MyBatis 实践 --Mapper与DAO

Posted 2020-06-17 菜鸟-翡青

MyBatis 实践

标签： Java与存储

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MyBatis简介

MyBatis前身是iBatis,是一个基于Java的数据持久层/对象关系映射(ORM)框架.

MyBatis是对JDBC的封装,使开发人员只需关注SQL本身,而不需花费过多的精力去处理如注册驱动、设置参数、创建Connection/Statement、解析结果集等JDBC过程性代码.MyBatis基于XML/注解的方式配置Statement,执行SQL,并将执行结果映射成Java对象, 大大降低了数据库开发的难度.

MyBatis is a first class persistence framework with support for custom SQL, stored procedures and advanced mappings. MyBatis eliminates almost all of the JDBC code and manual setting of parameters and retrieval of results. MyBatis can use simple XML or Annotations for configuration and map primitives, Map interfaces and Java POJOs (Plain Old Java Objects) to database records.
– MyBatis项目地址/在线文档.

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初识MyBatis

使用MyBatis需要在pom.xml中添加如下依赖:

<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>3.3.0</version>
</dependency>
<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>5.1.36</version>
</dependency>

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Select

• 配置mybatis/mybatis-configuration.xml
  作为MyBatis的全局配置文件,其配置了MyBatis的运行环境信息(如数据源/mapper文件等).

<?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">
            <!-- 配置JDBC事务管理-->
            <transactionManager type="JDBC"/>
            <!-- 配置数据源-->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://host:port/db?characterEncoding=utf-8"/>
                <property name="username" value="username"/>
                <property name="password" value="password"/>
            </dataSource>
        </environment>
    </environments>

    <!-- 加载mapper映射文件 -->
    <mappers>
        <mapper resource="mybatis/mapper/UserDAO.xml"/>
    </mappers>
</configuration>

• 书写UserDAO(mapper映射)
  最为MyBatis最核心的部分,配置了操作数据库的SQL语句:

<?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="namespace">

    <select id="selectUserById" parameterType="java.lang.Integer" resultType="com.fq.domain.User">
        SELECT * FROM user WHERE id = #{id};
    </select>

    <select id="selectUserByName" parameterType="java.lang.String" resultType="com.fq.domain.User">
        SELECT * FROM user WHERE name LIKE '%${value}%';
    </select>

</mapper>

属性	描述
namespace	命名空间,用于隔离SQL语句
parameterType	定义SQL输入映射类型,MyBatis通过OGNL从输入对象中获取参数传入SQL语句.
resultType	定义SQL输出映射类型,MyBatis将SQL查询结果的一行记录映射为resultType指定的类型.

mapper映射文件名有UserDAO.xml/UserMapper.xml/User.xml等几种形式, 其一般存放在与mybatis-configuration.xml同级的mapper目录下,由于其主要作用为定义SQL语句与映射关系, 因此一般统称为mapper映射文件.

• 定义PO类
  PO类主要作用为SQL(输入/输出)映射,通常与数据库表对应:

/**
 * @author jifang
 * @since 15/12/31 下午2:27.
 */
public class User {

    private Integer id;

    private String name;

    private String password;

    public User() {
    }

    public User(Integer id, String name, String password) {
        this.id = id;
        this.name = name;
        this.password = password;
    }

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPassword() {
        return password;
    }

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

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

• UserDAO(Java对象)
  获得SqlSession,执行SQL语句, 得到映射结果:

/**
 * @author jifang
 * @since 16/2/24 下午6:15.
 */
public class UserDAO {

    private SqlSessionFactory factory;

    @Before
    public void setUp() throws IOException {
        String resource = "mybatis/mybatis-configuration.xml";
        factory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream(resource));
    }

    @Test
    public void selectUserById() {
        try (SqlSession session = factory.openSession()) {
            User user = session.selectOne("namespace.selectUserById", 1);
            System.out.println(user);
        }
    }

