org.aksw.simba.bengal.triple2nl.converter.URIDereferencer.java Source code

Introduction

Here is the source code for org.aksw.simba.bengal.triple2nl.converter.URIDereferencer.java

Source

/*
 * #%L
 * Triple2NL
 * %%
 * Copyright (C) 2015 Agile Knowledge Engineering and Semantic Web (AKSW)
 * %%
 * Licensed 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.
 * #L%
 */
/**
 * 
 */
package org.aksw.simba.bengal.triple2nl.converter;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URI;
import java.net.URLConnection;

import org.apache.jena.rdf.model.Model;
import org.apache.jena.rdf.model.ModelFactory;
import org.apache.jena.riot.Lang;
import org.apache.jena.riot.RDFLanguages;
import org.apache.jena.riot.WebContent;
import org.apache.log4j.Logger;

import com.google.common.net.UrlEscapers;

/**
 * Class to retrieve triples based on the Linked Data dereferencing paradigm.
 * 
 * @author Lorenz Buehmann
 *
 */
public class URIDereferencer {

    private static final Logger logger = Logger.getLogger(URIDereferencer.class.getName());

    // the content type used in the accept header
    private String contentType = WebContent.contentTypeRDFXML;

    // settings for file based caching
    private File cacheDirectory;
    private boolean useCache = true;
    private Lang cacheFileLanguage = Lang.TURTLE;
    private String cacheFileExtension = cacheFileLanguage.getFileExtensions().get(0);

    public URIDereferencer(File cacheDirectory) {
        this.cacheDirectory = cacheDirectory;
        this.useCache = cacheDirectory != null;
    }

    public URIDereferencer() {
        this(null);
    }

    /**
     * Get the triples that describe the entity identified by the URI.
     * 
     * @param uri
     *            the URI of the entity
     * @return JENA model containing the triples
     * @throws DereferencingFailedException
     */
    public Model dereference(String uri) throws DereferencingFailedException {
        return dereference(URI.create(uri));
    }

    /**
     * Get the triples that describe the entity identified by the URI.
     * 
     * @param uri
     *            the URI of the entity
     * @return JENA model containing the triples
     * @throws DereferencingFailedException
     */
    public Model dereference(URI uri) throws DereferencingFailedException {
        logger.debug("Dereferencing " + uri + "...");
        Model model = null;

        // check if already cached
        if (useCache()) {
            model = loadFromDisk(uri);
        }

        // if we got nothing from cache
        if (model == null) {
            model = ModelFactory.createDefaultModel();
            try {
                URLConnection conn = uri.toURL().openConnection();
                conn.setRequestProperty("Accept", contentType);

                InputStream is = conn.getInputStream();
                model.read(is, null, RDFLanguages.contentTypeToLang(contentType).getLabel());
                is.close();

                if (useCache()) {
                    writeToDisk(uri, model);
                }
            } catch (IOException e) {
                throw new DereferencingFailedException(uri, e);
            }
        }

        logger.debug("Done. Got " + model.size() + " triples for " + uri);
        return model;
    }

    /**
     * Whether to use a file based caching solution, i.e. for each URI the
     * result is stored in a separate file on disk.
     * 
     * @param useCache
     *            use cache or not
     */
    public void setUseCache(boolean useCache) {
        this.useCache = useCache;
    }

    private boolean useCache() {
        return useCache && cacheDirectory != null;
    }

    private File getCacheFile(URI uri) {
        String filename = UrlEscapers.urlPathSegmentEscaper().escape(uri.toString()) + "." + cacheFileExtension;
        File cacheFile = new File(cacheDirectory, filename);
        return cacheFile;
    }

    private Model loadFromDisk(URI uri) {
        File cachedFile = getCacheFile(uri);

        if (cachedFile.exists()) {
            Model model = ModelFactory.createDefaultModel();
            try (InputStream is = new BufferedInputStream(new FileInputStream(cachedFile))) {
                model.read(is, null, cacheFileLanguage.getLabel());
                return model;
            } catch (IOException e) {
                logger.error("Failed loading from disk.", e);
            }

        }
        return null;
    }

    private void writeToDisk(URI uri, Model model) {
        logger.debug("Writing model for " + uri + "to disk.");

        File cacheFile = getCacheFile(uri);

        try (OutputStream os = new BufferedOutputStream(new FileOutputStream(cacheFile))) {
            model.write(os, cacheFileLanguage.getLabel());
        } catch (IOException e) {
            logger.error("Could not write to disk.", e);
        }
    }

    class DereferencingFailedException extends Exception {

        private static final long serialVersionUID = -1830907519484713882L;

        public DereferencingFailedException(URI uri, Exception cause) {
            super("Dereferencing " + uri + " failed.", cause);
        }
    }
}