Mybatis搭建第一个项目
1. 简介
1.1 什么是 MyBatis？
- MyBatis 是一款优秀的持久层框架，它支持定制化 SQL、存储过程以及高级映射。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以使用简单的 XML 或注解来配置和映射原生类型、接口和 Java 的 POJO（Plain Old Java Objects，普通老式 Java 对象）为数据库中的记录。
如何获取Mybatis？
-
对应的 maven 依赖
<dependency> <groupId>org.mybatis</groupId> <artifactId>mybatis</artifactId> <version>3.5.2</version> </dependency>
1.2 持久化
数据持久化
- 持久化就是将程序的数据在持久状态和瞬时状态转化的过程
- 内存：断电即失--->数据库，io 文件持久化
1.3 持久层
Dao 层
- 完成持久化工作的代码块
- 层界限十分明显
1.4 为什么需要Mybatis?
- 传统的 JDBC 代码太复杂，简化操作，减少代码冗余。
- 帮助开发人员将数据存入到数据库中
- 灵活，易于使用
- 解除 sql 与程序代码的耦合，sql 和代码的分离，提高了可维护性
- 提供 xml 标签，支持编写动态 sql
2. 搭建第一个Mybatis程序
- 配置
mybatis-confgi.xml文件
<?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>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/youzi/dao/UserMapper.xml"/>
</mappers>
</configuration>
- 编写工具类
public class MybatisUtils {
private static SqlSessionFactory sqlSessionFactory;
static {
try {
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
public static SqlSession getSqlSession() {
return sqlSessionFactory.openSession();
}
}
- 编写 pojo
package com.youzi.pojo;
public class User {
private int id;
private String name;
private String pwd;
public User() {
}
public User(int id, String name, String pwd) {
this.id = id;
this.name = name;
this.pwd = pwd;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
...
}
- DAO 接口
public interface UserMapper {
List<User> getUserList();
User getUserById(int uId);
int addUser(User user);
int updateUser(User user);
int deleteUser(int id);
}
- 接口实现类由原来 JDBC 中的 UserDaoImpl 转变为配置文件
<?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="com.youzi.dao.UserDao">
<select id="getUserList" resultType="com.youzi.pojo.User">
select * from mybatis_db.user;
</select>
</mapper>
3. CRUD
- select
<select id="getUserList" resultType="com.youzi.pojo.User">
select * from mybatis_db.user;
</select>
<select id="getUserById" parameterType="int" resultType="com.youzi.pojo.User">
select * from user where id = #{uId};
</select>
@Test
public void test_getUser() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
List<User> userList = mapper.getUserList();
for (User user : userList) {
System.out.println(user);
}
sqlSession.close();
}
@Test
public void test_getUserById() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
User user = mapper.getUserById(1);
System.out.println(user);
sqlSession.close();
}
修改，添加，删除因为有事务的存在所以必须 commit
- insert
<insert id="addUser" parameterType="com.youzi.pojo.User">
insert into user (id, name, pwd) values (#{id},#{name},#{pwd});
</insert>
@Test
public void test_addUser() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
int i = mapper.addUser(new User(4, "王五", "2323223"));
System.out.println(i);
//重要
sqlSession.commit();
sqlSession.close();
}
- update
<update id="updateUser" parameterType="com.youzi.pojo.User">
update user set name = #{name},pwd = #{pwd} where id = #{id};
</update>
@Test
public void test_updateUser() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
int i = mapper.updateUser(new User(2, "李四", "123123"));
System.out.println(i);
sqlSession.commit();
sqlSession.close();
}
- delete
<delete id="deleteUser" parameterType="int">
delete from user where id = #{id};
</delete>
@Test
public void test_deleteUser() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
int i = mapper.deleteUser(4);
System.out.println(i);
sqlSession.commit();
sqlSession.close();
}
上面的 sqlSession 也可以使用 try() 语句创建 即 try-with-resources 自动关闭资源
try(SqlSession sqlSession = MybatisUtils.getSqlSession()){
...
}
过程中出现的错误
org.apache.ibatis.binding.BindingException: Type interface com.youzi.dao.UserDao is not known to the MapperRegistry. 没有绑定 mapper
<mappers>
<mapper resource="com/youzi/dao/UserMap.xml"/>
</mappers>
使用 map 传参
例如此时 User 中存在 50 个字段，只需要修改其中的一个值，就可以采用 map 传参
<!--使用map的方式传值不用考虑其他的字段,只用考虑要传值的地方就可以-->
<update id="updateUser2" parameterType="map">
update user set name = #{userName} where id = #{userID};
</update>
@Test
public void test_updateUser2() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
Map<String, Object> map = new HashMap<>();
map.put("userName", "zhangsan");
map.put("userID", 1);
mapper.updateUser2(map);
sqlSession.commit();
sqlSession.close();
}
模糊查询怎么写？
-
Java 代码执行的时候，传递通配符 %value%
mapper.getUser("%wang%"); -
在 sql 语句拼接中使用通配符
<select id="getUserByNameLike" resultType="com.youzi.pojo.User"> select * from user where name like "%"#{name}"%"; </select>