MyBatis从入门到精通(八):MyBatis动态Sql之foreach标签的用法

最近在读刘增辉老师所著的《MyBatis从入门到精通》一书,很有收获,于是将自己学习的过程以博客形式输出,如有错误,欢迎指正,如帮助到你,不胜荣幸!

本篇博客主要讲解如何使用foreach标签生成动态的Sql,主要包含以下3个场景:

  1. foreach 实现in集合
  2. foreach 实现批量插入
  3. foreach 实现动态update

1. foreach 实现in集合

假设有这样1个需求:根据传入的用户id集合查询出所有符合条件的用户,此时我们需要使用到Sql中的IN,如 id in (1,1001)。

首先,我们在接口SysUserMapper中添加如下方法:

/**
 * 根据用户id集合查询用户
 *
 * @param idList
 * @return
 */
List<SysUser> selectByIdList(List<Long> idList);

然后在对应的SysUserMapper.xml中添加如下代码:

<select id="selectByIdList" resultType="com.zwwhnly.mybatisaction.model.SysUser">
    SELECT id,
    user_name,
    user_password,
    user_email,
    create_time
    FROM sys_user
    WHERE id IN
    <foreach collection="list" open="(" close=")" separator=","
             item="id" index="i">
        #{id}
    </foreach>
</select>

最后,在SysUserMapperTest测试类中添加如下测试方法:

@Test
public void testSelectByIdList() {
    SqlSession sqlSession = getSqlSession();

    try {
        SysUserMapper sysUserMapper = sqlSession.getMapper(SysUserMapper.class);

        List<Long> idList = new ArrayList<Long>();
        idList.add(1L);
        idList.add(1001L);

        List<SysUser> userList = sysUserMapper.selectByIdList(idList);
        Assert.assertEquals(2, userList.size());
    } finally {
        sqlSession.close();
    }
}

运行测试代码,测试通过,输出日志如下:

DEBUG [main] – ==> Preparing: SELECT id, user_name, user_password, user_email, create_time FROM sys_user WHERE id IN ( ? , ? )

DEBUG [main] – ==> Parameters: 1(Long), 1001(Long)

TRACE [main] – <== Columns: id, user_name, user_password, user_email, create_time

TRACE [main] – <== Row: 1, admin, 123456, admin@mybatis.tk, 2019-06-27 18:21:07.0

TRACE [main] – <== Row: 1001, test, 123456, test@mybatis.tk, 2019-06-27 18:21:07.0

DEBUG [main] – <== Total: 2

通过日志会发现,foreach元素中的内容最终生成的Sql语句为(1,1001)。

foreach包含属性讲解:

  • open:整个循环内容开头的字符串。
  • close:整个循环内容结尾的字符串。
  • separator:每次循环的分隔符。
  • item:从迭代对象中取出的每一个值。
  • index:如果参数为集合或者数组,该值为当前索引值,如果参数为Map类型时,该值为Map的key。
  • collection:要迭代循环的属性名。

也许有人会好奇,为什么collection的值是list?该值该如何设置呢?

上面的例子中只有一个集合参数,我们把collection属性的值设置为了list,其实也可以写成collection,为什么呢?让我们看下DefaultSqlSession中的默认处理逻辑:

private Object wrapCollection(Object object) {
    DefaultSqlSession.StrictMap map;
    if (object instanceof Collection) {
        map = new DefaultSqlSession.StrictMap();
        map.put("collection", object);
        if (object instanceof List) {
            map.put("list", object);
        }

        return map;
    } else if (object != null && object.getClass().isArray()) {
        map = new DefaultSqlSession.StrictMap();
        map.put("array", object);
        return map;
    } else {
        return object;
    }
}

虽然使用默认值,代码也可以正常运行,但还是推荐使用@Param来指定参数的名字,如下所示:

List<SysUser> selectByIdList(@Param("idList") List<Long> idList);
<foreach collection="idList" open="(" close=")" separator=","
         item="id" index="i">
    #{id}
</foreach>

如果参数是一个数组参数,collection可以设置为默认值array,看了上面的源码,应该不难理解。

/**
 * 根据用户id数组查询用户
 *
 * @param idArray
 * @return
 */
List<SysUser> selectByIdArray(Long[] idArray);
<select id="selectByIdArray" resultType="com.zwwhnly.mybatisaction.model.SysUser">
    SELECT id,
    user_name,
    user_password,
    user_email,
    create_time
    FROM sys_user
    WHERE id IN
    <foreach collection="array" open="(" close=")" separator=","
             item="id" index="i">
        #{id}
    </foreach>
</select>

虽然使用默认值,代码也可以正常运行,但还是推荐使用@Param来指定参数的名字,如下所示:

List<SysUser> selectByIdArray(@Param("idArray")Long[] idArray);
<foreach collection="idArray" open="(" close=")" separator=","
         item="id" index="i">
    #{id}
</foreach>

