents();
}

public interface StudentMapper { @Select("SELECT ADDR_ID AS ADDRID, STREET, CITY, STATE, ZIP, COUNTRY FROM ADDRESSES WHERE ADDR_ID=#{id}") Address findAddressById(int id); @Select("SELECT * FROM STUDENTS WHERE STUD_ID=#{studId} ") @Results( { @Result(id = true, column = "stud_id", property = "studId"), @Result(column = "name", property = "name"), @Result(column = "email", property = "email"), @Result(property = "address", column = "addr_id", one = @One(select = "com.mybatis3.mappers.StudentMapper. findAddressById")) }) Student selectStudentWithAddress(int studId); }
这里我们使用了@One注解的select属性来指定一个使用了完全限定名的方法上，该方法会返回一个Address对象。使用column=”addr_id”,则STUEDNTS表中列addr_id的值将会作为输入参数传递给findAddressById()方法。如果@OneSELECT查询返回了多行结果，则会抛出TooManyResultsException异常。

int studId = 1;
StudentMapper studentMapper = 
sqlSession.getMapper(StudentMapper.class);
Student student = studentMapper.selectStudentWithAddress(studId);
System.out.println("Student :"+student);
System.out.println("Address :"+student.getAddress());

在第三章，使用XML配置SQL映射器中我们讨论过，我们可以通过基于XML的映射器配置，使用嵌套结果ResultMap来加载一对一关联的查询。而MyBatis3.2.2版本，并没有对应的注解支持。但是我们可以在映射器Mapper配置文件中配置<resultMap>并且使用@ResultMap注解来引用它。

在StudentMapper.xml中配置<resultMap>，如下所示：

<mapper namespace="com.mybatis3.mappers.StudentMapper">
  <resultMap type="Address" id="AddressResult">
    <id property="addrId" column="addr_id" />
    <result property="street" column="street" />
    <result property="city" column="city" />
    <result property="state" column="state" />
    <result property="zip" column="zip" />
    <result property="country" column="country" />
  </resultMap>
  <resultMap type="Student" id="StudentWithAddressResult">
    <id property="studId" column="stud_id" />
    <result property="name" column="name" />
    <result property="email" column="email" />
    <association property="address" resultMap="AddressResult" />
  </resultMap>
</mapper>

public interface StudentMapper
{
    @Select("select stud_id, name, email, a.addr_id, street, city,
            state, zip, country" + " FROM students s left outer join addresses a
            on s.addr_id=a.addr_id" + " where stud_id=#{studId} ")
    @ResultMap("com.mybatis3.mappers.StudentMapper.
               StudentWithAddressResult")
    Student selectStudentWithAddress(int id);
}

public interface TutorMapper { @Select("select addr_id as addrId, street, city, state, zip, country from addresses where addr_id=#{id}") Address findAddressById(int id); @Select("select * from courses where tutor_id=#{tutorId}") @Results( { @Result(id = true, column = "course_id", property = "courseId"), @Result(column = "name", property = "name"), @Result(column = "description", property = "description"), @Result(column = "start_date" property = "startDate"), @Result(column = "end_date" property = "endDate") }) List<Course> findCoursesByTutorId(int tutorId); @Select("SELECT tutor_id, name as tutor_name, email, addr_id FROM tutors where tutor_id=#{tutorId}") @Results( { @Result(id = true, column = "tutor_id", property = "tutorId"), @Result(column = "tutor_name", property = "name"), @Result(column = "email", property = "email"), @Result(property = "address", column = "addr_id", one = @One(select = " com.mybatis3. mappers.TutorMapper.findAddressById")), @Result(property = "courses", column = "tutor_id", many = @Many(select = "com.mybatis3.mappers.TutorMapper. findCoursesByTutorId")) }) Tutor findTutorById(int tutorId); }

这里我们使用了@Many注解的select属性来指向一个完全限定名称的方法，该方法将返回一个List<Course>对象。使用column=”tutor_id”，TUTORS表中的tutor_id列值将会作为输入参数传递给findCoursesByTutorId()方法。

在第三章，使用XML配置SQL映射器中我们讨论过，我们可以通过基于XML的映射器配置，使用嵌套结果ResultMap来加载一对多关联的查询。而MyBatis3.2.2版本，并没有对应的注解支持。但是我们可以在映射器Mapper配置文件中配置<resultMap>并且使用@ResultMap注解来引用它。

