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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;
  
  import static io.netty.util.internal.ObjectUtil.checkPositive;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.channel.ChannelHandlerContext;
  
  import java.util.List;
  
  /**
   * A decoder that splits the received {@link ByteBuf}s by the fixed number
   * of bytes. For example, if you received the following four fragmented packets:
   * <pre>
   * +---+----+------+----+
   * | A | BC | DEFG | HI |
   * +---+----+------+----+
   * </pre>
   * A {@link FixedLengthFrameDecoder}{@code (3)} will decode them into the
   * following three packets with the fixed length:
   * <pre>
   * +-----+-----+-----+
   * | ABC | DEF | GHI |
   * +-----+-----+-----+
   * </pre>
   */
  public class FixedLengthFrameDecoder extends ByteToMessageDecoder {
  
      private final int frameLength;
  
      /**
       * Creates a new instance.
       *
       * @param frameLength the length of the frame
       */
      public FixedLengthFrameDecoder(int frameLength) {
          checkPositive(frameLength, "frameLength");
          this.frameLength = frameLength;
      }
  
      @Override
      protected final void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
          Object decoded = decode(ctx, in);
          if (decoded != null) {
              out.add(decoded);
          }
      }
  
      /**
       * Create a frame out of the {@link ByteBuf} and return it.
       *
       * @param   ctx             the {@link ChannelHandlerContext} which this {@link ByteToMessageDecoder} belongs to
       * @param   in              the {@link ByteBuf} from which to read data
       * @return  frame           the {@link ByteBuf} which represent the frame or {@code null} if no frame could
       *                          be created.
       */
      protected Object decode(
              @SuppressWarnings("UnusedParameters") ChannelHandlerContext ctx, ByteBuf in) throws Exception {
          if (in.readableBytes() < frameLength) {
              return null;
          } else {
              return in.readRetainedSlice(frameLength);
          }
      }
  }