MyBatis查询功能演示

Mybatis 2024-03-02

准备工作

① 创建数据库&数据表

## 创建数据库
CREATE DATABASE `dbtest1`;

## 创建数据表
CREATE TABLE `t_user` (
  `id` INT NOT NULL AUTO_INCREMENT,
  `username` VARCHAR(20) DEFAULT NULL,
  `password` VARCHAR(20) DEFAULT NULL,
  `age` INT DEFAULT NULL,
  `gender` CHAR(1) DEFAULT NULL,
  `email` VARCHAR(50) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=INNODB AUTO_INCREMENT=16 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

② 创建实体类

public class User {

    private int id;
    private String username;
    private String password;
    private int age;
    private String gender;
    private String email;

    public User() {
    }

    public User(int id, String username, String password, int age, String gender, String email) {
        this.id = id;
        this.username = username;
        this.password = password;
        this.age = age;
        this.gender = gender;
        this.email = email;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", password='" + password + '\'' +
                ", age=" + age +
                ", gender='" + gender + '\'' +
                ", email='" + email + '\'' +
                '}';
    }
}

③ 创建mybatis核心配置文件

<?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>
    <!-- 引入外部properties文件,此后就可以在当前文件的数据源连接信息使用${}的方式访问value -->
    <properties resource="jdbc.properties"/>
    <!-- 设置类型别名 -->
    <typeAliases>
        <!--以包为单位，将包下所有的类型设置默认的类型别名，即类名且不区分大小写-->
        <package name="com.evan.entity"/>
    </typeAliases>
    <!--配置多个连接数据库的环境-->
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <!--设置连接数据库的驱动-->
                <property name="driver" value="${jdbc.driver}"/>
                <!--设置连接数据库的连接地址-->
                <property name="url" value="${jdbc.url}"/>
                <!--设置连接数据库的用户名-->
                <property name="username" value="${jdbc.username}"/>
                <!--设置连接数据库的密码-->
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <!-- 引入mybatis的映射文件 -->
    <mappers>
        <!--
            以包为单位引入映射文件
            要求：
            1、mapper接口所在的包要和映射文件所在的包一致
            2、mapper接口要和映射文件的名字一致
        -->
        <package name="com.evan.mapper"/>
    </mappers>
</configuration>

④ 创建jdbc.properties

jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/dbtest1?serverTimezone=UTC
jdbc.username=root
jdbc.password=123456

⑤ 添加依赖

<dependencies>
<!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->
<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>3.5.7</version>
</dependency>
<!-- https://mvnrepository.com/artifact/junit/junit -->
<dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>4.13.2</version>
    <scope>test</scope>
</dependency>
<!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>8.0.32</version>
</dependency>
<!-- https://mvnrepository.com/artifact/log4j/log4j -->
<dependency>
    <groupId>log4j</groupId>
    <artifactId>log4j</artifactId>
    <version>1.2.17</version>
</dependency>
<!-- https://mvnrepository.com/artifact/commons-logging/commons-logging -->
<dependency>
    <groupId>commons-logging</groupId>
    <artifactId>commons-logging</artifactId>
    <version>1.2</version>
</dependency>
</dependencies>

⑥ 创建SqlSessionUtil

public class SqlSessionUtil {

    private static final Log logger = LogFactory.getLog(SqlSessionUtil.class);

    public static SqlSession getSqlSession() {
        SqlSession sqlSession = null;
        try (InputStream is = Resources.getResourceAsStream("mybatis-config.xml");) {
            SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
            SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
            sqlSession = sqlSessionFactory.openSession(true);
        } catch (IOException e) {
            logger.error(e);
        }
        return sqlSession;
    }
}

⑦ 创建log4j.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">
    <appender name="STDOUT" class="org.apache.log4j.ConsoleAppender">
        <param name="Encoding" value="UTF-8" />
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%-5p %d{MM-dd HH:mm:ss,SSS}%m (%F:%L) \n" />
        </layout>
    </appender>
    <logger name="java.sql">
        <level value="debug" />
    </logger>
    <logger name="org.apache.ibatis">
        <level value="info" />
    </logger>
    <root>
        <level value="debug" />
        <appender-ref ref="STDOUT" />
    </root>
</log4j:configuration>

