Examples for setting up custom service executors

jena-examples/src/main/java/arq/examples/service/CustomServiceExecutor.java

@@ -0,0 +1,140 @@
+/**
+ * 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 arq.examples.service;
+
+import java.util.Collections;
+
+import org.apache.jena.graph.Node;
+import org.apache.jena.graph.NodeFactory;
+import org.apache.jena.query.ARQ;
+import org.apache.jena.query.Dataset;
+import org.apache.jena.query.DatasetFactory;
+import org.apache.jena.sparql.algebra.op.OpService;
+import org.apache.jena.sparql.engine.iterator.QueryIterPlainWrapper;
+import org.apache.jena.sparql.exec.QueryExec;
+import org.apache.jena.sparql.exec.QueryExecDatasetBuilder;
+import org.apache.jena.sparql.service.ServiceExecutorFactory;
+import org.apache.jena.sparql.service.ServiceExecutorRegistry;
+import org.apache.jena.sparql.util.Context;
+import org.apache.jena.sparql.util.QueryExecUtils;
+
+public class CustomServiceExecutor {
+
+	/** Query for resources having the label "Apache Jena"en */
+    public static final String QUERY_STR = String.join("\n",
+            "PREFIX wd: <http://www.wikidata.org/entity/>",
+            "PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>",
+            "PREFIX dbr: <http://dbpedia.org/resource/>",
+            "SELECT * {",
+            "  SERVICE <http://query.wikidata.org/sparql> {",
+            "    SELECT * {",
+            "      ?s rdfs:label \"Apache Jena\"@en",
+            "     } LIMIT 10",
+            "  }",
+            "}");
+
+    public static void main(String[] args) {
+
+        Dataset dataset = DatasetFactory.empty();
+
+        conventionalExec(dataset);
+        relayWikidataToDBpedia(dataset);
+        suppressRemoteRequests(dataset);
+    }
+
+    /** Conventionally execute a query against Wikidata as per request */
+    public static void conventionalExec(Dataset dataset) {
+        // Default: Send request to Wikidata
+        execQueryAndShowResult(dataset, QUERY_STR, null);
+
+        /* ---------------
+         * | s           |
+         * ===============
+         * | wd:Q1686799 |
+         * ---------------
+         */
+    }
+
+    /** Relay requests for Wikidata to DBpedia instead */
+    public static void relayWikidataToDBpedia(Dataset dataset) {
+        Node WIKIDATA = NodeFactory.createURI("http://query.wikidata.org/sparql");
+        Node DBPEDIA = NodeFactory.createURI("http://dbpedia.org/sparql");
+
+        ServiceExecutorFactory relaySef = (opExecute, original, binding, execCxt) -> {
+                if (opExecute.getService().equals(WIKIDATA)) {
+                    opExecute = new OpService(DBPEDIA, opExecute.getSubOp(), opExecute.getSilent());
+                    return ServiceExecutorRegistry.httpService.createExecutor(opExecute, original, binding, execCxt);
+                }
+                return null;
+            };
+
+        Context cxt = ARQ.getContext().copy();
+        ServiceExecutorRegistry registry = new ServiceExecutorRegistry();
+        registry.add(relaySef);
+
+        ServiceExecutorRegistry.set(cxt, registry);
+        execQueryAndShowResult(dataset, QUERY_STR, cxt);
+
+        /*
+         * -------------------
+         * | s               |
+         * ===================
+         * | dbr:Apache_Jena |
+         * -------------------
+         */
+    }
+
+    /** Suppress remote requests - make any SERVICE request return the input binding */
+    public static void suppressRemoteRequests(Dataset dataset) {
+        ServiceExecutorFactory noop = (opExecute, original, binding, execCxt) ->
+            () -> QueryIterPlainWrapper.create(Collections.singleton(binding).iterator());
+
+        Context cxt = ARQ.getContext().copy();
+        ServiceExecutorRegistry registry = new ServiceExecutorRegistry();
+        registry.add(noop);
+
+        ServiceExecutorRegistry.set(cxt, registry);
+        execQueryAndShowResult(dataset, QUERY_STR, cxt);
+
+        /*
+         * -----
+         * | s |
+         * =====
+         * |   |
+         * -----
+         */
+    }
+
+    public static void execQueryAndShowResult(
+            Dataset dataset,
+            String queryStr,
+            Context cxt) {
+        try {
+            try (QueryExec exec = QueryExecDatasetBuilder.create()
+                    .dataset(dataset.asDatasetGraph())
+                    .query(queryStr)
+                    .context(cxt)
+                    .build()) {
+                QueryExecUtils.exec(exec);
+            }
+        } catch (Exception e) {
+            e.printStackTrace();
+        }
+    }
+}