MyBatis学习(四)

前言

　　最近比较松懈，下班回家后也懒得学习了。今晚实在是看不下去了，争取时间学习。社会上有这么多的资源，就看谁能抢的多吧。今天就说说MyBatis的动态SQL吧

正文

　　动态 SQL 通常要做的事情是有条件地包含 where 子句的一部分。比如:

<select id="findActiveBlogLike"
     resultType="Blog">
  SELECT * FROM BLOG 
  WHERE 
  <if test="state != null">
    state = #{state}
  </if> 
  <if test="title != null">
    AND title like #{title}
  </if>
  <if test="author != null and author.name != null">
    AND author_name like #{author.name}
  </if>
</select>

如果这些条件没有一个能匹配上将会怎样？最终这条 SQL 会变成这样：

SELECT * FROM BLOG
WHERE

这会导致查询失败。如果仅仅第二个条件匹配又会怎样？这条 SQL 最终会是这样:

SELECT * FROM BLOG
WHERE 
AND title like ‘someTitle’

这个查询也会失败。这个问题不能简单的用条件句式来解决，如果你也曾经被迫这样写过，那么你很可能从此以后都不想再这样去写了。

MyBatis 有一个简单的处理，这在90%的情况下都会有用。而在不能使用的地方，你可以自定义处理方式来令其正常工作。一处简单的修改就能得到想要的效果：

<select id="findActiveBlogLike"
     resultType="Blog">
  SELECT * FROM BLOG 
  <where> 
    <if test="state != null">
         state = #{state}
    </if> 
    <if test="title != null">
        AND title like #{title}
    </if>
    <if test="author != null and author.name != null">
        AND author_name like #{author.name}
    </if>
  </where>
</select>

where 元素知道只有在一个以上的if条件有值的情况下才去插入“WHERE”子句。而且，若最后的内容是“AND”或“OR”开头的，where 元素也知道如何将他们去除。

好了，上面都是摘自MyBatis官网，说明了动态sql在使用中存在的问题，那我就以where为例，跑一个小程序，还是基于前面几节中使用到的表。

<select id="getUserByObjInDynamicMethod" parameterType="org.tonny.entity.User" resultType="org.tonny.entity.User">
        SELECT * FROM users
        <where>
            <if test="id != null">
                id=#{id}
            </if>
            <if test="name != null">
                and name like #{name}
            </if>
            
        </where>
    </select>
    
    <select id="getUserByMapInDynamicMethod" parameterType="org.tonny.entity.User" resultType="org.tonny.entity.User">
        SELECT * FROM users
        <where>
            <if test="id != null">
                id=#{id}
            </if>
            <if test="name != null">
                and name like #{name}
            </if>
            
        </where>
    </select>

这部分配置与上面的一样，使用了where标签。对应的Java代码如下：

@Test
    public void getUserByObjInDynamicMethod()
    {
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 映射sql的标识字符串
        String sql = "org.tonny.mapper.UsersMapper.getUserByObjInDynamicMethod";
        User user = new User();
        user.setId(1);
        user.setName("%nny%");
        user.setAge(30);
        List<User> userList = sqlSession.selectList(sql, user);
        sqlSession.close();
        System.out.println(userList);
    }
    
    @Test
    public void getUserByMapInDynamicMethod()
    {
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 映射sql的标识字符串
        String sql = "org.tonny.mapper.UsersMapper.getUserByObjInDynamicMethod";
        Map<String,Object> map = new HashMap<String, Object>();
        map.put("id", 1);
        map.put("name", "%nny%");
        List<User> userList = sqlSession.selectList(sql, map);
        sqlSession.close();
        System.out.println(userList);
    }

这样就可以查出来了。对了，为了可以在控制台查看MyBatis的SQL，需要在mybatis.xml增加配置信息：

<!-- 设置日志打印 -->
    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>

这样就可以看到输出的sql语句了。测试结果：

Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
Opening JDBC Connection
Created connection 25193812.
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1806d54]
==>  Preparing: SELECT * FROM users WHERE id=? and name like ? 
==> Parameters: 1(Integer), %nny%(String)
<==    Columns: id, NAME, age
<==        Row: 1, Tonny Chien, 25
<==      Total: 1
Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1806d54]
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@1806d54]
Returned connection 25193812 to pool.
[User [id=1, name=Tonny Chien, age=25]]

还要介绍一下set标签，它对应update中的set

<update id="updateUserByMapInDynamicMethod" parameterType="java.util.Map">
        UPDATE users
        <set>
            <if test="name != null">
                name = #{name},
            </if>
            <if test="age != null">
                age = #{age}
            </if>
        </set>
        <where>
            <if test="id != null">
                id=#{id}
            </if>
            <if test="age != null">
                <![CDATA[
                    and age > #{age}
                ]]>
            </if>
        </where>
    </update>

Java测试代码如下：

@Test
    public void updateUserByMapInDynamicMethod()
    {
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 映射sql的标识字符串
        String sql = "org.tonny.mapper.UsersMapper.updateUserByMapInDynamicMethod";
        Map<String,Object> map = new HashMap<String, Object>();
        map.put("id", 1);
        map.put("age", 20);
        map.put("name", "北堂一刀");
        int result = sqlSession.update(sql, map);
        sqlSession.commit();
        sqlSession.close();
        System.out.println(result);
    }

执行结果：

Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
Opening JDBC Connection
Created connection 30587315.
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1d2b9b3]
==>  Preparing: UPDATE users SET name = ?, age = ? WHERE id=? and age > ? 
==> Parameters: 北堂一刀(String), 20(Integer), 1(Integer), 20(Integer)
<==    Updates: 1
Committing JDBC Connection [com.mysql.jdbc.JDBC4Connection@1d2b9b3]
Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1d2b9b3]
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@1d2b9b3]
Returned connection 30587315 to pool.
1

后记

就写到这儿吧，下次再详细些，休息了。