Add registry interface

Registry interface will be used to store and discover extensions

utils/src/com/cloud/utils/component/Registry.java

@@ -0,0 +1,50 @@
+/*
+ * 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 com.cloud.utils.component;
+
+import java.util.List;
+
+/**
+ * Simple interface to represents a registry of items
+ *
+ * @param <T>
+ */
+public interface Registry<T> extends Named {
+
+    /**
+     * Registers an item.  If the item has already been registered the implementation
+     * should detect that it is registered and not re-register it.
+     *
+     * @param type
+     * @return true if register, false if not registered or already exists
+     */
+    boolean register(T type);
+
+    void unregister(T type);
+
+    /**
+     * Returns a list that will dynamically change as items are registered/unregister.
+     * The list is thread safe to iterate upon.  Traversing the list using an index
+     * would not be safe as the size may changed during traversal.
+     *
+     * @return Unmodifiable list of registered items
+     */
+    List<T> getRegistered();
+
+}