Use HTTP 201, 204 response codes in REST API

When creating a new entity on the server respond HTTP 201 Created
with the JSON representation of the entity in the body.

When deleting an existing entity, respond with HTTP 204 No Content,
telling the client there is no more content behind the resources
that was just deleted. This is a hint that a future GET is likely
to return 404 Not Found.

Change-Id: Ia7b3964267fcd55b4abcc49dcd6ba4c61f61fd5d

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/restapi/Response.java

@@ -0,0 +1,93 @@
+// Copyright (C) 2012 The Android Open Source Project
+//
+// 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 com.google.gerrit.extensions.restapi;
+
+/** Special return value to mean specific HTTP status codes in a REST API. */
+public abstract class Response<T> {
+  @SuppressWarnings({"rawtypes"})
+  private static final Response NONE = new None();
+
+  /** HTTP 200 OK: pointless wrapper for type safety. */
+  public static <T> Response<T> ok(T value) {
+    return new Impl<T>(200, value);
+  }
+
+  /** HTTP 201 Created: typically used when a new resource is made. */
+  public static <T> Response<T> created(T value) {
+    return new Impl<T>(201, value);
+  }
+
+  /** HTTP 204 No Content: typically used when the resource is deleted. */
+  @SuppressWarnings("unchecked")
+  public static <T> Response<T> none() {
+    return NONE;
+  }
+
+  @SuppressWarnings({"unchecked", "rawtypes"})
+  public static <T> T unwrap(T obj) {
+    while (obj instanceof Response) {
+      obj = (T) ((Response) obj).value();
+    }
+    return obj;
+  }
+
+  public abstract int statusCode();
+  public abstract T value();
+  public abstract String toString();
+
+  private static final class Impl<T> extends Response<T> {
+    private final int statusCode;
+    private final T value;
+
+    private Impl(int sc, T val) {
+      statusCode = sc;
+      value = val;
+    }
+
+    @Override
+    public int statusCode() {
+      return statusCode;
+    }
+
+    @Override
+    public T value() {
+      return value;
+    }
+
+    @Override
+    public String toString() {
+      return "[" + statusCode() + "] " + value();
+    }
+  }
+
+  private static final class None extends Response<Object> {
+    private None() {
+    }
+
+    @Override
+    public int statusCode() {
+      return 204;
+    }
+
+    public Object value() {
+      throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public String toString() {
+      return "[204 No Content] None";
+    }
+  }
+}