Add basic fragment generation.

Author: RubenVerborgh

WebContent/WEB-INF/web.xml

@@ -5,12 +5,12 @@
 	id="WebApp_ID" version="2.5">
 	<display-name>Linked Data Fragments Server</display-name>
 	<servlet>
-		<display-name>FragmentServlet</display-name>
-		<servlet-name>FragmentServlet</servlet-name>
-		<servlet-class>org.linkeddatafragments.servlets.FragmentServlet</servlet-class>
+		<display-name>BasicLdfServlet</display-name>
+		<servlet-name>BasicLdfServlet</servlet-name>
+		<servlet-class>org.linkeddatafragments.servlets.BasicLdfServlet</servlet-class>
 	</servlet>
 	<servlet-mapping>
-		<servlet-name>FragmentServlet</servlet-name>
+		<servlet-name>BasicLdfServlet</servlet-name>
 		<url-pattern>/*</url-pattern>
 	</servlet-mapping>
 </web-app>

src/org/linkeddatafragments/servlets/BasicLdfServlet.java

@@ -0,0 +1,134 @@
+package org.linkeddatafragments.servlets;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import javax.servlet.ServletConfig;
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+import org.rdfhdt.hdt.hdt.HDT;
+import org.rdfhdt.hdt.hdt.HDTManager;
+import org.rdfhdt.hdtjena.HDTGraph;
+
+import com.hp.hpl.jena.rdf.model.Model;
+import com.hp.hpl.jena.rdf.model.ModelFactory;
+import com.hp.hpl.jena.rdf.model.Property;
+import com.hp.hpl.jena.rdf.model.RDFNode;
+import com.hp.hpl.jena.rdf.model.Resource;
+import com.hp.hpl.jena.rdf.model.StmtIterator;
+
+/**
+ * Servlet that responds with a Basic Linked Data Fragment.
+ * @author Ruben Verborgh
+ */
+public class BasicLdfServlet extends HttpServlet {
+	private final static long serialVersionUID = 1L;
+	private final static int TRIPLES_PER_PAGE = 100;
+	private final static Pattern STRINGPATTERN = Pattern.compile("^\"(.*)\"(?:\\^\\^<(.*)>|@(.*))?$");
+	
+	private Model model;
+
+	@Override
+	public void init(ServletConfig config) throws ServletException {
+		try {
+			final String dataFile = config.getInitParameter("dataFile");
+			final HDT hdt = HDTManager.mapIndexedHDT(dataFile, null);
+			final HDTGraph graph = new HDTGraph(hdt);
+			model = ModelFactory.createModelForGraph(graph);
+		}
+		catch (Exception e) {
+			throw new ServletException(e);
+		}
+	}
+	
+	@Override
+	public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException {
+		try {
+			final String path = request.getRequestURI().substring(request.getContextPath().length());
+			
+			// parse the subject, predicate, and object parameters
+			final Model output = ModelFactory.createDefaultModel();
+			final Resource subject = parseAsResource(request.getParameter("subject"), output);
+			final Property predicate = parseAsProperty(request.getParameter("predicate"), output);
+			final RDFNode object = parseAsNode(request.getParameter("object"), output);
+			
+			// add all statements with the given parameters to the output model
+			final StmtIterator statements = model.listStatements(subject, predicate, object);
+			for (int i = 0; i < TRIPLES_PER_PAGE && statements.hasNext(); i++)
+				output.add(statements.next());
+			
+			// serialize the output as Turtle
+			response.setHeader("Server", "Linked Data Fragments Server");
+			response.setHeader("Content-Type", "text/turtle");
+			output.write(response.getWriter(), "Turtle");
+		}
+		catch (Exception e) {
+			throw new ServletException(e);
+		}
+	}
+	
+	/**
+	 * Parses the given value as an RDF resource.
+	 * @param value the value
+	 * @param model the model
+	 * @return the parsed value, or null if unspecified
+	 */
+	private Resource parseAsResource(String value, Model model) {
+		final RDFNode subject = parseAsNode(value, model);
+		return subject instanceof Resource ? (Resource)subject : null;
+	}
+	
+	/**
+	 * Parses the given value as an RDF property.
+	 * @param value the value
+	 * @param model the model
+	 * @return the parsed value, or null if unspecified
+	 */
+	private Property parseAsProperty(String value, Model model) {
+		final RDFNode predicate = parseAsNode(value, model);
+		return predicate instanceof Resource ? model.createProperty(((Resource)predicate).getURI()) : null;
+	}
+	
+	/**
+	 * Parses the given value as an RDF node.
+	 * @param value the value
+	 * @param model the model
+	 * @return the parsed value, or null if unspecified
+	 */
+	private RDFNode parseAsNode(String value, Model model) {
+		// nothing or empty indicates an unknown
+		if (value == null || value.length() == 0)
+			return null;
+		// find the kind of entity based on the first character
+		final char firstChar = value.charAt(0);
+		switch(firstChar) {
+		// variable or blank node indicates an unknown
+		case '?':
+		case '_':
+			return null;
+		// angular brackets indicate a URI
+		case '<':
+			return model.createResource(value.substring(1, value.length() - 1));
+		// quotes indicate a string
+		case '"':
+			final Matcher matcher = STRINGPATTERN.matcher(value);
+			if (matcher.matches()) {
+				final String body = matcher.group(1);
+				final String type = matcher.group(2);
+				final String lang = matcher.group(3);
+				if (type != null)
+					return model.createTypedLiteral(body, type);
+				if (lang != null)
+					return model.createLiteral(body, lang);
+				return model.createLiteral(body);
+			}
+			return null;
+		// assume it's a URI without angular brackets
+		default:
+			return model.createResource(value);
+		}
+	}
+}