MyBatis的入门
MyBatis是什么
- 开源的持久层框架。
- jdbc 易掌握，性能好，代码繁琐，易出错
- hibernate 难掌握，性能不好，代码非常简洁，虽然不用写sql,但经常需要优化sql
- mybatis 比较容易掌握，性能一般，代码简洁，需要写sql
开发步骤
- 导包 mybatis,mysql,junit
- 添加配置文件 (SqlMapConfig.xml)
- 实体类,属性名要与表的字段名一样（大小写可以忽略)
- 添加映射文件(告诉mybatis,如何进行对象关系映射，里面主要是一些sql语句)
- 修改配置文件，添加映射文件的位置
- 调用mybatis提供的api访问数据库(主要是调用SqlSession类提供的方法）
jar包依赖
<dependencies>
<!-- mybatis的jar包 -->
<!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.5.4</version>
</dependency>
<!-- mysql的jar包 -->
<!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.19</version>
</dependency>
<!-- junit单元测试 -->
<!-- https://mvnrepository.com/artifact/junit/junit -->
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
</dependency>
</dependencies>
SqlMapConfig.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties resource="db.properties" />
<typeAliases>
<package name="entity"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}" />
<property name="url" value="${jdbc.url}" />
<property name="username" value="${jdbc.username}" />
<property name="password" value="${jdbc.password}" />
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="entity/EmployeeMapper.xml" />
</mappers>
</configuration>
db.properties
jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/ems?serverTimezone=Asia/Shanghai&useUnicode=true&characterEncoding=utf-8
jdbc.username=root
jdbc.password=root
数据库
//创建数据库
create database ems;
//创建表
CREATE TABLE t_emp (
id int(11) NOT NULL AUTO_INCREMENT COMMENT '主键',
ename varchar(255) DEFAULT NULL COMMENT '雇员名称',
salary decimal(10,2) DEFAULT NULL COMMENT '薪水',
age int(11) DEFAULT NULL COMMENT '年龄',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8
实体类
package entity;
public class Employee {
private int id;
private String ename;
private double salary;
private int age;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getEname() {
return ename;
}
public void setEname(String ename) {
this.ename = ename;
}
public double getSalary() {
return salary;
}
public void setSalary(double salary) {
this.salary = salary;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String toString() {
return "Employee [id=" + id + ", ename=" + ename + ", salary=" + salary + ", age=" + age + "]";
}
}
映射文件
EmployeeMapper.xml的配置
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="test">
<insert id="save" parameterType="Employee" useGeneratedKeys="true" keyProperty="id">
insert into t_emp values(null,#{ename},#{salary},#{age})
</insert>
<delete id="delete" parameterType="int">
delete from t_emp where id=#{id}
</delete>
<update id="update" parameterType="Employee">
update t_emp set ename=#{ename},salary=#{salary},age=#{age} where id=#{id}
</update>
<select id="findById" parameterType="int" resultType="Employee">
select * from t_emp where id=#{id}
</select>
<select id="findAll" resultType="Employee">
select * from t_emp
</select>
</mapper>
MyBatis配置文件SqlMapConfig.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties resource="db.properties" />
<typeAliases>
<package name="entity"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}" />
<property name="url" value="${jdbc.url}" />
<property name="username" value="${jdbc.username}" />
<property name="password" value="${jdbc.password}" />
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="entity/EmployeeMapper.xml" />
</mappers>
</configuration>
测试程序
package test;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;
import entity.Employee;
public class TestCase {
SqlSessionFactory sf;
@Before
public void before() throws IOException {
String resource = "SqlMapConfig.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sf = new SqlSessionFactoryBuilder().build(inputStream);
}
@Test
public void save() {
SqlSession ss = sf.openSession();
Employee e = new Employee();
e.setEname("小红");
e.setAge(16);
e.setSalary(2000);
ss.insert("test.save", e);
System.out.println(e.getId());
ss.commit();
ss.close();
}
@Test
public void delete() {
SqlSession ss = sf.openSession();
ss.delete("test.delete", 5);
ss.commit();
ss.close();
}
@Test
public void update() {
SqlSession ss = sf.openSession();
Employee e = new Employee();
e.setId(6);
e.setAge(23);
e.setEname("小红");
e.setSalary(2000);
ss.update("test.update", e);
ss.commit();
ss.close();
}
@Test
public void findById() {
SqlSession ss = sf.openSession();
Employee e = ss.selectOne("test.findById", 6);
System.out.println(e);
ss.close();
}
@Test
public void findAll() {
SqlSession ss = sf.openSession();
List<Employee> employees = ss.selectList("test.findAll");
for (Employee employee : employees) {
System.out.println(employee);
}
ss.close();
}
}
工作原理
返回Map类型的结果
mybatis会将查询到的记录先存放到map对象里面（以字段名作为
key,以字段值作为value,一条记录对应一个map）,再将map中存放的
数据添加到实体对象（默认以key作为属性名来赋值）。
注：
返回Map类型的结果，可以不用写实体类，但是取字段值时，
需要调用get方法，需要考虑字段名的差异（比如oracle数据库
会将字段名大写）。
还是返回实体对象取值方便。
映射文件EmployeeMapper.xml
添加如下代码
