MyBatis 实践

小咪咪 2022-04-24 00:42 130阅读 0赞

From： [https://www.cnblogs.com/luyiba/p/6303717.html][https_www.cnblogs.com_luyiba_p_6303717.html]

## 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**.  
![32957777.jpg][]  
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][Link 1]/[在线文档][Link 2].

--------------------

## 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>

--------------------

### 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">
                
                <transactionManager type="JDBC"/>
                
                <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>
    
        
        <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>

<table> 
 <thead> 
  <tr> 
   <th>属性</th> 
   <th>描述</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>namespace</code></td> 
   <td>命名空间,用于隔离SQL语句</td> 
  </tr> 
  <tr> 
   <td><code>parameterType</code></td> 
   <td>定义SQL输入映射类型,MyBatis通过<strong><em>OGNL</em></strong>从输入对象中获取参数传入SQL语句.</td> 
  </tr> 
  <tr> 
   <td><code>resultType</code></td> 
   <td>定义SQL输出映射类型,MyBatis将SQL查询结果的<strong>一行记录</strong>映射为<code>resultType</code>指定的类型.</td> 
  </tr> 
 </tbody> 
</table>

> 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);
                }
            }
        }
    }

--------------------

### 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();
        }
    }

--------------------

### 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>

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

*  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][MyBatis].

--------------------

### 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=="));
        }
    }

--------------------

### 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);
        }
    }

--------------------

### 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][JDBC]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.

--------------------

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][MyBatis 1].

--------------------

## 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**.

--------------------

### 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.

--------------------

### 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

> 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.

--------------------

## 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.

--------------------

### 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;

--------------------

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;

--------------------

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;

--------------------

### 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.

--------------------

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.

--------------------

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;

--------------------

### 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>

<table> 
 <thead> 
  <tr> 
   <th>属性</th> 
   <th>描述</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>&lt;id/&gt;</code></td> 
   <td>表示查询结果集的唯一标识;</td> 
  </tr> 
  <tr> 
   <td><code>&lt;result/&gt;</code></td> 
   <td>表示普通结果,即PO属性;</td> 
  </tr> 
  <tr> 
   <td><code>column</code></td> 
   <td>表示SQL查询出来的字段名,</td> 
  </tr> 
  <tr> 
   <td><code>property</code></td> 
   <td>表示PO属性.</td> 
  </tr> 
 </tbody> 
</table>

*  UserDAO接口同前.

[https_www.cnblogs.com_luyiba_p_6303717.html]: https://www.cnblogs.com/luyiba/p/6303717.html
[32957777.jpg]: http://7xrgh9.com1.z0.glb.clouddn.com/16-3-3/32957777.jpg
[Link 1]: https://github.com/mybatis/mybatis-3
[Link 2]: http://www.mybatis.org/mybatis-3/
[MyBatis]: http://www.mybatis.org/mybatis-3/zh/sqlmap-xml.html#insert_update_and_delete
[JDBC]: http://blog.csdn.net/zjf280441589/article/details/50714873
[MyBatis 1]: http://www.mybatis.org/mybatis-3/zh/logging.html