/projects/: Support JSON output format

Implement a machine readable JSON output for both ls-projects and
GET /projects/ interfaces. This will eventually permit replacing
the RPC used by ProjectListScreen to be this new common JSON API.

The JSON output is a single JSON object containing an object entry
for each matched project:

$ curl 'http://127.0.0.1:8080/projects/?format=JSON&d'
)]}'
{
  "external/bison": {
    "description": "A general-purpose parser generator"
  },
  "external/flex": {
    "description": "A fast lexical analyser generator"
  },
  "external/gcc": {},
  "tools/repo": {}
}

Change-Id: I2014a2209bab8be4fb9f61f15e3546c17a2debd5

gerrit-server/src/main/java/com/google/gerrit/server/OutputFormat.java

@@ -0,0 +1,71 @@
+// 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.server;
+
+import com.google.gson.FieldNamingPolicy;
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gwtjsonrpc.server.SqlTimestampDeserializer;
+
+import java.sql.Timestamp;
+
+/** Standard output format used by an API call. */
+public enum OutputFormat {
+  /**
+   * The output is a human readable text format. It may also be regular enough
+   * to be machine readable. Whether or not the text format is machine readable
+   * and will be committed to as a long term format that tools can build upon is
+   * specific to each API call.
+   */
+  TEXT,
+
+  /**
+   * Pretty-printed JSON format. This format uses whitespace to make the output
+   * readable by a human, but is also machine readable with a JSON library. The
+   * structure of the output is a long term format that tools can rely upon.
+   */
+  JSON,
+
+  /**
+   * Same as {@link #JSON}, but with unnecessary whitespace removed to save
+   * generation time and copy costs. Typically JSON_COMPACT format is used by a
+   * browser based HTML client running over the network.
+   */
+  JSON_COMPACT;
+
+  /** @return true when the format is either JSON or JSON_COMPACT. */
+  public boolean isJson() {
+    return this == JSON_COMPACT || this == JSON;
+  }
+
+  /** @return a new Gson instance configured according to the format. */
+  public GsonBuilder newGsonBuilder() {
+    if (!isJson()) {
+      throw new IllegalStateException(String.format("%s is not JSON", this));
+    }
+    GsonBuilder gb = new GsonBuilder()
+      .setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
+      .registerTypeAdapter(Timestamp.class, new SqlTimestampDeserializer());
+    if (this == OutputFormat.JSON) {
+      gb.setPrettyPrinting();
+    }
+    return gb;
+  }
+
+  /** @return a new Gson instance configured according to the format. */
+  public Gson newGson() {
+    return newGsonBuilder().create();
+  }
+}