    @Test
    public void selectUserByName() {
        try (SqlSession session = factory.openSession()) {
            List<User> users = session.selectList("namespace.selectUserByName", "student");
            for (User user : users) {
                System.out.println(user);
            }
        }
    }
}

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Insert

• mapper

<insert id="insertUser" parameterType="com.fq.domain.User">
    INSERT INTO user(name, password) VALUES(#{name}, #{password});
</insert>

• UserDAO

@Test
public void insertUser() {
    try (SqlSession session = factory.openSession()) {
        User user = new User();
        user.setName("new_name1");
        user.setPassword("new_password");
        session.insert("namespace.insertUser", user);
        session.commit();
    }
}

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自增主键返回

修改mapper文件,添加<selectKey/>,可以将MySQL的自增主键(即刚刚插入数据时生成的ID)返回:

<insert id="insertUser" parameterType="com.fq.domain.User">
    <selectKey keyProperty="id" order="AFTER" resultType="java.lang.Integer">
        SELECT LAST_INSERT_ID();
    </selectKey>
    INSERT INTO user(name, password) VALUES(#{name}, #{password});
</insert>

属性	描述
keyProperty	指定存储到DO中的哪个属性;
order	selectKey执行顺序(相对于insert语句),AFTER/BEFORE;
resultType	主键返回类型(DO中对应属性的类型);
LAST_INSERT_ID()	MySQL函数,返回auto_increment自增列新记录值.

• UserDAO

@Test
public void insertUser() {
    try (SqlSession session = factory.openSession()) {
        System.out.println(session);
        User user = new User(null, "new_name", "new_password");
        session.insert("namespace.insertUser", user);
        // 需要在commit之后才能获得自增主键
        session.commit();
        System.out.println(user.getId());
    }
}

该功能还可以通过<insert/>的useGeneratedKeys/keyProperty两个属性合作完成, 详见MyBatis文档.

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Update

• mapper

<update id="updateUserById" parameterType="com.fq.domain.User">
    UPDATE user SET name = #{name}, password = #{password} WHERE id = #{id};
</update>

• UserDAO

@Test
public void updateUserById() {
    try (SqlSession session = factory.openSession(true)) {
        session.update("namespace.updateUserById",
                new User(1, "feiqing", "ICy5YqxZB1uWSwcVLSNLcA=="));
    }
}

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Delete

• mapper

<delete id="deleteUserById" parameterType="java.lang.Integer">
    DELETE FROM user WHERE id = #{id};
</delete>

• UserDAO

@Test
public void deleteUserById() {
    try (SqlSession session = factory.openSession(true)) {
        session.delete("namespace.deleteUserById", 51615);
    }
}

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小结

• #{}/${}

  • #{}: 表示一个占位符号,实现向PreparedStatement占位符中设置值(#{}表示一个占位符?),自动进行Java类型到JDBC类型的转换(因此#{}可以有效防止SQL注入).#{}可以接收简单类型或PO属性值,如果parameterType传输的是单个简单类型值,#{}花括号中可以是value或其它名称.
  • ${}: 表示拼接SQL串,通过${}可将parameterType内容拼接在SQL中而不进行JDBC类型转换,${}可以接收简单类型或PO属性值,如果parameterType传输的是单个简单类型值,${}花括号中只能是value.
    虽然${}不能防止SQL注入,但有时${}会非常方便(如order by排序,需要将列名通过参数传入SQL,则用ORDER BY ${column},使用#{}则无法实现此功能(详见JDBC基础关于PreparedStatement的讨论).

• SqlSession
  提供操作数据库的方法(如:selectOne/selectList).但SqlSession是线程不安全的,因此最好将其定义成局部变量使用.

• MyBatis优点(与JDBC相比)
  
  • SQL写在Java代码中导致不易维护, 而MyBatis将SQL写在mapper中,XML与Java代码分离.
  • 向SQL语句传参繁琐(如:SQL的where条件不一,SQL数据类型与Java不同),MyBatis通过parameterType自动将Java对象映射至SQL语句.
  • 结果集解析麻烦(SQL变化导致解析代码变化,SQL数据类型与Java不同),MyBatis通过resultType自动将SQL执行结果映射成Java对象.

