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MyBatis基础1

发表于 更新于 分类于 Valine： 8.9k 8 分钟
MyBatis基础1

导入依赖

创建一个父项目，导入依赖

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<!--        mysql连接依赖-->
<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>8.0.21</version>
</dependency>
<!--        mybatis依赖-->
<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>3.5.2</version>
</dependency>
<!--        junit测试依赖-->
<dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>4.13</version>
</dependency>

MyBatis核心配置文件

在resource文件夹下创建核心配置文件，在这个文件下可以配置环境（数据源）、注册mapper、设置类别名等等。

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<?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>
<!--    在这里可以配置多个环境，开发环境和生产环境，可用于数据源的切换-->
    <environments default="development">
        <environment id="development">
<!--            事务管理-->
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=true&amp;useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
<!--            注册mapper-->
    <mappers>
        <mapper resource="com/muchlab/mapper/UserMapper.xml"/>
    </mappers>
</configuration>

读取MyBatis核心配置文件

在读取配置文件前,先介绍三个关键的MyBatis类:

  1. SqlSessionFactoryBuilder:通过读取配置文件,来创建SqlSessionFactory,这里使用了建造者模式,一旦创建了 SqlSessionFactory，就不再需要它了。 因此 SqlSessionFactoryBuilder 实例的最佳作用域是方法作用域（也就是局部方法变量）。但最好还是不要一直保留着它，以保证所有的 XML 解析资源可以被释放给更重要的事情。
  2. SqlSessionFactory:SqlSessionFactory 一旦被创建就应该在应用的运行期间一直存在，最好不要去释放它或重新创建一个新的实例,最简单的就是使用单例模式或者静态单例模式。
  3. SqlSession:每个线程都应该有它自己的 SqlSession 实例。SqlSession 的实例不是线程安全的，因此是不能被共享的，所以它的最佳的作用域是请求或方法作用域。换句话说，每次收到 HTTP 请求，就可以打开一个 SqlSession，返回一个响应后，就关闭它。 这个关闭操作很重要，为了确保每次都能执行关闭操作，你应该把这个关闭操作放到 finally 块中或使用try语法来释放该资源。

创建一个工具类,用来返回SqlSession实例

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public class MybatisUtils {
    private static SqlSessionFactory sqlSessionFactory;

    static {
        try{
            //根据mybatis-config.xml来获取SqlSessionFactory，
            // SqlSessionFactory是用来获取SqlSession
            // SqlSession 提供了在数据库执行 SQL 命令所需的所有方法
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    //返回创建好的SqlSession
    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

创建实体类和Mapper接口

Mapper接口就相当于Dao接口或Repository接口,是外界用来操作数据库的接口

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@Data
@AllArgsConstructor
@NoArgsConstructor
@ToString
public class User {
    private int id;
    private String name;
    private String pwd;
}

public interface UserMapper {
    //获取数据库中所有的User
    List<User> getAll();
}

创建对应的Mapper.xml

每一个Mapper接口都应该对应一个Mapper.xml文件,该文件是用来解释Mapper接口中方法对应的sql操作,文件里的具体细节请参考官方文档

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<?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.muchlab.mapper.UserMapper">
    <select id="getAll" resultType="com.muchlab.entity.User">
        select * from user
    </select>
</mapper>

编写测试类

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public class UserMapperTest {
    @Test
    public void test(){
        //获取SqlSession对象
        try (SqlSession sqlSession = MybatisUtils.getSqlSession()){
            //方式一：getMapper
            UserMapper mapper = sqlSession.getMapper(UserMapper.class);
            List<User> all = mapper.getAll();
            all.forEach(c->{
                System.out.println(c);
            });

            //方式二：涉及到类型强转，不推荐
            List<User> userlist = sqlSession.selectList("com.muchlab.mapper.UserMapper.getAll");
            userlist.forEach(c->{
                System.out.println(c);
            });
        }
    }
}

