mybatis-note-笔记
每日一图
Mysql连接出错
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/tmp/mysql.sock' (2)
有可能是mysql没有启动出错
Mybatis第一个代码：
设置POM，配置jar
这里是正确的dependency的maven
<dependencies>
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.4.6</version>
</dependency>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.11</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
</dependency>
</dependencies>
配置mybatis-config.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>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="com.mysql.cj.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/mybatis?characterEncoding=utf8&useSSL=false&serverTimezone=UTC&rewriteBatchedStatements=true"/>
<property name="username" value="root"/>
<property name="password" value="Mysql2486"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="userMapper.xml"/>
</mappers>
</configuration>
创建DAO、UTILS、POJO
Idea里面可以看到这个sqlSession的各种功能
public class mybatisutil {
private static SqlSessionFactory sqlSessionFactory;//将原本的sqlSessionFactory提到这里，提升作用域
static {
// static初始的代码块，一开始就会加载
InputStream inputStream = null;
try {
//获取sqlsession的对象，固定执行
String resource = "mybatis-config.xml";
inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
public static SqlSession getSqlSession(){
return sqlSessionFactory.openSession();//这个工长就是生产各种的功能
}
}
配置文件对应的就是dao的实现类，实现dao接口
<?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.userDao">
<!-- 相当于实现了接口 id就是函数的名字-->
<select id="getUserList" resultType="pojo.User" >
-- 结果集不要写ListUserzheg ，直接写user就可以了
select * from user_tb;
</select>
</mapper>
测试
对于测试的程序要放到绿色的test里面
这里是userDAO的测试方法
public class userDaoTest {
@Test
public void test(){
SqlSession sqlSession = mybatisutil.getSqlSession();
//第一种方法直接getMapper 就是获取dao的实现，然后获取对象。
userDao mapper = sqlSession.getMapper(DAO.userDao.class);
List<User> userList = mapper.getUserList();
for (User user : userList) {
System.out.println(user);
}
sqlSession.close();
}
}
Mybatis
注意！⚠️要提交事务才能使得结果发生改变，不然不会进入表里面
<select id="getUserList" resultType="pojo.User" >
-- 结果集不要写ListUserzheg ，直接写user就可以了
select * from mybatis.user_tb;
</select>
<select id="getUserById" resultType="pojo.User" parameterType="int">
select * from mybatis.user_tb where id = #{id};
</select>
<insert id="insertUser" parameterType="pojo.User">
insert into mybatis.user_tb values (#{id},#{name},#{pwd});
</insert>
<!--这里的上面的id name pwd就是User里面的属性，直接去取就是了-->
<update id="updateUser" parameterType="pojo.User">
update mybatis.user_tb
set name = #{name},pwd=#{pwd}
where id = #{id};
</update>
<delete id="deleteUser" parameterType="int">
delete
from mybatis.user_tb
where id= #{id};
</delete>
public List<User> getUserList();
public User getUserById(int id);
public void insertUser(User user);
void updateUser(User user);
void deleteUser(int i );
注意要提交事务
配置文件properties
数据库这些属性都是可外部配置且可动态替换的，既可以在典型的 Java 属性文件中配置，亦可通过 properties 元素的子元素来传递。具体的官方文档
我们来优化我们的配置文件
第一步 ; 在资源目录下新建一个db.properties
这里我的包错了因为这个转译符号&
driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis?characterEncoding=utf8&useSSL=false&serverTimezone=UTC&rewriteBatchedStatements=true
username=root
password=Mysql2486
第二步 : 将文件导入properties 配置文件
<configuration>
<!--导入properties文件-->
<properties resource="db.properties"/>
<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="mapper/UserMapper.xml"/>
</mappers>
</configuration>
配置文件优先级问题：优先外部文件
新特性：使用占位符
Mybatis注解模式
直接不用额外去实现，也不用谢标签了id和参数直接注解
@Select("select * from mybatis.user_tb")
List<User> getAll();
Association-Collection-resultMap
下面有一个经典的错误：类没有加POJO所以找不到类
Caused by: org.apache.ibatis.exceptions.PersistenceException:
### Error building SqlSession.
### The error may exist in studentMapper.xml
### The error occurred while processing mapper_resultMap[getAllMap]_association[teacher]
### Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML. The XML location is 'studentMapper.xml'. Cause: org.apache.ibatis.builder.BuilderException: Error resolving class. Cause: org.apache.ibatis.type.TypeException: Could not resolve type alias 'Teacher'. Cause: java.lang.ClassNotFoundException: Cannot find class: Teacher
上面那个错误
association
一个学生只有一个老师
多个学生共有一个老师
<?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.studentMapper">
<select id="getAll" resultMap="getAllMap">
SELECT s.id sid,s.name sname,t.name tname FROM mybatis.student s,mybatis.teacher t WHERE s.tid=t.id;
</select>
<!-- 这下面的Type就是resultType的意思，例如说本来是要对应Student的返回值
就是你要得到结果是Student的类，但因为有一个Teacher捣烂，所以
我们用resultMap来接受，这个resultMap里面的type就定义为Student-->
<resultMap id="getAllMap" type="pojo.Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<association property="teacher" javaType="pojo.Teacher">
<result property="name" column="tname"></result>
</association>
</resultMap>
</mapper>
collection
一个老师有多个学生 ，返回的使用collection来装
<?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.teacherMapper">
<select id="getById" resultMap="getByIdMap">
SELECT s.name sname,t.name tname FROM mybatis.student s,mybatis.teacher t WHERE s.tid=t.id AND t.id=#{id};
</select>
<resultMap id="getByIdMap" type="pojo.Teacher">
<result property="name" column="tname"/>
<collection property="students" ofType="pojo.Student">
<result property="name" column="sname"></result>
</collection>
</resultMap>
</mapper>
//在这个类teacher里面有一个students的成员，所以在resultMap里面使用collection 用ofType来制定返回的是什么
private int id;
private String name;
private List<Student> students;
动态sql
IF语句
关键词：IF(test = "title!=null")
<?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.blogMapper">
<select id="queryOne" parameterType="map" resultType="pojo.Blog">
select * from mybatis.blog where 1=1
<if test="title!=null">
and title = #{title}
</if>
<if test="author!=null">
and author=#{author}
</if>
</select>
</mapper>
气死我了，这个map.put的位置放错了
加载完我才put
SqlSession sqlSession = mybatisutil.getSqlSession();
blogMapper mapper = sqlSession.getMapper(blogMapper.class);
HashMap<String, String> map = new HashMap<>();
map.put("title","帅气");
List<Blog> blogs = mapper.queryOne(map);
System.out.println("====/====///====");
for (Blog blog : blogs) {
System.out.println(blog);
}
SQL片段-IF-WHEN -Otherwise 语句
IF语句
<update id="updateOne" parameterType="map">
update mybatis.blog
<set>
<if test="title != null">
title = #{title},
</if>
<if test="author != null">
author = #{author},
</if>
</set>
WHERE id = #{id}
</update>
CHOOSE
<choose>
<when test="title != null">
AND title like #{title}
</when>
<when test="author != null and author.name != null">
AND author_name like #{author.name}
</when>
<otherwise>
AND featured = 1
</otherwise>
</choose>
缓存
序列化是深拷贝，所以反序列化后的对像和原对象不是同一个对象，故哈希值不同
报错的地方
