MyBatis中的注解 之前都是在映射器MapperXML配置文件中配置映射语句的。除此之外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。
使用Person进行测试
drop table person;
create table person(
id number primary key,
name varchar2(15),
age number
);
public class Person{
private Integer id;
private String name;
private Integer age;
get/set
}
5.1 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。
5.2 映射语句
MyBatis提供了多种注解来支持不同类型的语句如:SELECT,INSERT,UPDATE,DELETE。
5.2.1 @Insert
我们可以使用 @Insert注解来定义一个INSERT映射语句：
package com.briup.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);
}
自动生成主键
可以使用 @Options注解的userGeneratedKeys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.addr Id},#{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 my_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.
@Insert("insert into students(name,email,addr_id, phone)
values(#{name},#{email},#{address.addrId},#{phone})")
@SelectKey(statement="select my_seq.currval from dual",
keyProperty="studId", resultType=int.class, before=false)
int insertStudent(Student student);
5.2.2 @Update
我们可以使用 @Update注解来定义一个UPDATE映射语句，如下所示：
@Update("update students set name=#{name}, email=#{email},
phone=#{phone} where stud_id=#{studId}")
int updateStudent(Student student);
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
int noOfRowsUpdated = mapper.updateStudent(student);
5.2.3 @Delete
我们可以使用 @Delete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);
5.2.4 @Select
我们可以使用 @Select注解来定义一个SELECT映射语句。
package com.itheima.mappers;
public interface StudentMapper{
@Select("select stud_id as studid, name, email, phone from
students where stud_id=#{studId}")
Student findStudentById(Integer studId);
}
为了将列名和对象中属性名匹配，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。
5.3 结果映射
我们可以将查询结果通过别名或者是 @Results注解与JavaBean属性映射起来。
package com.itheima.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();
}
注意:
@Results注解和映射器XML配置文件元素<resultMap> 对应。然而，My Batis3.2.2不能为 @Results注解赋予一个ID。所以，不像<resultMap>元素，我们不应在不同的映射语句中重用 @Results声明。这意味着即使 @Results注解完全相同，我们也需要(在不同的映射接口中)重复 @Results声明。
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>。
xml配置:
<mapper namespace="com.itheima.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 @Result Map引用名为StudentResult的resultMap:
public interface StudentMapper{
@Select("select * from students where stud_id=#{studId}")
@ResultMap("com.itheima.mappers.StudentMapper.StudentResult")
Student findStudentById(int studId);
@Select("select * from students")
@ResultMap("com.itheima.mappers.StudentMapper.StudentResult")
List<Student> findAllStudents();
}
5.4 一对一映射
MyBatis提供了 @One注解来使用嵌套select语句（Nested-Select）加载一对一关联查询数据。让我们看看怎样使用 @One注解获取学生及其地址信息
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.itheima.mappers.Student Mapper.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find Address By Id()方法。如果 @One SELECT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());
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.itheima.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.itheima.mappers.StudentMapper.StudentWithAddressResult")
Student selectStudentWithAddress(int id);
}
5.5 一对多映射
MyBatis提供了 @Many注解，用来使用嵌套Select语句加载一对多关联查询。现在让我们看一下如何使用 @Many注解获取一个讲师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.itheima.mappers.Tutor Mapper.findAddressById")),
@Result(property = "courses", column = "tutor_id",
many = @Many(select = "com.itheima.mappers.Tutor Mapper.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find Courses By Tutor Id()方法。
之前我们使用过嵌套结果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<result Map>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.itheima.mappers.Tutor Mapper">
<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="course Id" />
<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" result Map="AddressResult" />
<collection property="courses" result Map="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}")
@Result Map("com.itheima.mappers.TutorMapper.TutorResult")
Tutor selectTutorById(int tutorId);
}
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