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 * https://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 static io.netty.util.internal.ObjectUtil.checkPositive;
19
20 import io.netty.buffer.ByteBuf;
21 import io.netty.channel.ChannelHandlerContext;
22
23 import java.util.List;
24
25 /**
26 * A decoder that splits the received {@link ByteBuf}s by the fixed number
27 * of bytes. For example, if you received the following four fragmented packets:
28 * <pre>
29 * +---+----+------+----+
30 * | A | BC | DEFG | HI |
31 * +---+----+------+----+
32 * </pre>
33 * A {@link FixedLengthFrameDecoder}{@code (3)} will decode them into the
34 * following three packets with the fixed length:
35 * <pre>
36 * +-----+-----+-----+
37 * | ABC | DEF | GHI |
38 * +-----+-----+-----+
39 * </pre>
40 */
41 public class FixedLengthFrameDecoder extends ByteToMessageDecoder {
42
43 private final int frameLength;
44
45 /**
46 * Creates a new instance.
47 *
48 * @param frameLength the length of the frame
49 */
50 public FixedLengthFrameDecoder(int frameLength) {
51 checkPositive(frameLength, "frameLength");
52 this.frameLength = frameLength;
53 }
54
55 @Override
56 protected final void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
57 Object decoded = decode(ctx, in);
58 if (decoded != null) {
59 out.add(decoded);
60 }
61 }
62
63 /**
64 * Create a frame out of the {@link ByteBuf} and return it.
65 *
66 * @param ctx the {@link ChannelHandlerContext} which this {@link ByteToMessageDecoder} belongs to
67 * @param in the {@link ByteBuf} from which to read data
68 * @return frame the {@link ByteBuf} which represent the frame or {@code null} if no frame could
69 * be created.
70 */
71 protected Object decode(
72 @SuppressWarnings("UnusedParameters") ChannelHandlerContext ctx, ByteBuf in) throws Exception {
73 if (in.readableBytes() < frameLength) {
74 return null;
75 } else {
76 return in.readRetainedSlice(frameLength);
77 }
78 }
79 }