I have the same issue.

After playing around I was able to successfully import mysql via api following this guide http://solr.pl/en/2010/10/11/data-import-handler-%E2%80%93-how-to-import-data-from-sql-databases-part-1/

However in admin there is still notice "Sorry, no dataimport-handler defined!"
mysql-gradim.xml

<dataConfig>
    <dataSource type="JdbcDataSource"
   driver="com.mysql.jdbc.Driver"
   url="jdbc:mysql://localhost/mydatabase?zeroDateTimeBehavior=convertToNull"
   user="root"
   password=""/>

 <document name='tests'>
    <entity name='posts'
        query='select * from blog_posts'>
    <field column="ID" name="id" />
    <field column="TITLE" name="title" />
    </entity>
 </document>

</dataConfig>

schema.xml
default chema with added

    <field name="id" type="string" indexed="true" required="true" stored="true" />
<field name="title" type="string" indexed="true" required="true" stored="true" />

solrconfig.xml

<config>


<luceneMatchVersion>6.4.0</luceneMatchVersion>


<lib dir="${solr.install.dir:../../../..}/contrib/extraction/lib" regex=".*\.jar"/>
<lib dir="${solr.install.dir:../../../..}/dist/" regex="solr-cell-\d.*\.jar"/>

<^><lib dir="${solr.install.dir:../../../..}/dist/" regex="solr-dataimporthandler-\d.*\.jar"/>
<lib dir="${solr.install.dir:../../../..}/dist/" regex="solr-dataimporthandler-extras-\d.*\.jar"/>
<lib dir="${solr.install.dir:../../../..}/contrib/" regex="mysql-connector-java-.*\.jar"/>
<^>
<lib dir="${solr.install.dir:../../../..}/contrib/clustering/lib/" regex=".*\.jar"/>
<lib dir="${solr.install.dir:../../../..}/dist/" regex="solr-clustering-\d.*\.jar"/>
<lib dir="${solr.install.dir:../../../..}/contrib/langid/lib/" regex=".*\.jar"/>
<lib dir="${solr.install.dir:../../../..}/dist/" regex="solr-langid-\d.*\.jar"/>
<lib dir="${solr.install.dir:../../../..}/contrib/velocity/lib" regex=".*\.jar"/>
<lib dir="${solr.install.dir:../../../..}/dist/" regex="solr-velocity-\d.*\.jar"/>



<dataDir>${solr.data.dir:}</dataDir>

<directoryFactory name="DirectoryFactory" class="${solr.directoryFactory:solr.NRTCachingDirectoryFactory}"/>

<codecFactory class="solr.SchemaCodecFactory"/>

<indexConfig>













<lockType>${solr.lock.type:native}</lockType>











</indexConfig>

<jmx/>





<updateHandler class="solr.DirectUpdateHandler2">

<updateLog>
<str name="dir">${solr.ulog.dir:}</str>
<int name="numVersionBuckets">${solr.ulog.numVersionBuckets:65536}</int>
</updateLog>

<autoCommit>
<maxTime>${solr.autoCommit.maxTime:15000}</maxTime>
<openSearcher>false</openSearcher>
</autoCommit>

<autoSoftCommit>
<maxTime>${solr.autoSoftCommit.maxTime:-1}</maxTime>
</autoSoftCommit>




</updateHandler>



<query>

<maxBooleanClauses>1024</maxBooleanClauses>


<filterCache class="solr.FastLRUCache" size="512" initialSize="512" autowarmCount="0"/>

<queryResultCache class="solr.LRUCache" size="512" initialSize="512" autowarmCount="0"/>

<documentCache class="solr.LRUCache" size="512" initialSize="512" autowarmCount="0"/>

<cache name="perSegFilter" class="solr.search.LRUCache" size="10" initialSize="0" autowarmCount="10" regenerator="solr.NoOpRegenerator"/>





<enableLazyFieldLoading>true</enableLazyFieldLoading>



<queryResultWindowSize>20</queryResultWindowSize>

<queryResultMaxDocsCached>200</queryResultMaxDocsCached>


<listener event="newSearcher" class="solr.QuerySenderListener">
<arr name="queries">

</arr>
</listener>
<listener event="firstSearcher" class="solr.QuerySenderListener">
<arr name="queries">

</arr>
</listener>

<useColdSearcher>false</useColdSearcher>
</query>

<requestDispatcher handleSelect="false">

<requestParsers enableRemoteStreaming="true" multipartUploadLimitInKB="2048000" formdataUploadLimitInKB="2048" addHttpRequestToContext="false"/>

<httpCaching never304="true"/>




</requestDispatcher>


<requestHandler name="/select" class="solr.SearchHandler">

<lst name="defaults">
<str name="echoParams">explicit</str>
<int name="rows">10</int>

</lst>







</requestHandler>

<requestHandler name="/query" class="solr.SearchHandler">
<lst name="defaults">
<str name="echoParams">explicit</str>
<str name="wt">json</str>
<str name="indent">true</str>
</lst>
</requestHandler>

