fix(explore): make RdfSource serializable and answer a missing id plainly

src/main/java/com/knowledgepixels/nanodash/page/ExplorePage.java

@@ -19,6 +19,7 @@
 import org.apache.wicket.markup.html.link.ExternalLink;
 import org.apache.wicket.model.Model;
 import org.apache.wicket.request.flow.RedirectToUrlException;
+import org.apache.wicket.request.http.flow.AbortWithHttpErrorCodeException;
 import org.apache.wicket.request.mapper.parameter.INamedParameters.NamedPair;
 import org.apache.wicket.request.mapper.parameter.PageParameters;
 import org.commonjava.mimeparse.MIMEParse;
@@ -157,9 +158,15 @@ public ExplorePage(final Nanopub publishedNanopub, final PageParameters paramete
     private void initPage() {
         PageParameters parameters = getPageParameters();
 
-        String tempRef = parameters.get("id").toString();
+        String tempRef = parameters.get("id").toString("");
         // Sometimes these Wicket session IDs end up here and they can mess up the query cache:
         tempRef = tempRef.replaceFirst(";jsessionid.*$", "");
+        // There is nothing to explore without an id. Answering that plainly keeps a request
+        // that lost the parameter -- a stale link, or an Ajax call to a page that is no longer
+        // in the page store -- from failing as a server error.
+        if (tempRef.isEmpty()) {
+            throw new AbortWithHttpErrorCodeException(400, "Parameter 'id' is required");
+        }
 
         String contextId = parameters.get("context").toString("");
 

src/main/java/com/knowledgepixels/nanodash/page/RdfSource.java

@@ -15,6 +15,7 @@
 import org.nanopub.Nanopub;
 import org.nanopub.NanopubWithNs;
 
+import java.io.Serializable;
 import java.io.StringWriter;
 import java.util.LinkedHashMap;
 import java.util.List;
@@ -24,13 +25,19 @@
  * What a resource page has to offer as RDF (issue #710): the download page's view of the
  * resource, plus the nanopublications that declare the resource itself, whose assertions
  * are small enough to embed in the page.
+ * <p>
+ * Pages keep this in a field, and Wicket serializes the pages it stores, so this has to be
+ * {@link Serializable}: a page holding something that is not never reaches the page store, and
+ * the next request for it then re-instantiates the page without its parameters, which is how the
+ * field added for issue #710 made the Explore page's lazy-loaded panels fail.
+ * The nanopublications it carries are serializable themselves ({@code NanopubImpl}).
  *
  * @param type         the download page's {@code type}: user, space, resource or part
  * @param id           the resource IRI
  * @param contextId    the containing resource for a part, null otherwise
  * @param declarations the nanopublications declaring the resource; empty when unknown
  */
-public record RdfSource(String type, String id, String contextId, List<Nanopub> declarations) {
+public record RdfSource(String type, String id, String contextId, List<Nanopub> declarations) implements Serializable {
 
     /**
      * Prefixes a nanopublication declares for its own URI space, which mean nothing outside it.

src/test/java/com/knowledgepixels/nanodash/page/ExplorePageTest.java

@@ -1,16 +1,30 @@
 package com.knowledgepixels.nanodash.page;
 
+import com.knowledgepixels.nanodash.WicketApplication;
+import org.apache.wicket.util.tester.WicketTester;
 import org.eclipse.rdf4j.model.vocabulary.DCTERMS;
 import org.eclipse.rdf4j.model.vocabulary.RDFS;
 import org.eclipse.rdf4j.model.vocabulary.SKOS;
 import org.junit.jupiter.api.Test;
 
 import static org.eclipse.rdf4j.model.util.Values.iri;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 class ExplorePageTest {
 
+    // A request that lost its id -- a stale link, or an Ajax call to a page that is no longer
+    // in the page store -- used to fail as a server error, which left the status line and the
+    // other lazy-loaded panels spinning forever.
+    @Test
+    void anExploreRequestWithoutAnIdIsRejected() {
+        WicketTester tester = new WicketTester(new WicketApplication());
+        tester.executeUrl("." + ExplorePage.MOUNT_PATH);
+        assertEquals(400, tester.getLastResponse().getStatus(),
+                "an explore URL without an 'id' parameter should be rejected, not fail as a server error");
+    }
+
     @Test
     void membershipIsDeclaredByPartVersionAndSchemeRelations() {
         assertTrue(ExplorePage.declaresMembership(DCTERMS.IS_PART_OF));

src/test/java/com/knowledgepixels/nanodash/page/RdfSourceTest.java

@@ -5,7 +5,11 @@
 import org.nanopub.Nanopub;
 import org.nanopub.NanopubImpl;
 
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
 import java.io.File;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
 import java.util.List;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
@@ -93,6 +97,31 @@ void nothingToEmbedWithoutDeclarations() {
         assertNull(new RdfSource("space", RESOURCE_IRI, null, List.of()).toEmbeddedJsonLd(DUMP_URL));
     }
 
+    /**
+     * Wicket serializes the pages it stores, and a page keeps its RdfSource in a field, so
+     * one has to survive serialization: a page holding something that does not never reaches
+     * the page store, and the next request for it re-instantiates the page without its
+     * parameters -- which is how the Explore page's lazy-loaded panels came to fail.
+     */
+    @Test
+    void survivesPageSerialization() throws Exception {
+        RdfSource source = new RdfSource("space", RESOURCE_IRI, null, List.of(declaration()));
+        ByteArrayOutputStream bytes = new ByteArrayOutputStream();
+        try (ObjectOutputStream out = new ObjectOutputStream(bytes)) {
+            out.writeObject(source);
+        }
+        RdfSource restored;
+        try (ObjectInputStream in = new ObjectInputStream(new ByteArrayInputStream(bytes.toByteArray()))) {
+            restored = (RdfSource) in.readObject();
+        }
+        assertEquals("space", restored.type());
+        assertEquals(RESOURCE_IRI, restored.id());
+        assertEquals(1, restored.declarations().size());
+        String doc = restored.toEmbeddedJsonLd(DUMP_URL);
+        assertTrue(doc.contains(RESOURCE_IRI), doc);
+        assertTrue(doc.contains("Test Workshop"), doc);
+    }
+
     /**
      * The download parameters name the resource, its context for a part, and the variant's
      * format and assertion switch.