遇到的问题

  1. Mapper接口定义后,必须在核心配置文件中注册Mapper.xml
  2. 资源过滤问题,程序运行后会生成目标文件,因为编译后默认只包含.class文件,如果没配置资源过滤,在target文件夹中会找不到Mapper.xml文件,在pom文件中添加下面代码:
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<build>
    <resources>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>false</filtering>
        </resource>
        <resource>
            <directory>src/main/resources</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>false</filtering>
        </resource>
    </resources>
</build>

select、insert、update、delete

  1. id：对应于Mapper接口中的方法名
  2. resultType：接口中方法的返回类型
  3. parameterType：接口中方法的传入参数类型
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<?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.muchlab.mapper.UserMapper">
    <insert id="addUser" parameterType="com.muchlab.entity.User">
        insert into user (id, name, pwd) values (#{id}, #{name}, #{pwd});
    </insert>
    <update id="updateUser" parameterType="com.muchlab.entity.User">
        update user set name=#{name}, pwd=#{pwd} where id=#{id} ;
    </update>
    <delete id="deleteUser" parameterType="int">
        delete from user where id=#{id}
    </delete>
    <select id="getAll" resultType="com.muchlab.entity.User">
        select * from user
    </select>
    <select id="getById" resultType="com.muchlab.entity.User" parameterType="int">
        select * from user alias where alias.id = #{id};
    </select>
</mapper>

插入、更新、删除需要执行commit方法，执行commit方法才会把数据提交到数据库中。

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public interface UserMapper {
    List<User> getAll();
    User getById(int id);
    int addUser(User user);
    int updateUser(User user);
    int deleteUser(int id);
}


public class UserMapperTest {
    @Test
    public void getAll(){
        //获取SqlSession对象
        try (SqlSession sqlSession = MybatisUtils.getSqlSession()){
            //方式一：getMapper
            UserMapper mapper = sqlSession.getMapper(UserMapper.class);
            List<User> all = mapper.getAll();
            all.forEach(c->{
                System.out.println(c);
            });

            //方式二：涉及到类型强转，不推荐
            List<User> userlist = sqlSession.selectList("com.muchlab.mapper.UserMapper.getAll");
            userlist.forEach(c->{
                System.out.println(c);
            });
        }
    }
    @Test
    public void getById(){
        try(SqlSession sqlSession = MybatisUtils.getSqlSession()){
            UserMapper mapper = sqlSession.getMapper(UserMapper.class);
            User user = mapper.getById(1);
            System.out.println(user);
        }
    }
    @Test
    public void addUser(){
        try(SqlSession sqlSession = MybatisUtils.getSqlSession()){
            UserMapper mapper = sqlSession.getMapper(UserMapper.class);
            int result = mapper.addUser(new User(6,"John Lennon", "12345"));
            if (result>0){
                System.out.println("插入成功");
            }
            List<User> all = mapper.getAll();
            for (User user : all) {
                System.out.println(user);
            }
            //执行commit才把插入的数据提交上去
            sqlSession.commit();

        }
    }
    @Test
    public void updateUser(){
        try (SqlSession sqlSession = MybatisUtils.getSqlSession()){
            UserMapper mapper = sqlSession.getMapper(UserMapper.class);
            User user = mapper.getById(1);
            user.setName("Cobain");
            int i = mapper.updateUser(user);
            if (i>0){
                System.out.println("修改成功");
            }
            List<User> all = mapper.getAll();
            for (User user1 : all) {
                System.out.println(user1);
            }
            sqlSession.commit();
        }
    }
    @Test
    public void deleteUser(){
        try(SqlSession sqlSession = MybatisUtils.getSqlSession()){
            UserMapper mapper = sqlSession.getMapper(UserMapper.class);
            int i = mapper.deleteUser(0);
            if (i>0){
                System.out.println("删除成功");
            }
            List<User> all = mapper.getAll();
            for (User user : all) {
                System.out.println(user);
            }
            sqlSession.commit();
        }
    }
}

万能的Map

使用实体类进行传参，在sql中是取出它的字段名；二map是取出它的key；

map可以进行数据塑形，根据你需要的来插入、更新，一般在局部更新和多参数中比较有用。

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int addUserMap(Map<String, Object> map);
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<insert id="addUserMap">
    insert into user (id, name, pwd) values (#{a}, #{b}, #{c});
</insert>
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@Test
public void addUserMap(){
    try (SqlSession sqlSession = MybatisUtils.getSqlSession()){
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        Map<String, Object> map = new HashMap<>();
        map.put("a", 7);
        map.put("b", "ChenZheKai");
        map.put("c", "123456");
        int i = mapper.addUserMap(map);
        if (i>0){
            System.out.println("插入成功");
        }
        sqlSession.commit();
    }
}

模糊查询

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List<User> getUserLike(String name);
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<select id="getUserLike" resultType="com.muchlab.entity.User">
    select * from user alias where alias.name like "%"#{name}"%";
</select>
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@Test
public void addUserLike(){
    try (SqlSession sqlSession = MybatisUtils.getSqlSession()){
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        List<User> userLike = mapper.getUserLike("b");
        for (User user : userLike) {
            System.out.println(user);
        }
    }
}

参考资料:B站狂神

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