2. foreach 实现批量插入

假设有这样1个需求:将传入的用户集合批量写入数据库。

首先,我们在接口SysUserMapper中添加如下方法:

/**
 * 批量插入用户信息
 *
 * @param userList
 * @return
 */
int insertList(List<SysUser> userList);

然后在对应的SysUserMapper.xml中添加如下代码:

<insert id="insertList" useGeneratedKeys="true" keyProperty="id">
    INSERT INTO sys_user(user_name, user_password, user_email, user_info, head_img, create_time)
    VALUES
    <foreach collection="list" item="user" separator=",">
        (#{user.userName},#{user.userPassword},#{user.userEmail},#{user.userInfo},#{user.headImg,jdbcType=BLOB},#{user.createTime,jdbcType=TIMESTAMP})
    </foreach>
</insert>

最后,在SysUserMapperTest测试类中添加如下测试方法:

@Test
public void testInsertList() {
    SqlSession sqlSession = getSqlSession();

    try {
        SysUserMapper sysUserMapper = sqlSession.getMapper(SysUserMapper.class);

        List<SysUser> sysUserList = new ArrayList<SysUser>();
        for (int i = 0; i < 2; i++) {
            SysUser sysUser = new SysUser();
            sysUser.setUserName("test" + i);
            sysUser.setUserPassword("123456");
            sysUser.setUserEmail("test@mybatis.tk");

            sysUserList.add(sysUser);
        }

        int result = sysUserMapper.insertList(sysUserList);

        for (SysUser sysUser : sysUserList) {
            System.out.println(sysUser.getId());
        }

        Assert.assertEquals(2, result);
    } finally {
        sqlSession.close();
    }
}

运行测试代码,测试通过,输出日志如下:

DEBUG [main] – ==> Preparing: INSERT INTO sys_user(user_name, user_password, user_email, user_info, head_img, create_time) VALUES (?,?,?,?,?,?) , (?,?,?,?,?,?)

DEBUG [main] – ==> Parameters: test0(String), 123456(String), test@mybatis.tk(String), null, null, null, test1(String), 123456(String), test@mybatis.tk(String), null, null, null

DEBUG [main] – <== Updates: 2

1035

1036

3. foreach 实现动态update

假设有这样1个需求:根据传入的Map参数更新用户信息。

首先,我们在接口SysUserMapper中添加如下方法:

/**
 * 通过Map更新列
 *
 * @param map
 * @return
 */
int updateByMap(Map<String, Object> map);

然后在对应的SysUserMapper.xml中添加如下代码:

<update id="updateByMap">
    UPDATE sys_user
    SET
    <foreach collection="_parameter" item="val" index="key" separator=",">
        ${key} = #{val}
    </foreach>
    WHERE id = #{id}
</update>

最后,在SysUserMapperTest测试类中添加如下测试方法:

@Test
public void testUpdateByMap() {
    SqlSession sqlSession = getSqlSession();

    try {
        SysUserMapper sysUserMapper = sqlSession.getMapper(SysUserMapper.class);

        Map<String, Object> map = new HashMap<String, Object>();
        map.put("id", 1L);
        map.put("user_email", "test@mybatis.tk");
        map.put("user_password", "12345678");

        Assert.assertEquals(1, sysUserMapper.updateByMap(map));

        SysUser sysUser = sysUserMapper.selectById(1L);
        Assert.assertEquals("test@mybatis.tk", sysUser.getUserEmail());
        Assert.assertEquals("12345678", sysUser.getUserPassword());
    } finally {
        sqlSession.close();
    }
}

运行测试代码,测试通过,输出日志如下:

DEBUG [main] – ==> Preparing: UPDATE sys_user SET user_email = ? , user_password = ? , id = ? WHERE id = ?

DEBUG [main] – ==> Parameters: test@mybatis.tk(String), 12345678(String), 1(Long), 1(Long)

DEBUG [main] – <== Updates: 1

DEBUG [main] – ==> Preparing: SELECT id, user_name, user_password, user_email, create_time FROM sys_user WHERE id = ?

DEBUG [main] – ==> Parameters: 1(Long)

TRACE [main] – <== Columns: id, user_name, user_password, user_email, create_time

TRACE [main] – <== Row: 1, admin, 12345678, test@mybatis.tk, 2019-06-27 18:21:07.0

DEBUG [main] – <== Total: 1

上面示例中,collection使用的是默认值_parameter,也可以使用@Param指定参数名字,如下所示:

int updateByMap(@Param("userMap") Map<String, Object> map);
<update id="updateByMap">
    UPDATE sys_user
    SET
    <foreach collection="userMap" item="val" index="key" separator=",">
        ${key} = #{val}
    </foreach>
    WHERE id = #{userMap.id}
</update>

4. 源码

源码地址:https://github.com/zwwhnly/mybatis-action.git,欢迎下载。

5. 参考

刘增辉《MyBatis从入门到精通》