在TutorMapper.xml中配置<resultMap>,如下所示：

<mapper namespace="com.mybatis3.mappers.TutorMapper">
  <resultMap type="Address" id="AddressResult">
    <id property="addrId" column="addr_id" />
    <result property="street" column="street" />
    <result property="city" column="city" />
    <result property="state" column="state" />
    <result property="zip" column="zip" />
    <result property="country" column="country" />
  </resultMap>
  <resultMap type="Course" id="CourseResult">
    <id column="course_id" property="courseId" />
    <result column="name" property="name" />
    <result column="description" property="description" />
    <result column="start_date" property="startDate" />
    <result column="end_date" property="endDate" />
  </resultMap>
  <resultMap type="Tutor" id="TutorResult">
    <id column="tutor_id" property="tutorId" />
    <result column="tutor_name" property="name" />
    <result column="email" property="email" />
    <association property="address" resultMap="AddressResult" />
    <collection property="courses" resultMap="CourseResult" />
  </resultMap>
</mapper>

public interface TutorMapper
{
    @Select("SELECT T.TUTOR_ID, T.NAME AS TUTOR_NAME, EMAIL,
            A.ADDR_ID, STREET, CITY, STATE, ZIP, COUNTRY, COURSE_ID, C.NAME,
            DESCRIPTION, START_DATE, END_DATE  FROM TUTORS T LEFT OUTER
            JOIN ADDRESSES A ON T.ADDR_ID=A.ADDR_ID LEFT OUTER JOIN COURSES
            C ON T.TUTOR_ID=C.TUTOR_ID WHERE T.TUTOR_ID=#{tutorId}")
    @ResultMap("com.mybatis3.mappers.TutorMapper.TutorResult")
    Tutor selectTutorById(int tutorId);
}

package com.mybatis3.sqlproviders; import org.apache.ibatis.jdbc.SQL; public class TutorDynaSqlProvider { public String findTutorByIdSql(int tutorId) { return "SELECT TUTOR_ID AS tutorId, NAME, EMAIL FROM TUTORS WHERE TUTOR_ID=" + tutorId; } } 在TutorMapper.java接口中创建一个映射语句，如下：

@SelectProvider(type=TutorDynaSqlProvider.class, method="findTutorByIdSql")
Tutor findTutorById(int tutorId);

这里我们使用了@SelectProvider来指定了一个类，及其内部的方法，用来提供需要执行的SQL语句。

但是使用字符串拼接的方法唉构建SQL语句是非常困难的，并且容易出错。所以MyBaits提供了一个SQL工具类不使用字符串拼接的方式，简化构造动态SQL语句。

现在，让我们看看如何使用org.apache.ibatis.jdbc.SQL工具类来准备相同的SQL语句。

package com.mybatis3.sqlproviders;
import org.apache.ibatis.jdbc.SQL;
public class TutorDynaSqlProvider
{
    public String findTutorByIdSql(final int tutorId)
    {
        return new SQL()
        {
            {
                SELECT("tutor_id as tutorId, name, email");
                FROM("tutors");
                WHERE("tutor_id=" + tutorId);
            }
        } .toString();
    }
}

SQL工具类会处理以合适的空格前缀和后缀来构造SQL语句。

动态SQL provider方法可以接收以下其中一种参数：

? 无参数
? 和映射器Mapper接口的方法同类型的参数
? java.util.Map

如果SQL语句的准备不取决于输入参数，你可以使用不带参数的SQL Provider方法。

例如：

public String findTutorByIdSql()
{
    return new SQL()
    {
        {
            SELECT("tutor_id as tutorId, name, email");
            FROM("tutors");
            WHERE("tutor_id = #{tutorId}");
        }
    } .toString();
}

这里我们没有使用输入参数构造SQL语句，所以它可以是一个无参方法。

如果映射器Mapper接口方法只有一个参数，那么可以定义SQLProvider方法，它接受一个与Mapper接口方法相同类型的参数。

例如映射器Mapper接口有如下定义：

<span style="font-family:Microsoft YaHei;font-size:12px;">Tutor findTutorById(int tutorId);</span>

这里findTutorById(int)方法只有一个int类型的参数。我们可以定义findTutorByIdSql(int)方法作为SQL provider方法。

public String findTutorByIdSql(final int tutorId)
{
    return new SQL()
    {
        {
            SELECT("tutor_id as tutorId, name, email");
            FROM("tutors");
            WHERE("tutor_id=" + tutorId);
        }
    } .toString();
}

如果映射器Mapper接口有多个输入参数，我们可以使用参数类型为java.util.Map的方法作为SQLprovider方法。然后映射器Mapper接口方法所有的输入参数将会被放到map中，以param1,param2等等作为key，将输入参数按序作为value。你也可以使用0，1，2等作为key值来取的输入参数。

