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
4.1 在映射器Mapper接口上使用注解
MyBatis对于大部分的基于XML的映射器元素（包括<select>,<update>）提供了对应的基于注解的配置项。然而在某些情况下，基于注解配置 还不能支持基于XML的一些元素。
4.2 映射语句
MyBatis提供了多种注解来支持不同类型的语句(statement)如SELECT,INSERT,UPDATE,DELETE。让我们看一下具体怎样配置映射语句。
4.2.1 @Insert
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);}[email protected]()方法将返回insert语句执行后影响的行数。[自动生成主键]
在上一章中我们讨论过主键列值[email protected]atedKeys 和keyProperty属性让数据库产生auto_increment（自增长）列的值，然后将生成的值设置到输入参数对象的属性中。
@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。
[email protected]??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);[email protected]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);4.2.2 @Update
[email protected]，如下所示：
@Update("UPDATE STUDENTS SET NAME=#{name}, EMAIL=#{email}, PHONE=#{phone} WHERE STUD_ID=#{studId}")int updateStudent(Student student);[email protected]()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);4.2.3 @Delete
[email protected] 注解来定义一个DELETE映射语句，如下所示：
@Delete("DELETE FROM STUDENTS WHERE STUD_ID=#{studId}")int deleteStudent(int studId);[email protected]()方法将会返回执行了update语句后影响的行数。4.2.4 @Select
我们可以使用@ Select注解来定义一个SELECT映射语句。
让我们看一下怎样使用注解配置一个简单的select查询。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。
4.3 结果映射
[email protected]JavaBean属性映射起来。
[email protected]定JavaBean属性映射器来，执行SELECT查询的：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();}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();
[email protected]，[email protected]?可以创建一个映射器Mapper配置文件， 然后配置<resultMap>元素，[email protected]<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，[email protected]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();}4.3.1 一对一映射
[email protected]（Nested-Select）加载一对一关联查询数据。[email protected]??。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);}
[email protected]??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()[email protected]，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>[email protected]
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);}4.3.2 一对多映射
[email protected]，用来使用嵌套Select语句加载一对多关联查询。
[email protected]及其教授课程列表信息：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);}[email protected]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>[email protected]
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);}
4.4 动态SQL
有时候我们需要根据输入条件动态地构建SQL语句。[email protected],@UpdateProvider,@[email protected]，来帮助构建动态SQL语句，然后让MyBatis执行这些SQL语句。
4.4.1 @SelectProvider
[email protected]?个简单的SELECT映射语句的例子。
创建一个TutorDynaSqlProvider.java类，以及findTutorByIdSql()方法，如下所示：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);[email protected]，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，[email protected]<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。4.4.2 @InsertProvider
[email protected]?，如下所示：
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);}4.4.3 @UpdateProvider
[email protected]，如下所示：
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);}4.4.4 @DeleteProvider
[email protected]?,如下所示：
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：Java Persistence with MyBatis 3(中文版) 第三章 使用XML配置SQL映射器
下一章：Java Persistence with MyBatis 3(中文版) 第五章 与Spring集成
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