How can I enable GeoSPARQL in my Dockerized Apache Jena Fuseki setup?

The error

  "the root file:///fuseki/config.ttl#geo_ds has no most specific type that is a subclass of ja:Object"

indicates that when the assembler is loading the configuration, it does not recognize the resource ­– in this case, your “geo_ds” node – as an object type it can assemble. In other words, the automatic registration (via the ServiceLoader mechanism) that should recognize geosparql:geosparqlDataset isn’t taking effect. This situation is common when using a minimal runtime (via jlink) or tweaking the classpath manually: the extra module (or service registration files) required by the GeoSPARQL assembler may not be “visible” or active.

There are two main ways to proceed:

1. Explicitly register the GeoSPARQL assembler class in your configuration.
  You can add a triple to force the use of GeoSPARQL’s assembler. For example, modify your configuration so that the dataset resource explicitly loads the correct assembler class:

TURTLECopied
PREFIX fuseki:    <http://jena.apache.org/fuseki#>
PREFIX rdf:       <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs:      <http://www.w3.org/2000/01/rdf-schema#>
PREFIX tdb2:      <http://jena.apache.org/2016/tdb#>
PREFIX ja:        <http://jena.hpl.hp.com/2005/11/Assembler#>
PREFIX geosparql: <http://jena.apache.org/geosparql#>

<#service> rdf:type fuseki:Service;
    fuseki:name "geo";
    fuseki:endpoint [ fuseki:operation fuseki:query; ] ;
    fuseki:dataset <#geo_ds> .

<#geo_ds> rdf:type geosparql:geosparqlDataset ;
    ja:loadClass "org.apache.jena.geosparql.assembler.GeoSPARQLAssembler" ;
    geosparql:spatialIndexFile "DB/spatial.index";
    geosparql:dataset <#baseDataset> ;
    .

<#baseDataset> rdf:type tdb2:DatasetTDB2 ;
    tdb2:location "DB/" ;
    .

This explicit ja:loadClass triple tells the assembler which implementation to use, bypassing any issues with automatic discovery.

2. Use the self-contained integration (jena-fuseki-geosparql).
  Alternatively, the original jena-fuseki-geosparql package integrates GeoSPARQL “out of the box” with Fuseki and may avoid the need to adjust the classpath or modify the configuration. If you choose that approach, simply follow its instructions for adding it to your Fuseki server without splitting out the jena-geosparql jar and its dependencies manually.

Additional notes:

• When using jlink (or a minimized Java runtime), it is important to ensure that all services and modules required for your use case are really present in the runtime image. Sometimes the minimization process may remove files (like META-INF/services entries) that the ServiceLoader relies on.
• The explicit ja:loadClass declaration is a good workaround when service registration is compromised.

In summary, your configuration must “tell” Fuseki (via the assembler) what to do with geosparql:geosparqlDataset. Explicitly adding ja:loadClass to the configuration is a straightforward solution if your current minimal runtime configuration is not automatically detecting the GeoSPARQL assembler.