<select id="findById2" parameterType="int" resultType="map">
select * from t_emp where id=#{id}
</select>
测试程序
@Test
public void findById2() {
SqlSession ss = sf.openSession();
Map map = ss.selectOne("test.findById2", 6);
System.out.println(map);
ss.close();
}
测试结果
解决实体类的属性名与表的字段名不一致的情况
方式一 使用别名
SELECT name as ename…
方式二 使用resultMap元素
实体类修改
package entity;
public class Emp2 {
private int empNo;
private String name;
private double salary;
private int age;
public int getEmpNo() {
return empNo;
}
public void setEmpNo(int empNo) {
this.empNo = empNo;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public double getSalary() {
return salary;
}
public void setSalary(double salary) {
this.salary = salary;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String toString() {
return "Emp2 [empNo=" + empNo + ", name=" + name + ", salary=" + salary + ", age=" + age + "]";
}
}
映射文件EmployeeMapper.xml修改
<resultMap type="Emp2" id="Emp2ResultMap">
<id column="id" property="empNo"/>
<result column="ename" property="name"/>
<result column="salary" property="salary"/>
<result column="age" property="age"/>
</resultMap>
<select id="findAll2" resultMap="Emp2ResultMap">
select * from t_emp
</select>
测试程序
SqlSessionFactory sf;
@Before
public void before() throws IOException {
String resource = "SqlMapConfig.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sf = new SqlSessionFactoryBuilder().build(inputStream);
}
@Test
public void findAll2() {
SqlSession ss = sf.openSession();
List<Emp2> employees = ss.selectList("test.findAll2");
for (Emp2 employee : employees) {
System.out.println(employee);
}
ss.close();
}
测试结果
Mapper映射器
Mapper映射器是什么
是一个符合映射文件要求的接口。
注：mybatis会生成一个符合该接口的实现类。
使用Mapper映射器
- 定义接口
方法名必须与sqlId一样
参数类型必须与parameterType一样
返回类型必须与resultType一样
2、映射文件中的namespace必须等于接口名(全限定名)
3、调用SqlSession提供的getMapper方法。
修改映射文件Emp2.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="dao.EmployeeDAO">
<insert id="save" parameterType="Employee" useGeneratedKeys="true" keyProperty="id">
insert into t_emp values(null,#{ename},#{salary},#{age})
</insert>
<delete id="delete" parameterType="int">
delete from t_emp where id=#{id}
</delete>
<update id="update" parameterType="Employee">
update t_emp set ename=#{ename},salary=#{salary},age=#{age} where id=#{id}
</update>
<select id="findById" parameterType="int" resultType="Employee">
select * from t_emp where id=#{id}
</select>
<select id="findAll" resultType="Employee">
select * from t_emp
</select>
</mapper>
接口EmployeeDAO
package dao;
import java.util.List;
import entity.Employee;
public interface EmployeeDAO {
public void save(Employee e);
public void delete(int id);
public void update(Employee e);
public Employee findById(int id);
public List<Employee>findAll();
}
mybatis配置文件中添加Emp2.xml的映射
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties resource="db.properties" />
<typeAliases>
<package name="entity"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}" />
<property name="url" value="${jdbc.url}" />
<property name="username" value="${jdbc.username}" />
<property name="password" value="${jdbc.password}" />
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="entity/EmployeeMapper.xml" />
<mapper resource="entity/Emp2.xml" />
</mappers>
</configuration>
测试程序
package test;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;
import dao.EmployeeDAO;
import entity.Employee;
public class TestCase2 {
SqlSessionFactory sf;
@Before
public void before() throws IOException {
String resource = "SqlMapConfig.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sf = new SqlSessionFactoryBuilder().build(inputStream);
}
@Test
public void save() {
SqlSession ss = sf.openSession();
EmployeeDAO employeeDAO = ss.getMapper(EmployeeDAO.class);
Employee e = new Employee();
e.setEname("腊月红");
e.setAge(24);
e.setSalary(40000);
employeeDAO.save(e);
System.out.println(e.getId());
ss.commit();
ss.close();
}
@Test
public void delete() {
SqlSession ss = sf.openSession();
EmployeeDAO employeeDAO = ss.getMapper(EmployeeDAO.class);
employeeDAO.delete(7);
ss.commit();
ss.close();
}
@Test
public void update() {
SqlSession ss = sf.openSession();
EmployeeDAO employeeDAO = ss.getMapper(EmployeeDAO.class);
Employee e = new Employee();
e.setId(8);
e.setEname("呼呼");
e.setAge(17);
e.setSalary(4000);
employeeDAO.update(e);
ss.commit();
ss.close();
}
@Test
public void findById() {
SqlSession ss = sf.openSession();
EmployeeDAO employeeDAO = ss.getMapper(EmployeeDAO.class);
Employee e = employeeDAO.findById(8);
System.out.println(e);
ss.close();
}
@Test
public void findAll() {
SqlSession ss = sf.openSession();
EmployeeDAO employeeDAO = ss.getMapper(EmployeeDAO.class);
List<Employee> employees = employeeDAO.findAll();
for (Employee employee : employees) {
System.out.println(employee);
}
ss.close();
}
}
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