1.结果集映射ResultMap
（1）resultMap 元素是 MyBatis 中最重要最强大的元素。它可以让你从 90% 的 JDBC ResultSets 数据提取代码中解放出来
（2）ResultMap 的设计思想是，对简单的语句做到零配置，对于复杂一点的语句，只需要描述语句之间的关系就行了
（1）手动映射
用于解决实体类和数据库字段名称不匹配的问题
具体步骤：1.返回值类型为 resultMap ； 2.编写resultMap，实现手动映射
手动映射代码案例
（2）高级的结果集映射
解决数据库的查询牵扯到多表嵌套查询，如一对多和多对一
eg：学生表 & 老师表
需求：学生对应的老师
当一个实体类的属性时对象时，mapper文件中使用association标签
案例就是Student类中的属性时Teacher
学生类，注意看属性
@Alias("student")
public class Student {
private int id;
private String name;
private Teacher teacher;
public Student() {
}
public Student(int id, String name, Teacher teacher) {
this.id = id;
this.name = name;
this.teacher = teacher;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
@Override
public String toString() {
return "Student{" +
"id=" + id +
", name='" + name + '\'' +
", teacher=" + teacher +
'}';
}
}
老师类
package pojo;
import org.apache.ibatis.type.Alias;
@Alias("teacher")
public class Teacher {
private int id;
private String name;
public Teacher() {
}
public Teacher(int id, String name) {
this.id = id;
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Teacher{" +
"id=" + id +
", name='" + name + '\'' +
'}';
}
}
映射配置文件StudentMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="dao.StudentMapper">
<!-- 方法一-->
<!-- 子查询-->
<resultMap id="StudentTeacher" type="student">
<result column="id" property="id"></result>
<result column="name" property="name"></result>
<association property="teacher" column="tid" javaType="teacher" select="getTeacher"></association>
</resultMap>
<select id="getStudentInfo" resultMap="StudentTeacher">
select * from student;
</select>
<select id="getTeacher" resultType="teacher">
select * from teacher where id=#{id};
</select>
<!-- ===================================== -->
<!-- 方法二-->
<!-- 联表查询-->
<select id="getStudentInfo2" resultMap="StudentTeacher2">
select s.id,s.name,t.name tname
from student s,teacher t
where s.tid = t.id;
</select>
<resultMap id="StudentTeacher2" type="pojo.Student">
<result column="id" property="id"></result>
<result column="name" property="name"></result>
<association property="teacher" javaType="pojo.Teacher">
<result column="tname" property="name"></result>
</association>
</resultMap>
</mapper>
图解
需求：老师对应的学生
当Teacher属性中有一个属性的类型是集合时，mapper中就要使用collection标签
老师类
package pojo;
import org.apache.ibatis.type.Alias;
import java.util.List;
@Alias("teacher")
public class Teacher {
private int id;
private String name;
private List<Student> students;
public Teacher() {
}
public Teacher(int id, String name, List<Student> students) {
this.id = id;
this.name = name;
this.students = students;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Student> getStudents() {
return students;
}
public void setStudents(List<Student> students) {
this.students = students;
}
@Override
public String toString() {
return "Teacher{" +
"id=" + id +
", name='" + name + '\'' +
", students=" + students +
'}';
}
}
学生类
package pojo;
import org.apache.ibatis.type.Alias;
@Alias("student")
public class Student {
private int id;
private String name;
private int tid;
public Student() {
}
public Student(int id, String name, int tid) {
this.id = id;
this.name = name;
this.tid = tid;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getTid() {
return tid;
}
public void setTid(int tid) {
this.tid = tid;
}
@Override
public String toString() {
return "Student{" +
"id=" + id +
", name='" + name + '\'' +
", teacher=" + tid +
'}';
}
}
TeacherMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="dao.TeacherMapper">
<!-- 按结果嵌套查询-->
<select id="findByID" resultMap="TeacherStudent">
select s.id sid,s.name sname,t.name tname,t.id tid
from student s,teacher t
where s.tid=t.id and t.id=#{tid};
</select>
<resultMap id="TeacherStudent" type="teacher">
<result property="id" column="tid"></result>
<result property="name" column="tname"></result>