查询返回实体类对象的用户信息

查询返回一条数据使用实体类接收。

/**
  * 根据id查询用户信息
  * @param id
  * @return
  */
User getUserById(@Param("id") Integer id);

<select id="getUserById" resultType="com.evan.entity.User">
	select * from t_user where id = #{id}
</select>

@Test
public void test1() {
	SqlSession sqlSession = SqlSessionUtil.getSqlSession();
	SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
	System.out.println(mapper.getUserById(17));
	sqlSession.close();
}

查询返回List集合的所有用户信息

使用List集合接收查询返回的多条数据。

    /**
     * 查询所有用户信息
     * @return
     */
    List<User> getAllUser();

    <select id="getAllUser" resultType="com.evan.entity.User">
        select * from t_user
    </select>

@Test
public void test2() {
	SqlSession sqlSession = SqlSessionUtil.getSqlSession();
	SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
	List<User> users = mapper.getAllUser();
	users.forEach(System.out::println);
	sqlSession.close();
}

说明:
当查询的数据为多条时，不能使用实体类作为返回值，否则会抛出异常TooManyResultsException；
但是若查询的数据只有一条，可以使用实体类或集合作为返回值。

查询返回Map集合的所有用户信息

Map集合返回单条记录

    /**
     * 根据id查询用户信息(Map集合接收)
     * @param id
     * @return
     */
    Map<String,Object> getUserByIdMap(@Param("id") Integer id);

<!--Map<String,Object> getUserByIdMap(@Param("id") Integer id);-->
<select id="getUserByIdMap" resultType="map">
    select * from t_user where id = #{id}
</select>

@Test
public void test3() {
    SqlSession sqlSession = SqlSessionUtil.getSqlSession();
    SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
    Map<String, Object> users = mapper.getUserByIdMap(1002);
    System.out.println(users);
    sqlSession.close();
}

Map集合返回多条记录

方式1：使用List集合接收Map查询的数据

/**
  * 查询所有用户信息(list集合接收Map集合查询的所有数据)
  * @return
  */
List<Map<String,Object>> getAllUserMap();

<!-- List<Map<String,Object>> getAllUserMap(); -->
<select id="getAllUserMap" resultType="map">
    select * from t_user
</select>

@Test
public void test5() {
    SqlSession sqlSession = SqlSessionUtil.getSqlSession();
    SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
    List<Map<String, Object> > userList = mapper.getAllUserMap();
    userList.forEach(System.out::println);
    sqlSession.close();
}

方式2：@MapKey注解

@MapKey("id")
Map<String,Object> getAllUserMap();

<!-- Map<String,Object> getAllUserMap(); -->
<select id="getAllUserMap" resultType="map">
    select * from t_user
</select>

@Test
public void test5() {
    SqlSession sqlSession = SqlSessionUtil.getSqlSession();
    SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
    Map<String, Object> userMap = mapper.getAllUserMap();
    System.out.println(userMap);
    sqlSession.close();
}

结论:
查询所有的用户信息为map集合时：
若查询的数据有多条时，并且要将每条数据转换为map集合，此时有两种解决方案：
1、将mapper接口方法的返回值设置为泛型是map的list集合
List<Map<String,Object>> getAllUserMap();
显示结果的格式(无序结果集)：
{password=123456,gender=男,id=1001,age=18,email=lisi@qq.com,username=李四}
2、可以将每条数据转换的map集合放到一个大的Map集合中，但是必须要通过@MapKey注解将查询的某个字段的值作为大的Map集合的键。
@MapKey("id")
Map<String,Object> getAllUserMap();
相当于：Map<String,Map<String,Object>>
显示结果格式：
{
1001={password=123456, gender=男, id=1001, age=15, email=root@qq.com, username=root},
1002={password=123456, gender=女, id=1002, age=20, email=zh3@sina.cn, username=张三}
}

查询总记录数

/**
* 查询用户总记录数
* @return
*/
Integer getCount();

<!--int getCount();-->
<!-- 在MyBatis中，默认对于Java中常用的类型都设置了类型别名 -->
<select id="getCount" resultType="integer">
    select count(1) from t_user
</select>

@Test
public void test4() {
    SqlSession sqlSession = SqlSessionUtil.getSqlSession();
    SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
    Integer count = mapper.getCount();
    System.out.println(count);
    sqlSession.close();
}

MyBatis对Java常用类型设置的类型别名

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