<requestHandler name="/browse" class="solr.SearchHandler" useParams="query,facets,velocity,browse">
<lst name="defaults">
<str name="echoParams">explicit</str>
</lst>
</requestHandler>

<requestHandler name="/dataimport" class="org.apache.solr.handler.dataimport.DataImportHandler">
<lst name="defaults">
<str name="config">
/opt/solr-6.4.0/server/solr/gradim/conf/mysql-gradim.xml
</str>
</lst>
</requestHandler>
<initParams path="/update/**,/query,/select,/tvrh,/elevate,/spell,/browse">
<lst name="defaults">
<str name="df">_text_</str>
</lst>
</initParams>
<initParams path="/update/**">
<lst name="defaults">
<str name="update.chain">add-unknown-fields-to-the-schema</str>
</lst>
</initParams>

<requestHandler name="/update/extract" startup="lazy" class="solr.extraction.ExtractingRequestHandler">
<lst name="defaults">
<str name="lowernames">true</str>
<str name="fmap.meta">ignored_</str>
<str name="fmap.content">_text_</str>
</lst>
</requestHandler>


<searchComponent name="spellcheck" class="solr.SpellCheckComponent">
<str name="queryAnalyzerFieldType">text_general</str>


<lst name="spellchecker">
<str name="name">default</str>
<str name="field">_text_</str>
<str name="classname">solr.DirectSolrSpellChecker</str>

<str name="distanceMeasure">internal</str>

<float name="accuracy">0.5</float>

<int name="maxEdits">2</int>

<int name="minPrefix">1</int>

<int name="maxInspections">5</int>

<int name="minQueryLength">4</int>

<float name="maxQueryFrequency">0.01</float>

</lst>


</searchComponent>

<requestHandler name="/spell" class="solr.SearchHandler" startup="lazy">
<lst name="defaults">

<str name="spellcheck.dictionary">default</str>
<str name="spellcheck">on</str>
<str name="spellcheck.extendedResults">true</str>
<str name="spellcheck.count">10</str>
<str name="spellcheck.alternativeTermCount">5</str>
<str name="spellcheck.maxResultsForSuggest">5</str>
<str name="spellcheck.collate">true</str>
<str name="spellcheck.collateExtendedResults">true</str>
<str name="spellcheck.maxCollationTries">10</str>
<str name="spellcheck.maxCollations">5</str>
</lst>
<arr name="last-components">
<str>spellcheck</str>
</arr>
</requestHandler>

<searchComponent name="tvComponent" class="solr.TermVectorComponent"/>

<requestHandler name="/tvrh" class="solr.SearchHandler" startup="lazy">
<lst name="defaults">
<bool name="tv">true</bool>
</lst>
<arr name="last-components">
<str>tvComponent</str>
</arr>
</requestHandler>


<searchComponent name="terms" class="solr.TermsComponent"/>

<requestHandler name="/terms" class="solr.SearchHandler" startup="lazy">
<lst name="defaults">
<bool name="terms">true</bool>
<bool name="distrib">false</bool>
</lst>
<arr name="components">
<str>terms</str>
</arr>
</requestHandler>

<searchComponent name="elevator" class="solr.QueryElevationComponent">

<str name="queryFieldType">string</str>
<str name="config-file">elevate.xml</str>
</searchComponent>

<requestHandler name="/elevate" class="solr.SearchHandler" startup="lazy">
<lst name="defaults">
<str name="echoParams">explicit</str>
</lst>
<arr name="last-components">
<str>elevator</str>
</arr>
</requestHandler>

<searchComponent class="solr.HighlightComponent" name="highlight">
<highlighting>


<fragmenter name="gap" default="true" class="solr.highlight.GapFragmenter">
<lst name="defaults">
<int name="hl.fragsize">100</int>
</lst>
</fragmenter>

<fragmenter name="regex" class="solr.highlight.RegexFragmenter">
<lst name="defaults">

<int name="hl.fragsize">70</int>

<float name="hl.regex.slop">0.5</float>

<str name="hl.regex.pattern">[-\w ,/\n\"']{20,200}</str>
</lst>
</fragmenter>

<formatter name="html" default="true" class="solr.highlight.HtmlFormatter">
<lst name="defaults">
<str name="hl.simple.pre">
<![CDATA[ <em> ]]>
</str>
<str name="hl.simple.post">
<![CDATA[ </em> ]]>
</str>
</lst>
</formatter>

<encoder name="html" class="solr.highlight.HtmlEncoder"/>

<fragListBuilder name="simple" class="solr.highlight.SimpleFragListBuilder"/>

<fragListBuilder name="single" class="solr.highlight.SingleFragListBuilder"/>

<fragListBuilder name="weighted" default="true" class="solr.highlight.WeightedFragListBuilder"/>

<fragmentsBuilder name="default" default="true" class="solr.highlight.ScoreOrderFragmentsBuilder">

