Added RDF/JSON syntax support for writing OWL data

sansa-owl-spark/src/main/scala/net/sansa_stack/owl/spark/package.scala

@@ -5,7 +5,7 @@ import org.apache.spark.sql.SparkSession
 import org.semanticweb.owlapi.formats._
 import org.semanticweb.owlapi.model.OWLDocumentFormat
 import net.sansa_stack.owl.spark.rdd._
-import net.sansa_stack.owl.spark.writers.{DLSyntaxWriter, KRSS2SyntaxWriter, KRSSSyntaxWriter, OBOWriter, OWLFunctionalSyntaxWriter}
+import net.sansa_stack.owl.spark.writers.{DLSyntaxWriter, KRSS2SyntaxWriter, KRSSSyntaxWriter, OBOWriter, OWLFunctionalSyntaxWriter, RDFJSonWriter}
 
 
 class UnknownOWLFormatException(msg: String) extends Exception
@@ -181,8 +181,7 @@ package object owl {
           case _: KRSSDocumentFormat => KRSSSyntaxWriter.save(path, axioms)
           case _: DLSyntaxDocumentFormat => DLSyntaxWriter.save(path, axioms)
           case _: OBODocumentFormat => OBOWriter.save(path, axioms)
-          case format: RDFJsonDocumentFormat =>
-            throw new NotImplementedError(s"Support for ${format.getClass.getName} not implemented, yet")
+          case _: RDFJsonDocumentFormat => RDFJSonWriter.save(path, axioms)
           case format: RDFJsonLDDocumentFormat =>
             throw new NotImplementedError(s"Support for ${format.getClass.getName} not implemented, yet")
           case format: NQuadsDocumentFormat =>

sansa-owl-spark/src/main/scala/net/sansa_stack/owl/spark/writers/RDFJSonWriter.scala

@@ -0,0 +1,57 @@
+package net.sansa_stack.owl.spark.writers
+
+import java.io.{BufferedWriter, ByteArrayOutputStream, OutputStreamWriter, PrintWriter}
+import java.util.Collections
+
+import scala.collection.JavaConverters._
+
+import org.eclipse.rdf4j.rio.RDFFormat
+import org.eclipse.rdf4j.rio.rdfjson.RDFJSONWriter
+import org.semanticweb.owlapi.apibinding.OWLManager
+import org.semanticweb.owlapi.formats.RDFJsonDocumentFormat
+import org.semanticweb.owlapi.model.{OWLAxiom, OWLOntology}
+import org.semanticweb.owlapi.rio.RioRenderer
+
+import net.sansa_stack.owl.spark.rdd.OWLAxiomsRDD
+
+
+protected class SANSARioRenderer(ont: OWLOntology, rdfJsonWriter: RDFJSONWriter, docFormat: RDFJsonDocumentFormat)
+  extends RioRenderer(ont, rdfJsonWriter, docFormat) {
+
+  override def renderOntologyHeader(): Unit = None
+}
+
+object RDFJSonWriter extends OWLWriterBase {
+  override def save(filePath: String, owlAxioms: OWLAxiomsRDD): Unit = {
+    owlAxioms.mapPartitionsWithIndex((idx: Int, partition: Iterator[OWLAxiom]) => if (partition.hasNext) {
+      val snippets = partition.map(axiom => {
+        // writer stuff...
+        val os = new ByteArrayOutputStream()
+        val osWriter = new OutputStreamWriter(os)
+        val buffPrintWriter = new PrintWriter(new BufferedWriter(osWriter))
+
+        val man = OWLManager.createOWLOntologyManager()
+        val ont = man.createOntology(Seq(axiom).asJava)
+
+        val renderer = new SANSARioRenderer(
+          ont,
+          new RDFJSONWriter(osWriter, RDFFormat.RDFJSON),
+          new RDFJsonDocumentFormat)
+
+        renderer.render()
+        buffPrintWriter.flush()
+
+        os.toString("UTF-8")
+          .replaceFirst("\\{", "")  // remove per-partition opening curly brace
+          .reverse.replaceFirst("\\}", "").reverse  // remove per-partition closing curly brace
+          // make blank node IDs unique (by appending the partition ID)
+          .replaceAll("_:genid([0-9]+)", "_:genid$1" + s"_$idx")
+          .replaceAll("\\s+$", "")  // trim end
+      })
+      Collections.singleton(snippets.mkString(",")).iterator().asScala
+
+    } else {
+      Iterator()
+    }).saveAsTextFile(filePath)
+  }
+}