mybatis if test 不为空字符串或null

<sql id="public_content">
  	<if test="productId != null and productId !=" >
        and  a.product_id = #{productId,jdbcType=VARCHAR}
      </if>
      <if test="productDefinId != null" >
        and a.product_defin_id = #{productDefinId,jdbcType=VARCHAR}
      </if>
      <if test="productUid != null and productUid !=">
        and a.product_uid = #{productUid,jdbcType=VARCHAR}
      </if>
      <if test="productKey != null" >
        and a.product_key = #{productKey,jdbcType=VARCHAR}
      </if>
      <if test="prouuctSecret != null" >
        and a.prouuct_secret = #{prouuctSecret,jdbcType=VARCHAR}
      </if>
      <if test="productPass != null" >
        and a.product_pass = #{productPass,jdbcType=VARCHAR}
      </if>
      <if test="productVisitPass != null" >
        and a.product_visit_pass = #{productVisitPass,jdbcType=VARCHAR}
      </if>
      <if test="createTime != null and createTime !=" >
        and a.create_time = #{createTime,jdbcType=VARCHAR}
      </if>
      <if test="pageSize != null and pageNum !=null " >      
         ORDER BY a.product_id DESC limit #{pageNum},#{pageSize}
      </if>
  </sql>