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附: 最好在pom.xml中添加一个日志系统实现(logback/log4j), 这样会在调试程序时打印日志信息,便于查错, 以logback为例:

• pom.xml

<dependency>
    <groupId>ch.qos.logback</groupId>
    <artifactId>logback-classic</artifactId>
    <version>1.1.2</version>
</dependency>

• logback.xml

<configuration>

    <property name="logRoot" value="/data/logs"/>
    <property name="pattern" value="%d{HH:mm:ss.SSS} [%thread] %-5level %logger{0} - %msg%n"/>

    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>${pattern}</pattern>
        </encoder>
    </appender>
    <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
            <fileNamePattern>${logRoot}/common-server.%d{yyyy-MM-dd}.log</fileNamePattern>
            <maxHistory>7</maxHistory>
        </rollingPolicy>
        <encoder>
            <pattern>${pattern}</pattern>
        </encoder>
    </appender>

    <root level="DEBUG">
        <appender-ref ref="STDOUT"/>
        <appender-ref ref="FILE"/>
    </root>

</configuration>

其他关于MyBatis日志的详细信息可参考MyBatis文档日志部分.

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DAO开发

使用MyBatis开发DAO有两个方法,原始DAO开发与Mapper映射DAO开发.

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原始DAO开发

原始DAO开发需要开发人员编写DAO接口与DAO实现,如根据ID查询用户信息:

• mapper(同前)

<select id="selectUserById" parameterType="java.lang.Integer" resultType="com.fq.domain.User">
    SELECT * FROM user WHERE id = #{id};
</select>

• UserDAO接口

/**
 * @author jifang
 * @since 16/2/22 上午10:20.
 */
public interface UserDAO {
    User selectUserById(Integer id) throws Exception;
}

• UserDAO实现

public class UserDAOImpl implements UserDAO {

    private SqlSessionFactory factory;

    public UserDAOImpl(SqlSessionFactory factory) {
        this.factory = factory;
    }

    @Override
    public User selectUserById(Integer id) throws Exception {
        SqlSession session = factory.openSession();
        User user = session.selectOne("namespace.selectUserById", id);
        session.close();
        return user;
    }
}

• Client

public class MyBatisClient {

    @Test
    public void originalClient() throws Exception {
        UserDAO dao = new UserDAOImpl(new SqlSessionFactoryBuilder().
                build(ClassLoader.getSystemResourceAsStream("mybatis/mybatis-configuration.xml")));
        User user = dao.selectUserById(1);
        System.out.println(user);
    }
}

• 原始DAO开发中存在的问题:
  1) DAO实现方法体中存在很多过程性代码.
  2) 调用SqlSession的方法(select/insert/update)需要指定Statement的id,存在硬编码,不利于代码维护.

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Mapper映射开发

mapper映射开发方法只需编写DAO接口,MyBatis根据接口定义与mapper文件中的SQL语句动态创建接口实现.

• mapper

<?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.fq.mybatis.UserDAO">
    <select id="selectUserById" parameterType="java.lang.Integer" resultType="com.fq.domain.User">
        SELECT * FROM user WHERE id = #{id};
    </select>
</mapper>

注意: 此时namespace必须与UserDAO接口的全限定名相同.

• UserDAO接口与前面相同, 但不再使用UserDAOImpl
• Client

/**
 * @author jifang
 * @since 16/2/22 下午2:57.
 */
public class MyBatisClient {

    private SqlSession session;

    private SqlSessionFactory factory;

    @Before
    public void setUp() {
        factory = new SqlSessionFactoryBuilder().
                build(ClassLoader.getSystemResourceAsStream("mybatis/mybatis-configuration.xml"));
        session = factory.openSession();
    }

    @Test
    public void mapperClient() throws Exception {
        UserDAO dao = session.getMapper(UserDAO.class);
        User user = dao.selectUserById(1);
        System.out.println(user);
    }

    @After
    public void tearDown() {
        session.close();
    }
}

• mapper映射开发方法需要遵循以下规范:
  
  1. mapper文件中的namespace与DAO接口的全限定名相同;
  2. mapper文件中的Statement的id与DAO接口方法名相同;
  3. mapper文件中的Statement的parameterType/resultType与DAO方法的入参/回参类型相同.