@SelectProvider(type = TutorDynaSqlProvider.class,
                method = "findTutorByNameAndEmailSql")
Tutor findTutorByNameAndEmail(String name, String email);
public String findTutorByNameAndEmailSql(Map<String, Object> map)
{
    String name = (String) map.get("param1");
    String email = (String) map.get("param2");
    //you can also get those values using 0,1 keys
    //String name = (String) map.get("0");
    //String email = (String) map.get("1");
    return new SQL()
    {
        {
            SELECT("tutor_id as tutorId, name, email");
            FROM("tutors");
            WHERE("name=#{name} AND email=#{email}");
        }
    } .toString();
}

SQL工具类也提供了其他的方法来表示JOINS，ORDER_BY，GROUP_BY等等。

让我们看一个使用LEFT_OUTER_JOIN的例子：

public class TutorDynaSqlProvider
{
    public String selectTutorById()
    {
        return new SQL()
        {
            {
                SELECT("t.tutor_id, t.name as tutor_name, email");
                SELECT("a.addr_id, street, city, state, zip, country");
                SELECT("course_id, c.name as course_name, description,
                       start_date, end_date");
                FROM("TUTORS t");
                LEFT_OUTER_JOIN("addresses a on t.addr_id=a.addr_id");
                LEFT_OUTER_JOIN("courses c on t.tutor_id=c.tutor_id");
                WHERE("t.TUTOR_ID = #{id}");
            }
        } .toString();
    }
}

public interface TutorMapper
{
    @SelectProvider(type = TutorDynaSqlProvider.class,
                    method = "selectTutorById")
    @ResultMap("com.mybatis3.mappers.TutorMapper.TutorResult")
    Tutor selectTutorById(int tutorId);
}

由于没有支持使用内嵌结果ResultMap的一对多关联映射的注解支持，我们可以使用基于XML的<resultMap>配置，然后与@ResultMap映射。

<mapper namespace="com.mybatis3.mappers.TutorMapper">
  <resultMap type="Address" id="AddressResult">
    <id property="id" column="addr_id" />
    <result property="street" column="street" />
    <result property="city" column="city" />
    <result property="state" column="state" />
    <result property="zip" column="zip" />
    <result property="country" column="country" />
  </resultMap>
  <resultMap type="Course" id="CourseResult">
    <id column="course_id" property="id" />
    <result column="course_name" property="name" />
    <result column="description" property="description" />
    <result column="start_date" property="startDate" />
    <result column="end_date" property="endDate" />
  </resultMap>
  <resultMap type="Tutor" id="TutorResult">
    <id column="tutor_id" property="id" />
    <result column="tutor_name" property="name" />
    <result column="email" property="email" />
    <association property="address" resultMap="AddressResult" />
    <collection property="courses" resultMap="CourseResult"></collection>
  </resultMap>
</mapper>

使用了动态的SQL provider，我们可以取得讲师及其地址和课程明细。

public class TutorDynaSqlProvider { public String insertTutor(final Tutor tutor) { return new SQL() { { INSERT_INTO("TUTORS"); if (tutor.getName() != null) { VALUES("NAME", "#{name}"); } if (tutor.getEmail() != null) { VALUES("EMAIL", "#{email}"); } } } .toString(); } } public interface TutorMapper { @InsertProvider(type = TutorDynaSqlProvider.class, method = "insertTutor") @Options(useGeneratedKeys = true, keyProperty = "tutorId") int insertTutor(Tutor tutor); }

public class TutorDynaSqlProvider { public String updateTutor(final Tutor tutor) { return new SQL() { { UPDATE("TUTORS"); if (tutor.getName() != null) { SET("NAME = #{name}"); } if (tutor.getEmail() != null) { SET("EMAIL = #{email}"); } WHERE("TUTOR_ID = #{tutorId}"); } } .toString(); } } public interface TutorMapper { @UpdateProvider(type = TutorDynaSqlProvider.class, method = "updateTutor") int updateTutor(Tutor tutor); }

public class TutorDynaSqlProvider { public String deleteTutor(int tutorId) { return new SQL() { { DELETE_FROM("TUTORS"); WHERE("TUTOR_ID = #{tutorId}"); } } .toString(); } } public interface TutorMapper { @DeleteProvider(type = TutorDynaSqlProvider.class, method = "deleteTutor") int deleteTutor(int tutorId); }

4.5 总结

在本章中，我们学习了怎样使用注解书写SQL映射语句。讨论了如何配置简单语句，一对一关系语句和一对多关系语句。我们还探讨了怎样使用SqlProvider注解来构建动态SQL语句。在下一章，我们将讨论如何将MyBatis与Spring框架集成。

前一章：
下一章：Java Persistence with MyBatis 3(中文版) 第五章与Spring集成

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