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JSON-LD Java integration module for Jena
There is a newer version: 0.4.1
/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.github.jsonldjava.jena;

import java.util.Iterator;

import org.apache.jena.atlas.logging.Log;
import org.apache.jena.riot.out.NodeToLabel;
import org.apache.jena.riot.system.SyntaxLabels;

import com.github.jsonldjava.core.JsonLdError;
import com.github.jsonldjava.core.RDFDataset;
import com.hp.hpl.jena.datatypes.xsd.XSDDatatype;
import com.hp.hpl.jena.graph.Node;
import com.hp.hpl.jena.sparql.core.DatasetGraph;
import com.hp.hpl.jena.sparql.core.Quad;

// From RDF to JSON-LD java structure.
class JenaRDF2JSONLD implements com.github.jsonldjava.core.RDFParser {
    NodeToLabel labels = SyntaxLabels.createNodeToLabel();

    @Override
    public RDFDataset parse(Object object) throws JsonLdError {
        final RDFDataset result = new RDFDataset();
        if (object instanceof DatasetGraph) {
            final DatasetGraph dsg = (DatasetGraph) object;

            final Iterator iter = dsg.find();
            for (; iter.hasNext();) {
                final Quad q = iter.next();
                final Node s = q.getSubject();
                final Node p = q.getPredicate();
                final Node o = q.getObject();
                final Node g = q.getGraph();

                final String gq = (g == null || Quad.isDefaultGraph(g)) ? null : g.getURI();
                final String sq = resourceString(s);
                final String pq = p.getURI();
                if (o.isLiteral()) {
                    final String lex = o.getLiteralLexicalForm();
                    String lang = o.getLiteralLanguage();
                    String dt = o.getLiteralDatatypeURI();
                    if (lang != null && lang.length() == 0) {
                        lang = null;
                        // dt = RDF.getURI()+"langString" ;
                    }
                    if (dt == null) {
                        dt = XSDDatatype.XSDstring.getURI();
                    }

                    result.addQuad(sq, pq, lex, dt, lang, gq);
                } else {
                    final String oq = resourceString(o);
                    result.addQuad(sq, pq, oq, gq);
                }
            }
        } else {
            Log.warn(JenaRDF2JSONLD.class, "unknown");
        }
        return result;
    }

    private String resourceString(Node x) {
        if (x.isURI()) {
            return x.getURI();
        }
        if (x.isBlank()) {
            return labels.get(null, x);
        }
        return null;
    }
}