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https://issues.apache.org/jira/browse/JENA-1634?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16689538#comment-16689538
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Andy Seaborne commented on JENA-1634:
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{{RDFWriterRegistry}} holds all the registered writers; {{StreamRDFWriter}}
gives a stronger guarantee about a subset of these - that the output streams.
The default RDFFormat for RDF/XML does not stream; RDFFormat.RDFXML_PLAIN does.
{{RDFDataMgr}} uses {{RDFWriterRegistry}}.
If you want a fallback, then check with {{StreamRDFWriter.registered}} and if
it returns null, use {{RDFWriterRegistry}}.
Or just lookup in {{RDFWriterRegistry}}. For a streaming format, like
RDFFormat.TURTLE_BLOCKS, you will get a streaming writer (in fact the same
writer code).
See {{ModLangOutput}} and how it handles {{argOutput}}.
On reading all RDF formats stream.
> StreamRDFWriter doesn't work with Lang or RDFFormat default instances.
> ----------------------------------------------------------------------
>
> Key: JENA-1634
> URL: https://issues.apache.org/jira/browse/JENA-1634
> Project: Apache Jena
> Issue Type: New Feature
> Components: RIOT
> Affects Versions: Jena 3.9.0
> Reporter: Marco Brandizi
> Priority: Major
>
> I have [some
> code|https://github.com/Rothamsted/ondex-knet-builder/blob/master/modules/rdf-export-2/src/main/java/net/sourceforge/ondex/rdf/export/RDFFileExporter.java]
> that writes RDF to a file, starting from a Model. This is invoked many times
> over the same FileOutput stream, by many threads that are producing RDF in
> parallel.
> The output type can be selected by the invoker, by passing Lang or RDFFormat
> instances. Because of the way it works, that output will be
> RDFFormat.TURTLE_BLOCKS most of times. However, there might be cases of small
> output, where the user might want to send in formats like Lang.RDFXML.
> Now, the problem is in the latter case the StreamRDFWriter.getWriterStream()
> doesn't work. I've isolated the issue
> [here|https://github.com/marco-brandizi/jena-stream-writer-issue/blob/master/src/test/java/info/marcobrandizi/rdf/test/JenaWritersTest.java]:
> I get messages like _"No serialization for language Lang:RDF/XML"_ and,
> looking at the sources, it seems that StreamRDFWriter recognises only
> Lang/RDFFormat instances set in its own registry.
> The same languages/variants work fine when I use the RDFDataMgr approach.
> This makes me guess/hope that data manager is able to work in a stream
> fashion, when the received RDF variants supports it.
> Whatever it is, I think this is wrong, or at least should be documented
> better, in particular, the documentation should say what to do in a situation
> like mine, where it's not known in advance if we're going to deal with
> streaming or not. Ideally, StreamRDFWriter should fallback to non-streaming,
> or trigger a specific exception when dealing with a streaming-incompatible
> format, so that the invoker can take fallback actions.
> I'm using the whole apache-jena-libs as Jena dependency.
> I suppose this applies to reading too.
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