/**
 * 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.apache.hadoop.conf;

import java.io.IOException;
import java.io.Writer;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.ws.rs.core.HttpHeaders;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.http.HttpServer2;

import org.apache.hadoop.classification.VisibleForTesting;

/**
 * A servlet to print out the running configuration data.
 */
@InterfaceAudience.LimitedPrivate({"HDFS", "MapReduce"})
@InterfaceStability.Unstable
public class ConfServlet extends HttpServlet {
  private static final long serialVersionUID = 1L;

  protected static final String FORMAT_JSON = "json";
  protected static final String FORMAT_XML = "xml";

  /**
   * Return the Configuration of the daemon hosting this servlet.
   * This is populated when the HttpServer starts.
   */
  private Configuration getConfFromContext() {
    Configuration conf = (Configuration)getServletContext().getAttribute(
        HttpServer2.CONF_CONTEXT_ATTRIBUTE);
    assert conf != null;
    return conf;
  }

  @Override
  public void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {

    if (!HttpServer2.isInstrumentationAccessAllowed(getServletContext(),
                                                   request, response)) {
      return;
    }

    String format = parseAcceptHeader(request);
    if (FORMAT_XML.equals(format)) {
      response.setContentType("text/xml; charset=utf-8");
    } else if (FORMAT_JSON.equals(format)) {
      response.setContentType("application/json; charset=utf-8");
    }

    String name = request.getParameter("name");
    Writer out = response.getWriter();
    try {
      writeResponse(getConfFromContext(), out, format, name);
    } catch (BadFormatException bfe) {
      response.sendError(HttpServletResponse.SC_BAD_REQUEST, bfe.getMessage());
    } catch (IllegalArgumentException iae) {
      response.sendError(HttpServletResponse.SC_NOT_FOUND, iae.getMessage());
    }
    out.close();
  }

  @VisibleForTesting
  static String parseAcceptHeader(HttpServletRequest request) {
    String format = request.getHeader(HttpHeaders.ACCEPT);
    return format != null && format.contains(FORMAT_JSON) ?
        FORMAT_JSON : FORMAT_XML;
  }

  /**
   * Guts of the servlet - extracted for easy testing.
   */
  static void writeResponse(Configuration conf,
      Writer out, String format, String propertyName)
          throws IOException, IllegalArgumentException, BadFormatException {
    if (FORMAT_JSON.equals(format)) {
      Configuration.dumpConfiguration(conf, propertyName, out);
    } else if (FORMAT_XML.equals(format)) {
      conf.writeXml(propertyName, out, conf);
    } else {
      throw new BadFormatException("Bad format: " + format);
    }
  }

  static void writeResponse(Configuration conf, Writer out, String format)
      throws IOException, BadFormatException {
    writeResponse(conf, out, format, null);
  }

  public static class BadFormatException extends Exception {
    private static final long serialVersionUID = 1L;

    public BadFormatException(String msg) {
      super(msg);
    }
  }

}
