[ONOS-5012] implement RESTconf server

 - fix javadoc longer than 80 char limit
 - fix javadoc that missing @params
 - chain calls to StringBuilder.append()
 - combine constant strings in place

Change-Id: Ie2ef4fd4c19e955ad2d5a5584f5017a842abb790
diff --git a/protocols/restconf/server/utils/src/main/java/org/onosproject/protocol/restconf/server/utils/parser/json/DefaultJsonWalker.java b/protocols/restconf/server/utils/src/main/java/org/onosproject/protocol/restconf/server/utils/parser/json/DefaultJsonWalker.java
new file mode 100644
index 0000000..c1802ac
--- /dev/null
+++ b/protocols/restconf/server/utils/src/main/java/org/onosproject/protocol/restconf/server/utils/parser/json/DefaultJsonWalker.java
@@ -0,0 +1,109 @@
+/*
+ * Copyright 2016-present Open Networking Laboratory
+ *
+ * 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.
+ */
+
+package org.onosproject.protocol.restconf.server.utils.parser.json;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.node.ArrayNode;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+import org.onosproject.protocol.restconf.server.utils.parser.api.JsonWalker;
+import org.onosproject.protocol.restconf.server.utils.parser.api.JsonListener;
+
+import java.util.Iterator;
+import java.util.Map;
+
+/**
+ * Represents implementation of JSON walk, which walks the JSON object node.
+ */
+public class DefaultJsonWalker implements JsonWalker {
+    @Override
+    public void walk(JsonListener jsonListener, String fieldName,
+                     ObjectNode objectNode) {
+
+        //enter the object node, the original ObjectNode should have a module
+        //name as fieldName.
+        jsonListener.enterJsonNode(fieldName, objectNode);
+        //the node has no children, then exist and return.
+        if (!objectNode.isContainerNode()) {
+            jsonListener.exitJsonNode(objectNode);
+            return;
+        }
+
+        Iterator<Map.Entry<String, JsonNode>> fields = objectNode.fields();
+        while (fields.hasNext()) {
+            //get the children entry of the node
+            Map.Entry<String, JsonNode> currentChild = fields.next();
+            String key = currentChild.getKey();
+            JsonNode value = currentChild.getValue();
+            //if the entry's value has its own children, do a recursion.
+            //if the entry has no children, store the key and value.
+            //for we don't know the specific type of the entry's value, we
+            // should give it to a method which can handle JsonNode
+            if (value.isContainerNode()) {
+                walkJsonNode(jsonListener, key, value);
+            } else {
+                jsonListener.enterJsonNode(key, value);
+                jsonListener.exitJsonNode(value);
+            }
+        }
+        jsonListener.exitJsonNode(objectNode);
+    }
+
+    /**
+     * Walks the JSON data tree. This method is called when we don't know
+     * the exact type of a json node.
+     *
+     * @param jsonListener Json listener implemented by the user
+     * @param fieldName    the original object node field
+     * @param rootNode     the json node which needs to be walk
+     */
+    private void walkJsonNode(JsonListener jsonListener, String fieldName,
+                              JsonNode rootNode) {
+        if (rootNode.isObject()) {
+            walk(jsonListener, fieldName, (ObjectNode) rootNode);
+            return;
+        }
+
+        if (rootNode.isArray()) {
+            walkArrayNode(jsonListener, fieldName, (ArrayNode) rootNode);
+        }
+    }
+
+    /**
+     * Walks the JSON data tree. This method is called when the user knows the
+     * json node type is ArrayNode.
+     *
+     * @param jsonListener Json listener implemented by the user
+     * @param fieldName    the original object node field
+     * @param rootNode     the json node which needs to be walk
+     */
+    private void walkArrayNode(JsonListener jsonListener, String fieldName,
+                               ArrayNode rootNode) {
+        if (rootNode == null) {
+            return;
+        }
+        //enter the array node.
+        jsonListener.enterJsonNode(fieldName, rootNode);
+        Iterator<JsonNode> children = rootNode.elements();
+        while (children.hasNext()) {
+            JsonNode currentChild = children.next();
+            if (currentChild.isContainerNode()) {
+                walkJsonNode(jsonListener, null, currentChild);
+            }
+        }
+        jsonListener.exitJsonNode(rootNode);
+    }
+}