Ostrzyciel commented on issue #2578:
URL: https://github.com/apache/jena/issues/2578#issuecomment-2233481730
@afs Thank you for the detailed answer, that actually explains a lot!
I've updated the code to read directly from `StreamRDF` instead of
constructing a dataset – the results now are different:
<details>
Empty output for null graph in TriG/pretty
Failed to write null graph in JSON-LD-11/pretty
org.apache.jena.shared.JenaException: Exception while writing
JSON-LD 1.1
at
org.apache.jena.riot.writer.JsonLD11Writer.write$(JsonLD11Writer.java:123)
at
org.apache.jena.riot.writer.JsonLD11Writer.write(JsonLD11Writer.java:73)
at org.apache.jena.riot.RDFWriter.write$(RDFWriter.java:261)
at org.apache.jena.riot.RDFWriter.output(RDFWriter.java:219)
at org.apache.jena.riot.RDFWriter.output(RDFWriter.java:158)
at
org.apache.jena.riot.RDFWriterBuilder.output(RDFWriterBuilder.java:207)
at org.apache.jena.riot.RDFDataMgr.write$(RDFDataMgr.java:809)
at org.apache.jena.riot.RDFDataMgr.write(RDFDataMgr.java:754)
at org.example.Main.main(Main.java:68)
Caused by: java.lang.NullPointerException: Cannot invoke
"org.apache.jena.graph.Node.isBlank()" because "node" is null
at
org.apache.jena.riot.system.JenaTitanium.resource(JenaTitanium.java:133)
at
org.apache.jena.riot.system.JenaTitanium.lambda$convert$0(JenaTitanium.java:60)
at
org.apache.jena.atlas.iterator.Iter$IterMap.lambda$forEachRemaining$0(Iter.java:432)
at
org.apache.jena.mem2.store.fast.FastArrayBunch$1.forEachRemaining(FastArrayBunch.java:164)
at
org.apache.jena.mem2.iterator.IteratorOfJenaSets.forEachRemaining(IteratorOfJenaSets.java:71)
at
org.apache.jena.atlas.iterator.Iter$IterMap.forEachRemaining(Iter.java:432)
at
org.apache.jena.atlas.iterator.Iter.forEachRemaining(Iter.java:927)
at
org.apache.jena.atlas.iterator.IteratorConcat.forEachRemaining(IteratorConcat.java:99)
at
org.apache.jena.riot.system.JenaTitanium.convert(JenaTitanium.java:49)
at
org.apache.jena.riot.writer.JsonLD11Writer.write$(JsonLD11Writer.java:91)
... 8 more
To serialize Format
Status/deserialized
null graph TriG/pretty empty
output
null graph JSON-LD-11/pretty
serialization failed
null graph N-Quads/utf-8
Quad.defaultGraphNodeGenerated
null graph RDF-PROTO null
graph
null graph RDF-THRIFT triple
instead of quad
Quad.defaultGraphNodeGenerated TriG/pretty
Quad.defaultGraphNodeGenerated
Quad.defaultGraphNodeGenerated JSON-LD-11/pretty triple
instead of quad
Quad.defaultGraphNodeGenerated N-Quads/utf-8
Quad.defaultGraphNodeGenerated
Quad.defaultGraphNodeGenerated RDF-PROTO triple
instead of quad
Quad.defaultGraphNodeGenerated RDF-THRIFT triple
instead of quad
Quad.defaultGraphIRI TriG/pretty
Quad.defaultGraphNodeGenerated
Quad.defaultGraphIRI JSON-LD-11/pretty triple
instead of quad
Quad.defaultGraphIRI N-Quads/utf-8
Quad.defaultGraphNodeGenerated
Quad.defaultGraphIRI RDF-PROTO triple
instead of quad
Quad.defaultGraphIRI RDF-THRIFT triple
instead of quad
</details>
- TriG and N-Quads parsers output `Quad.defaultGraphNodeGenerated`
- JSON-LD and Thrift parsers output a triple instead of a quad
- Protobuf parser behaves like Thrift, except the case where the file was
serialized from a quad that had null for the graph, then the parser output is
also null.
So, to summarize (please correct me if I'm wrong here):
- Parsers when handling quads that the source file explicitly specifies as
being in the default graph, should call `StreamRDF.quad(Quad(_, _, _,
Quad.defaultGraphNodeGenerated))` – this is something to fix in my parser.
- Parsers when handling triples that may or may not be in the default graph,
should call `StreamRDF.triple`.
- Writers when handling triples should write them as triples.
- Writers when handling quads:
- If `quad.isTriple()`, should output a triple (if the format supports
it), or a quad in the default graph otherwise.
- If `Quad.isDefaultGraph()`, should output a quad in the default graph
(if the format supports it), or a triple otherwise.
I will try to fix the JSON-LD and TriG issues in a PR.
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