</fragmentsBuilder>

<fragmentsBuilder name="colored" class="solr.highlight.ScoreOrderFragmentsBuilder">
<lst name="defaults">
<str name="hl.tag.pre">
<![CDATA[
<b style="background:yellow">,<b style="background:lawgreen">, <b style="background:aquamarine">,<b style="background:magenta">, <b style="background:palegreen">,<b style="background:coral">, <b style="background:wheat">,<b style="background:khaki">, <b style="background:lime">,<b style="background:deepskyblue">
]]>
</str>
<str name="hl.tag.post">
<![CDATA[ </b> ]]>
</str>
</lst>
</fragmentsBuilder>
<boundaryScanner name="default" default="true" class="solr.highlight.SimpleBoundaryScanner">
<lst name="defaults">
<str name="hl.bs.maxScan">10</str>
<str name="hl.bs.chars">.,!?</str>
</lst>
</boundaryScanner>
<boundaryScanner name="breakIterator" class="solr.highlight.BreakIteratorBoundaryScanner">
<lst name="defaults">

<str name="hl.bs.type">WORD</str>


<str name="hl.bs.language">en</str>
<str name="hl.bs.country">US</str>
</lst>
</boundaryScanner>
</highlighting>
</searchComponent>


<updateRequestProcessorChain name="add-unknown-fields-to-the-schema">

<processor class="solr.UUIDUpdateProcessorFactory"/>
<processor class="solr.RemoveBlankFieldUpdateProcessorFactory"/>
<processor class="solr.FieldNameMutatingUpdateProcessorFactory">
<str name="pattern">[^\w-\.]</str>
<str name="replacement">_</str>
</processor>
<processor class="solr.ParseBooleanFieldUpdateProcessorFactory"/>
<processor class="solr.ParseLongFieldUpdateProcessorFactory"/>
<processor class="solr.ParseDoubleFieldUpdateProcessorFactory"/>
<processor class="solr.ParseDateFieldUpdateProcessorFactory">
<arr name="format">
<str>yyyy-MM-dd'T'HH:mm:ss.SSSZ</str>
<str>yyyy-MM-dd'T'HH:mm:ss,SSSZ</str>
<str>yyyy-MM-dd'T'HH:mm:ss.SSS</str>
<str>yyyy-MM-dd'T'HH:mm:ss,SSS</str>
<str>yyyy-MM-dd'T'HH:mm:ssZ</str>
<str>yyyy-MM-dd'T'HH:mm:ss</str>
<str>yyyy-MM-dd'T'HH:mmZ</str>
<str>yyyy-MM-dd'T'HH:mm</str>
<str>yyyy-MM-dd HH:mm:ss.SSSZ</str>
<str>yyyy-MM-dd HH:mm:ss,SSSZ</str>
<str>yyyy-MM-dd HH:mm:ss.SSS</str>
<str>yyyy-MM-dd HH:mm:ss,SSS</str>
<str>yyyy-MM-dd HH:mm:ssZ</str>
<str>yyyy-MM-dd HH:mm:ss</str>
<str>yyyy-MM-dd HH:mmZ</str>
<str>yyyy-MM-dd HH:mm</str>
<str>yyyy-MM-dd</str>
</arr>
</processor>
<processor class="solr.AddSchemaFieldsUpdateProcessorFactory">
<str name="defaultFieldType">strings</str>
<lst name="typeMapping">
<str name="valueClass">java.lang.Boolean</str>
<str name="fieldType">booleans</str>
</lst>
<lst name="typeMapping">
<str name="valueClass">java.util.Date</str>
<str name="fieldType">tdates</str>
</lst>
<lst name="typeMapping">
<str name="valueClass">java.lang.Long</str>
<str name="valueClass">java.lang.Integer</str>
<str name="fieldType">tlongs</str>
</lst>
<lst name="typeMapping">
<str name="valueClass">java.lang.Number</str>
<str name="fieldType">tdoubles</str>
</lst>
</processor>
<processor class="solr.LogUpdateProcessorFactory"/>
<processor class="solr.DistributedUpdateProcessorFactory"/>
<processor class="solr.RunUpdateProcessorFactory"/>
</updateRequestProcessorChain>









<queryResponseWriter name="json" class="solr.JSONResponseWriter">

<str name="content-type">text/plain; charset=UTF-8</str>
</queryResponseWriter>

<queryResponseWriter name="velocity" class="solr.VelocityResponseWriter" startup="lazy">
<str name="template.base.dir">${velocity.template.base.dir:}</str>
<str name="solr.resource.loader.enabled">${velocity.solr.resource.loader.enabled:true}</str>
<str name="params.resource.loader.enabled">${velocity.params.resource.loader.enabled:false}</str>
</queryResponseWriter>

<queryResponseWriter name="xslt" class="solr.XSLTResponseWriter">
<int name="xsltCacheLifetimeSeconds">5</int>
</queryResponseWriter>








</config>