数组判空

传过来的数组 object[] ，在mapper中判空时先判断是否为null，再判断数组长度 object.length是否大于0.

    <if test=" object !=null and object.length > 0">
		你的逻辑sql
	</if>

集合判空

比如参数为List集合，在mybatis中先判断是否为null，不为null再判断集合的长度 object.size() 是否大于0即可。

    <if test=" object != null and object.size() > 0">
		你的逻辑sql
	</if>

不为空循环 使用forech 

foreach

如果collection的类型为List

List<UserList> getUserInfo(@Param("userName") List<String> userName);

使用@Param注解自定义keyName; 

<if test="userName!= null and userName.size() >0">
	   USERNAME IN
	   <foreach collection="userName" item="value" separator="," open="(" close=")">
	        #{value}
	   </foreach>
 </if>

也可以使用默认属性值list作为keyname

<select id="selectByIds" resultType="com.olive.pojo.User">
        select * from t_user where id in
        <foreach collection="list" index="index" item="item" open="(" separator="," close=")">
            #{item}
        </foreach>
</select>

如果collection的属性为array

List<UserList> getUserInfo(@Param("userName") String[] userName);

使用@Param注解自定义keyName; 

	 <select id="getUserInfo" resultType="com.test.UserList">
	        SELECT
	        	*
	        FROM user_info
	        where
	        <if test="userName!= null and userName.length >0">
	            USERNAME IN
	            <foreach collection="userName" item="value" separator="," open="(" close=")">
	                #{value}
	            </foreach>
	        </if>
	 </select>

也可以，使用默认属性值array作为keyname

<select id="selectByIds" resultType="com.olive.pojo.User">
        select * from t_user where id in
        <foreach collection="array" index="index" item="item" open="(" separator="," close=")">
            #{item}
        </foreach>
    </select>

如果collection的属性为Map

List<UserList> getUserInfo(@Param("user") Map<String,String> user);

第一种：获取Map的键值对

多字段组合条件情况下，一定要注意书写格式：括号（）

eg: SELECT * FROM user_info WHERE (USERNAME,AGE) IN (('张三','26'),('李四','58'),('王五','27'),......);

 <select id="getUserInfo" resultType="com.test.UserList">
	        SELECT
	        	*
	        FROM user_info
	        where
	        <if test="user!= null and user.size() >0">
	            (USERNAME,AGE) IN
	            <foreach collection="user.entrySet()" item="value" index="key" separator="," open="(" close=")">
	                (#{key},#{value})
	            </foreach>
	        </if>
</select>

第二种：参数Map类型，只需要获取key值或者value值

key:

 <select id="getUserInfo" resultType="com.test.UserList">
	        SELECT
	        	*
	        FROM user_info
	        where
	        <if test="user!= null and user.size() >0">
	            (USERNAME) IN
	            <foreach collection="user.keys" item="key"  separator="," open="(" close=")">
	                #{key}
	            </foreach>
	        </if>
</select>

value:

	 <select id="getUserInfo" resultType="com.test.UserList">
	        SELECT
	        	*
	        FROM user_info
	        where
	        <if test="user!= null and user.size() >0">
	            (USERNAME) IN
	            <foreach collection="user.values" item="value"  separator="," open="(" close=")">
	                #{key}
	            </foreach>
	        </if>
</select>