Java的MyBatis快速入门和实战详解

什么是MyBatis

MyBatis 是一款轻量级的 Java 持久层框架。 它支持自定义 SQL、存储过程以及高级映射。MyBatis 通过简化 JDBC 编程来实现对数据库的操作，并将 SQL 语句与程序代码分离，使应用程序的开发和维护更加简单。

MyBatis快速入门

环境搭建

• 安装 JDK
• 安装 Maven
• 创建 Maven 项目

Maven 配置

<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>3.5.6</version>
</dependency>

数据库配置

使用 MyBatis 操作数据库需要在 mybatis-config.xml 中配置数据库连接信息和映射文件路径。配置示例如下：

<configuration>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/testdb"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/example/mapper/BookMapper.xml"/>
    </mappers>
</configuration>

映射文件编写

MyBatis 通过自定义 XML 配置文件进行数据操作。映射文件中主要包含了要执行的 SQL 语句、参数映射、结果映射等内容。

以下示例演示了如何编写一个查询 SQL 的映射文件：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.mapper.BookMapper">
    <select id="getBookById" resultType="com.example.model.Book">
        SELECT * FROM book WHERE book_id = #{bookId}
    </select>
</mapper>

Java 代码编写

在 Java 代码中创建并使用 MyBatis 的 SQL 会话工厂来执行 SQL 命令。下面的示例展示如何在 Java 代码中调用查询 SQL：

public class BookDao {

    private SqlSessionFactory sqlSessionFactory;

    public BookDao(SqlSessionFactory sqlSessionFactory) {
        this.sqlSessionFactory = sqlSessionFactory;
    }

    public Book getBookById(int bookId) {
        try (SqlSession session = sqlSessionFactory.openSession()) {
            return session.selectOne("com.example.mapper.BookMapper.getBookById", bookId);
        }
    }

}

MyBatis实战详解

使用 MyBatis 进行增删改查

在 MyBatis 中，增删改查对应的 SQL 语句分别是 insert、update、delete 和 select，每个 SQL 对应一个映射文件。

以下示例演示了如何编写和调用 insert SQL：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.mapper.BookMapper">
    <insert id="addBook" parameterType="com.example.model.Book">
        INSERT INTO book (book_id, book_name, author, price, invent_date) 
        VALUES (#{bookId}, #{bookName}, #{author}, #{price}, #{inventDate})
    </insert>
</mapper>

public class BookDao {

    private SqlSessionFactory sqlSessionFactory;

    public BookDao(SqlSessionFactory sqlSessionFactory) {
        this.sqlSessionFactory = sqlSessionFactory;
    }

    public void addBook(Book book) {
        try (SqlSession session = sqlSessionFactory.openSession()) {
            session.insert("com.example.mapper.BookMapper.addBook", book);
            session.commit();
        }
    }

}

使用 MyBatis 进行动态 SQL

MyBatis 提供了一套强大的动态 SQL 语言来帮助我们编写灵活的 SQL 语句。使用动态 SQL 可以根据不同的条件组合生成不同的 SQL 语句。

以下示例演示了如何使用动态 SQL 编写查询 SQL：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.mapper.BookMapper">
    <select id="getBookList" resultType="com.example.model.Book">
        SELECT * FROM book
        <where>
            <if test="bookId != null">
                AND book_id = #{bookId}
            </if>
            <if test="bookName != null">
                AND book_name LIKE CONCAT('%', #{bookName}, '%')
            </if>
        </where>
    </select>
</mapper>

public class BookDao {

    private SqlSessionFactory sqlSessionFactory;

    public BookDao(SqlSessionFactory sqlSessionFactory) {
        this.sqlSessionFactory = sqlSessionFactory;
    }

    public List<Book> getBookList(Integer bookId, String bookName) {
        try (SqlSession session = sqlSessionFactory.openSession()) {
            BookMapper mapper = session.getMapper(BookMapper.class);
            return mapper.getBookList(bookId, bookName);
        }
    }

}

示例1：使用 MyBatis 进行分页查询

以下示例演示如何使用 MyBatis 进行分页查询：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.mapper.BookMapper">
    <select id="getBookListByPage" resultType="com.example.model.Book">
        SELECT * FROM book 
        <where>
            <if test="bookName != null">
                AND book_name LIKE CONCAT('%', #{bookName}, '%')
            </if>
        </where>
        ORDER BY book_id
        LIMIT #{start}, #{size}
    </select>
</mapper>

public class BookDao {

    private SqlSessionFactory sqlSessionFactory;

    public BookDao(SqlSessionFactory sqlSessionFactory) {
        this.sqlSessionFactory = sqlSessionFactory;
    }

    public List<Book> getBookListByPage(String bookName, int start, int size) {
        try (SqlSession session = sqlSessionFactory.openSession()) {
            BookMapper mapper = session.getMapper(BookMapper.class);
            return mapper.getBookListByPage(bookName, start, size);
        }
    }

}

示例2：使用 MyBatis 进行一对多关联查询

以下示例演示如何使用 MyBatis 进行一对多关联查询：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.mapper.BookMapper">
    <resultMap id="bookMap" type="com.example.model.Book">
        <id property="bookId" column="book_id"/>
        <result property="bookName" column="book_name"/>
        <result property="author" column="author"/>
        <result property="price" column="price"/>
        <result property="inventDate" column="invent_date"/>
        <collection property="bookItems" ofType="com.example.model.BookItem">
            <id property="itemId" column="item_id"/>
            <result property="itemName" column="item_name"/>
            <result property="count" column="count"/>
        </collection>
    </resultMap>
    <select id="getBookWithItemsById" resultMap="bookMap">
        SELECT book.book_id, book.book_name, book.author, book.price, book.invent_date,
               book_item.item_id, book_item.item_name, book_item.count
        FROM book
        LEFT JOIN book_item ON book.book_id = book_item.book_id
        WHERE book.book_id = #{bookId}
    </select>
</mapper>

public class BookDao {

    private SqlSessionFactory sqlSessionFactory;

    public BookDao(SqlSessionFactory sqlSessionFactory) {
        this.sqlSessionFactory = sqlSessionFactory;
    }

    public Book getBookWithItemsById(int bookId) {
        try (SqlSession session = sqlSessionFactory.openSession()) {
            BookMapper mapper = session.getMapper(BookMapper.class);
            return mapper.getBookWithItemsById(bookId);
        }
    }

}