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   /*
    * Copyright 2012 The Netty Project
    *
    * The Netty Project 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:
    *
    *   https://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 io.netty.handler.codec.http;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.buffer.ByteBufUtil;
  
  import static io.netty.handler.codec.http.HttpConstants.*;
  
  /**
   * Encodes an {@link HttpResponse} or an {@link HttpContent} into
   * a {@link ByteBuf}.
   */
  public class HttpResponseEncoder extends HttpObjectEncoder<HttpResponse> {
  
      @Override
      public boolean acceptOutboundMessage(Object msg) throws Exception {
          // JDK type checks vs non-implemented interfaces costs O(N), where
          // N is the number of interfaces already implemented by the concrete type that's being tested.
          // !(msg instanceof HttpRequest) is supposed to always be true (and meaning that msg isn't a HttpRequest),
          // but sadly was part of the original behaviour of this method and cannot be removed.
          // We place here exact checks vs DefaultHttpResponse and DefaultFullHttpResponse because bad users can
          // extends such types and make them to implement HttpRequest (non-sense, but still possible).
          final Class<?> msgClass = msg.getClass();
          if (msgClass == DefaultFullHttpResponse.class || msgClass == DefaultHttpResponse.class) {
              return true;
          }
          return super.acceptOutboundMessage(msg) && !(msg instanceof HttpRequest);
      }
  
      @Override
      protected void encodeInitialLine(ByteBuf buf, HttpResponse response) throws Exception {
          response.protocolVersion().encode(buf);
          buf.writeByte(SP);
          response.status().encode(buf);
          ByteBufUtil.writeShortBE(buf, CRLF_SHORT);
      }
  
      @Override
      protected void sanitizeHeadersBeforeEncode(HttpResponse msg, boolean isAlwaysEmpty) {
          if (isAlwaysEmpty) {
              HttpResponseStatus status = msg.status();
              if (status.codeClass() == HttpStatusClass.INFORMATIONAL ||
                      status.code() == HttpResponseStatus.NO_CONTENT.code()) {
  
                  // Stripping Content-Length:
                  // See https://tools.ietf.org/html/rfc7230#section-3.3.2
                  msg.headers().remove(HttpHeaderNames.CONTENT_LENGTH);
  
                  // Stripping Transfer-Encoding:
                  // See https://tools.ietf.org/html/rfc7230#section-3.3.1
                  msg.headers().remove(HttpHeaderNames.TRANSFER_ENCODING);
              } else if (status.code() == HttpResponseStatus.RESET_CONTENT.code()) {
  
                  // Stripping Transfer-Encoding:
                  msg.headers().remove(HttpHeaderNames.TRANSFER_ENCODING);
  
                  // Set Content-Length: 0
                  // https://httpstatuses.com/205
                  msg.headers().setInt(HttpHeaderNames.CONTENT_LENGTH, 0);
              }
          }
      }
  
      @Override
      protected boolean isContentAlwaysEmpty(HttpResponse msg) {
          // Correctly handle special cases as stated in:
          // https://tools.ietf.org/html/rfc7230#section-3.3.3
          HttpResponseStatus status = msg.status();
  
          if (status.codeClass() == HttpStatusClass.INFORMATIONAL) {
  
              if (status.code() == HttpResponseStatus.SWITCHING_PROTOCOLS.code()) {
                  // We need special handling for WebSockets version 00 as it will include an body.
                  // Fortunally this version should not really be used in the wild very often.
                  // See https://tools.ietf.org/html/draft-ietf-hybi-thewebsocketprotocol-00#section-1.2
                  return msg.headers().contains(HttpHeaderNames.SEC_WEBSOCKET_VERSION);
              }
              return true;
          }
          return status.code() == HttpResponseStatus.NO_CONTENT.code() ||
                  status.code() == HttpResponseStatus.NOT_MODIFIED.code() ||
                  status.code() == HttpResponseStatus.RESET_CONTENT.code();
      }
  }