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Mapper映射

mapper映射文件(如UserDAO.xml)主要作用是定义SQL语句(每个SQL是一个Statement),是MyBatis的核心.

MyBatis官方推荐使用mapper映射的方法来开发DAO,因此我们以后就不再过多介绍原始DAO的开发.

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输入映射

多个形参

传递简单类型前面示例已经使用过,在此就不再赘述.当需要传递多个形参时,不再需要设置parameterType参数:

• mapper

<update id="updateUserById">
    UPDATE user SET name = #{1}, password = #{2} WHERE id = #{0};
</update>

• UserDAO

void updateUserById(Integer id, String name, String password) throws Exception;

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传入PO

MyBatis使用OGNL表达式解析对象属性值:

• mapper

<select id="selectUserByNamePassword" parameterType="com.fq.domain.User" resultType="com.fq.domain.User">
    SELECT *
    FROM user
    WHERE name = #{name} AND password = #{password};
</select>

• UserDAO

User selectUserByNamePassword(User user) throws Exception;

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传入Map

• mapper

<select id="selectUserByMap" parameterType="java.util.Map" resultType="com.fq.domain.User">
    SELECT *
    FROM user
    WHERE name = #{name} AND password = #{password};
</select>

#{}花括号内对应Map的key.

• UserDAO

User selectUserByMap(Map<String, Object> map) throws Exception;

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输出映射

输出简单类型

• mapper

<select id="selectUserCount" parameterType="java.lang.String" resultType="java.lang.Integer">
    SELECT count(*)
    FROM user
    WHERE name LIKE '%${value}%';
</select>

• UserDAO

Integer selectUserCount(String name) throws Exception;

返回简单类型必须保证查询结果只有一行记录,最终将第一个字段的值转换为输出类型.

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输出PO对象/列表

• 前面已经演示过输出两种类型(selectUserById/selectUserByName虽然当时使用的是原始DAO开发方法, 但mapper定义形式大同小异),因此在这儿只做简单总结:
  
  1. 输出单个PO对象和输出PO列表在mapper中定义的resultType是一样的;
  2. 输出单个PO对象要保证SQL查询结果为单条数据,其内部使用selectOne方法调用;
  3. 输出PO列表表示查询结果可能为多条,其内部使用selectList方法调用,接口返回值可用List<PO>/Set<PO>承载.

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输出Map

输出PO对象完全可以改用Map输出,字段名作key,字段值作value.

• mapper

<select id="selectUserLikeName" resultType="java.util.Map">
    SELECT *
    FROM user
    WHERE name LIKE '%${value}%';
</select>

• UserDAO

List<Map<String, Object>> selectUserLikeName(String name) throws Exception;

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resultMap

resultType可将查询结果映射为PO,但前提是PO属性名与SQL字段名必须一致,如不一致,则可通过resultMap作对应映射:

• mapper

<resultMap id="userMap" type="com.fq.domain.User">
    <id column="user_id" property="id"/>
    <result column="user_name" property="name"/>
    <result column="user_password" property="password"/>
</resultMap>

<select id="selectUserByName" parameterType="java.lang.String" resultMap="userMap">
    SELECT
        id       user_id,
        name     user_name,
        password user_password
    FROM user
    WHERE name = #{name};
</select>

属性	描述
<id/>	表示查询结果集的唯一标识;
<result/>	表示普通结果,即PO属性;
column	表示SQL查询出来的字段名,
property	表示PO属性.

• UserDAO接口同前.

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