Sketched out TopologyResource for the new GUI.

+/*
+ * 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 org.onlab.onos.gui;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.node.ArrayNode;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+import org.onlab.onos.net.ConnectPoint;
+import org.onlab.onos.net.Device;
+import org.onlab.onos.net.Host;
+import org.onlab.onos.net.HostLocation;
+import org.onlab.onos.net.Link;
+import org.onlab.onos.net.device.DeviceService;
+import org.onlab.onos.net.host.HostService;
+import org.onlab.onos.net.link.LinkService;
+import org.onlab.onos.net.topology.Topology;
+import org.onlab.onos.net.topology.TopologyGraph;
+import org.onlab.onos.net.topology.TopologyService;
+import org.onlab.onos.net.topology.TopologyVertex;
+import org.onlab.packet.IpPrefix;
+import org.onlab.packet.MacAddress;
+import org.onlab.rest.BaseResource;
+
+import javax.ws.rs.GET;
+import javax.ws.rs.Produces;
+import javax.ws.rs.core.Response;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Set;
+
+/**
+ * Topology viewer resource.
+ */
+@javax.ws.rs.Path("topology")
+public class TopologyResource extends BaseResource {
+
+    @javax.ws.rs.Path("/graph")
+    @GET
+    @Produces("application/json")
+    public Response graph() {
+        // Fetch the services we'll be using.
+        DeviceService deviceService = get(DeviceService.class);
+        HostService hostService = get(HostService.class);
+        TopologyService topologyService = get(TopologyService.class);
+
+        // Fetch the current topology and its graph that we'll use to render.
+        Topology topo = topologyService.currentTopology();
+        TopologyGraph graph = topologyService.getGraph(topo);
+
+        ObjectMapper mapper = new ObjectMapper();
+        ObjectNode rootNode = mapper.createObjectNode();
+        rootNode.set("devices", getDevices(mapper, deviceService, graph));
+        rootNode.set("links", getLinks(mapper, topo, graph));
+        rootNode.set("hosts", getHosts(mapper, hostService));
+        return Response.ok(rootNode.toString()).build();
+    }
+
+    // Encodes all infrastructure devices.
+    private ArrayNode getDevices(ObjectMapper mapper, DeviceService deviceService,
+                                 TopologyGraph graph) {
+        ArrayNode devices = mapper.createArrayNode();
+        for (TopologyVertex vertex : graph.getVertexes()) {
+            devices.add(json(mapper, deviceService.getDevice(vertex.deviceId()),
+                             deviceService.isAvailable(vertex.deviceId())));
+        }
+        return devices;
+    }
+
+    // Encodes all infrastructure links.
+    private ArrayNode getLinks(ObjectMapper mapper, Topology topo, TopologyGraph graph) {
+        // Now scan all links and count number of them between the same devices
+        // using a normalized link key.
+        Map<String, AggLink> linkRecords = aggregateLinks();
+
+        // Now build all interior edges using the aggregated links.
+        ArrayNode links = mapper.createArrayNode();
+        for (AggLink lr : linkRecords.values()) {
+            links.add(json(mapper, lr));
+        }
+        return links;
+    }
+
+    // Encodes all end-station hosts.
+    private ArrayNode getHosts(ObjectMapper mapper, HostService hostService) {
+        ArrayNode hosts = mapper.createArrayNode();
+        for (Host host : hostService.getHosts()) {
+            Set<IpPrefix> ipAddresses = host.ipAddresses();
+            IpPrefix ipAddress = ipAddresses.isEmpty() ? null : ipAddresses.iterator().next();
+            String label = ipAddress != null ? ipAddress.toString() : host.mac().toString();
+            hosts.add(json(mapper, host));
+        }
+        return hosts;
+    }
+
+    // Scan all links and counts number of them between the same devices
+    // using a normalized link key.
+    private Map<String, AggLink> aggregateLinks() {
+        Map<String, AggLink> aggLinks = new HashMap<>();
+        LinkService linkService = get(LinkService.class);
+        for (Link link : linkService.getLinks()) {
+            String key = key(link);
+            AggLink lr = aggLinks.get(key);
+            if (lr == null) {
+                lr = new AggLink(key);
+                aggLinks.put(key, lr);
+            }
+            lr.addLink(link);
+        }
+        return aggLinks;
+    }
+
+    // Produces JSON for a device.
+    private ObjectNode json(ObjectMapper mapper, Device device, boolean isOnline) {
+        ObjectNode node = mapper.createObjectNode()
+                .put("id", device.id().toString())
+                .put("type", device.type().toString().toLowerCase())
+                .put("online", isOnline);
+        node.set("labels", labels(mapper,
+                                  device.id().uri().getSchemeSpecificPart(),
+                                  MacAddress.valueOf(device.chassisId().value()).toString(),
+                                  device.serialNumber()));
+        return node;
+    }
+
+    // Produces JSON for a link.
+    private ObjectNode json(ObjectMapper mapper, AggLink aggLink) {
+        Link link = aggLink.link;
+        return mapper.createObjectNode()
+                .put("src", link.src().deviceId().toString())
+                .put("dst", link.dst().deviceId().toString())
+                .put("type", link.type().toString().toLowerCase())
+                .put("linkWidth", aggLink.links.size());
+    }
+
+    // Produces JSON for a device.
+    private ObjectNode json(ObjectMapper mapper, Host host) {
+        ObjectNode json = mapper.createObjectNode()
+                .put("id", host.id().toString());
+        json.set("cp", location(mapper, host.location()));
+        json.set("labels", labels(mapper, ip(host.ipAddresses()),
+                                  host.mac().toString()));
+        return json;
+    }
+
+    private String ip(Set<IpPrefix> ipPrefixes) {
+        Iterator<IpPrefix> it = ipPrefixes.iterator();
+        return it.hasNext() ? it.next().toString() : "unknown";
+    }
+
+    private ObjectNode location(ObjectMapper mapper, HostLocation location) {
+        return mapper.createObjectNode()
+                .put("device", location.deviceId().toString())
+                .put("port", location.port().toLong());
+    }
+
+    private ArrayNode labels(ObjectMapper mapper, String... labels) {
+        ArrayNode json = mapper.createArrayNode();
+        for (String label : labels) {
+            json.add(label);
+        }
+        return json;
+    }
+
+    // Aggregate link of all links between the same devices regardless of
+    // their direction.
+    private class AggLink {
+        Link link; // representative links
+
+        final String key;
+        final Set<Link> links = new HashSet<>();
+
+        AggLink(String key) {
+            this.key = key;
+        }
+
+        void addLink(Link link) {
+            links.add(link);
+            if (this.link == null) {
+                this.link = link;
+            }
+        }
+    }
+
+    // Returns a canonical key for the specified link.
+    static String key(Link link) {
+        String s = id(link.src());
+        String d = id(link.dst());
+        return s.compareTo(d) > 0 ? d + s : s + d;
+    }
+
+    // Returns a formatted string for the element associated with the given
+    // connection point.
+    private static String id(ConnectPoint cp) {
+        return cp.elementId().toString();
+    }
+
+}

@@ -14,6 +14,7 @@
                 layering: true,
                 collisionPrevention: true
             },
+            XjsonUrl: 'rs/topology/graph',
             jsonUrl: 'network.json',
             iconUrl: {
                 device: 'img/device.png',