1 /*
2 * Copyright 2012 The Netty Project
3 *
4 * The Netty Project licenses this file to you under the Apache License,
5 * version 2.0 (the "License"); you may not use this file except in compliance
6 * with the License. You may obtain a copy of the License at:
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
12 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
13 * License for the specific language governing permissions and limitations
14 * under the License.
15 */
16 package io.netty.handler.codec;
17
18 import io.netty.buffer.ByteBuf;
19 import io.netty.channel.ChannelHandlerContext;
20
21 import java.util.List;
22
23 /**
24 * A decoder that splits the received {@link ByteBuf}s by the fixed number
25 * of bytes. For example, if you received the following four fragmented packets:
26 * <pre>
27 * +---+----+------+----+
28 * | A | BC | DEFG | HI |
29 * +---+----+------+----+
30 * </pre>
31 * A {@link FixedLengthFrameDecoder}{@code (3)} will decode them into the
32 * following three packets with the fixed length:
33 * <pre>
34 * +-----+-----+-----+
35 * | ABC | DEF | GHI |
36 * +-----+-----+-----+
37 * </pre>
38 */
39 public class FixedLengthFrameDecoder extends ByteToMessageDecoder {
40
41 private final int frameLength;
42
43 /**
44 * Creates a new instance.
45 *
46 * @param frameLength the length of the frame
47 */
48 public FixedLengthFrameDecoder(int frameLength) {
49 if (frameLength <= 0) {
50 throw new IllegalArgumentException(
51 "frameLength must be a positive integer: " + frameLength);
52 }
53 this.frameLength = frameLength;
54 }
55
56 @Override
57 protected final void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
58 Object decoded = decode(ctx, in);
59 if (decoded != null) {
60 out.add(decoded);
61 }
62 }
63
64 /**
65 * Create a frame out of the {@link ByteBuf} and return it.
66 *
67 * @param ctx the {@link ChannelHandlerContext} which this {@link ByteToMessageDecoder} belongs to
68 * @param in the {@link ByteBuf} from which to read data
69 * @return frame the {@link ByteBuf} which represent the frame or {@code null} if no frame could
70 * be created.
71 */
72 protected Object decode(
73 @SuppressWarnings("UnusedParameters") ChannelHandlerContext ctx, ByteBuf in) throws Exception {
74 if (in.readableBytes() < frameLength) {
75 return null;
76 } else {
77 return in.readSlice(frameLength).retain();
78 }
79 }
80 }