<!-- ofType可以理解为泛型 -->
<collection property="students" ofType="student">
<result property="id" column="sid"></result>
<result property="name" column="sname"></result>
<result property="tid" column="tid"></result>
</collection>
</resultMap>
<!-- ===================================================-->
<select id="findByID2" resultMap="TeacherStudent2">
select *
from teacher t
where t.id=#{tid};
</select>
<resultMap id="TeacherStudent2" type="teacher">
<!-- javaType是集合的类型 -->
<!-- ofType可以理解为泛型 -->
<collection property="students" javaType="ArrayList" ofType="student" select="getStudents" column="id"></collection>
</resultMap>
<select id="getStudents" resultType="student">
select *
from student s
where s.tid = #{id}
</select>
</mapper>
2.分页的实现
意义：每一次只处理少部分的数据，减轻数据库的压力
方式一：在SQL上实现分页
第一步：修改mapper.xml文件
<!-- 分页查询的实现方法 1-->
<select id="findByLimit" parameterType="map" resultMap="InfoMap">
-- limit a,b
select * from db.info limit #{startIndex},#{pageSize};
</select>
第二步：在接口中定义该方法
List<Info> findByLimit(Map<String,Integer> map);
第三步：修改测试代码
@Test
public void fingByLimitTest(){
SqlSession sqlSession = MyBatisUtils.getSqlSession();
InfoMapper mapper = sqlSession.getMapper(InfoMapper.class);
Map<String,Integer> map = new HashMap<>();
map.put("startIndex",0);
map.put("pageSize",2);
List<Info> info = mapper.findByLimit(map);
for (Info info1 : info) {
System.out.println(info1);
}
sqlSession.close();
}
方式二：在Java上做修改
映射文件还是之前的正规书写，这里只需要修改Java测试的代码即可
import org.apache.ibatis.session.RowBounds;
@Test
public void findByRowBoundsTest(){
SqlSession sqlSession = MyBatisUtils.getSqlSession();
//使用RowBounds
List<Info> info = sqlSession.selectList("dao.InfoMapper.findByRowBounds", null, new RowBounds(2, 2));
for (Info info1 : info) {
System.out.println(info1);
}
sqlSession.close();
}
方式三：分页插件
3.日志
MyBatis多使用接口，使用日志工具有利于我们的排错处理
MyBatis的日志分类：
SLF4J
Apache Commons Logging
Log4j 2
Log4j
JDK logging
MyBatis 内置日志工厂会基于运行时检测信息选择日志委托实现。它会（按上面罗列的顺序）使用第一个查找到的实现。当没有找到这些实现时，将会禁用日志功能。
- 标准日志实现
<settings>
<!-- 标准日志实现-->
<setting name="logImpl" value="STDOUT_LOGGING"/>
</settings>
- LOG4J
1.log4j可以控制日志信息输出的目的地
2.可以控制每一条日志的输出格式
3.定义日志信息的级别，能够更加细致地控制日志地生成过程，可使用配置文件来配置这些设置，不用修改代码
log4j的使用步骤
- 在Maven工程的pom.xml文件中导入依赖
<!-- https://mvnrepository.com/artifact/log4j/log4j -->
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.17</version>
</dependency>
- 编写配置文件：log4j.properties
#将等级为DEBUG的日志信息输出到console和file这两个目的地，console和file的定义在下面的代码
log4j.rootLogger=DEBUG,console,file
#控制台输出的相关设置
log4j.appender.console = org.apache.log4j.ConsoleAppender
log4j.appender.console.Target = System.out
log4j.appender.console.Threshold=DEBUG
log4j.appender.console.layout = org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern=[%c]-%m%n
#文件输出的相关设置
log4j.appender.file = org.apache.log4j.RollingFileAppender
log4j.appender.file.File=./log/log4j.log
log4j.appender.file.MaxFileSize=10mb
log4j.appender.file.Threshold=DEBUG
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=[%p][%d{yy-MM-dd}][%c]%m%n
#日志输出级别
log4j.logger.org.mybatis=DEBUG
log4j.logger.java.sql=DEBUG
log4j.logger.java.sql.Statement=DEBUG
log4j.logger.java.sql.ResultSet=DEBUG
log4j.logger.java.sql.PreparedStatement=DEBUG
- setting设置日志的实现方式
<setting name="logImpl" value="LOG4J"/>
- 在测试代码中修改
static Logger logger = Logger.getLogger(当前类名.class);
public void findByNameTest(){
logger.info("info：进入方法");
logger.debug("debug：进入方法");
logger.error("error：进入方法");
SqlSession sqlSession = MyBatisUtils.getSqlSession();
InfoMapper mapper = sqlSession.getMapper(InfoMapper.class);
Info info = mapper.findByName("张三");
System.out.println(info);
sqlSession.close();
}
@Test
public void log4j(){
logger.info("info：进入方法");
logger.debug("debug：进入方法");
logger.error("error：进入方法");
}
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