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Java康复性训练——MyBatis3(3) - Zhan's Blog
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<h1>Java康复性训练——MyBatis3(3)</h1>
<div class="read-more clearfix">
<span class="date">2016/7/20</span>
<span>posted in </span>
<span class="posted-in"><a href='Java.html'>Java</a></span>
<span class="comments">
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<p>接上篇，继续对Mybatis的使用进行介绍。</p>
<blockquote>
<p>代码展示：<a href="https://github.com/swuzjb/MybatisDemo">Github地址</a></p>
</blockquote>
<h2 id="toc_0">动态SQL</h2>
<p>在实际开发过程中，静态的SQL写法并没有什么灵活性。当我们对User进行检索时，可能是依据id，role，username等等参数，如果使用静态SQL语句，我们就需要重复写大量的代码。为了解决这类问题，我们引入动态SQL。</p>
<p>Mybatis对动态SQL有很不错的支持，增加了很多扩展。</p>
<h3 id="toc_1">IF标签</h3>
<p>当我们开发过程中，遇到上述的多条件或指定条件查询时，我们可以使用IF标签来扩展。<br/>
以实现一个用户检索为例：</p>
<ol>
<li>在UserMapper接口中定义查询方法：<code>public List<User> searchUser(Map<String, Object> searchMap);</code></li>
<li><p>在UserMapper.xml中对接口进行实现：</p>
<pre><code class="language-xml"> <select id="searchUser" parameterType="Map" resultMap="UserResult">
select * from user where 1=1
<if test="username!=null">
and username like #{username}
</if>
<if test="id!=null">
and id=#{id}
</if>
<if test="role!=null">
and roleId=#{role.id}
</if>
</select>
</code></pre></li>
<li><p>调用查询，传入查询条件的Map即可。</p>
<pre><code class="language-java">public void testSearch(){
Role role = new Role();
role.setId(1);
Map<String, Object> map = new HashMap<String, Object>();
map.put("role", role);
List<User> userList = userMapper.searchUser(map);
for(User user:userList){
System.out.println(user);
}
}
</code></pre></li>
</ol>
<h3 id="toc_2">Choose、when和otherwise标签</h3>
<p>上面的IF标签类似于我们结构控制的IF语句，那么现在介绍的这个标签就类似于我们的结构控制中的Switch-Case语句。<br/>
当我们需要对条件进行限制的时候，我们就可以使用这个标签。标签的结构如下：</p>
<pre><code class="language-xml"><choose>
<when test=""></when>
<when test=""></when>
<otherwise></otherwise>
</choose>
</code></pre>
<h3 id="toc_3">where标签</h3>
<p>where条件是对Where语句的一个优化，可以自动给SQL语句加上Where。如果Where子句以and或者or开头时，则自动删除第一个and或or。</p>
<pre><code class="language-xml"> <select id="searchUser" parameterType="Map" resultMap="UserResult">
select * from user
<where>
<if test="username!=null">
and username like #{username}
</if>
<if test="id!=null">
and id=#{id}
</if>
<if test="role!=null">
and roleId=#{role.id}
</if>
</where>
</select>
</code></pre>
<h3 id="toc_4">trim标签</h3>
<p>与where语句类似，给语句增加前后缀。</p>
<h3 id="toc_5">Foreach标签</h3>
<p>用于拼接SQL语句的in语句，对传入的集合进行遍历。集合名称collectionName，元素名称itemName，拼接时以<code>(</code>开始，以<code>）</code>结束，用<code>，</code>分割。</p>
<pre><code class="language-xml"><foreach collection="collectionName" item="itemName" open="(" close=")" separator=",">
#{itemName}
</foreach>
</code></pre>
<h3 id="toc_6">set标签</h3>
<p>和where子句类似，用于update语句。可以自动加上set，并自动删除最后一个<code>，</code>。</p>
<pre><code class="language-xml"><update id="updateUser" parameterType="User">
update user
<set>
<if test="username!=null">
username=#{username},
</if>
<if test="password!=null">
password=#{password},
</if>
<if test="role!=null">
roleId=#{role.id},
</if>
</set>
where id=#{id}
</update>
</code></pre>
<h3 id="toc_7">处理CLOB、BLOB类型数据</h3>
<ul>
<li>BLOB映射为<code>byte[]</code>,CLOB映射为<code>String</code>。</li>
<li>数据插入和读取时，参照对应的映射类型进行处理即可。</li>
</ul>
<h3 id="toc_8">传入多个输入参数</h3>
<ul>
<li>使用<code><key,value></code>键值对的Map进行传递</li>
<li>在SQL拼接时，直接使用#{param1}、#{param2}...来获取</li>
</ul>
<h3 id="toc_9">MyBatis分页</h3>
<ul>
<li>逻辑分页，即把所有的数据加入内存，每次取出若干条。通过<code>RowBounds</code>对象即可实现（需要指定offset,limit两个值）</li>
<li>物理分页，使用limit子句来实现分页</li>
</ul>
<h3 id="toc_10">MyBatis缓存</h3>
<p>MyBatis默认情况下，启用一级缓存，即同一个SqlSession接口对象调用了相同的select语句，则直接会从缓存中返回结果，而不再查询一次数据库。<br/>
开发者可以自己配置二级缓存，二级缓存是全局的。默认情况下，select使用缓存，insert、update、delete不使用缓存。</p>
<h3 id="toc_11">使用注解配置SQL映射器</h3>
<h4 id="toc_12">基本映射语句</h4>
<ol>
<li>@Insert：声明在插入的接口上方，参数为sql语句，和XML配置时相同；</li>
<li>@Update：同上；</li>
<li>@delete：同上；</li>
<li>@select：简单的返回单个对象时，和在查询方法上配置sql语句即可。</li>
<li><p>@select：当需要返回集合时：</p>
<pre><code class="language-java">@Select("select * from user where id=#{id}")
@Results({
@Result(id=true,column="id",property="id"),
@Result(column="username",property="username"),
@Result(column="password",property="password"),
})
public User getUserById(Integer id);
</code></pre></li>
</ol>
<h4 id="toc_13">关系映射</h4>
<ol>
<li><p>一对一映射：<code>@Result(column="roleId",property="role" one=@One(select="site.zhanjingbo.mapper.RoleMapper.getRoleById"))</code>;</p></li>
<li><p>一对多： <code>@Result(column="id",property="userList" many=@Many(select="site.zhanjingbo.mapper.StudentMapper.getUserbyRole"))</code></p></li>
</ol>
<h4 id="toc_14">动态SQL</h4>
<p>实现比较复杂，建议使用XML实现。使用注解效果不是很好。</p>
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