netty源码阅读之解码之基于固定长度解码器分析

布满荆棘的人生 2022-05-10 03:56 184阅读 0赞

固定长度解码器FixedLengthFrameDecoder比较简单，我们看下它类的注释：

/**
     * 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 {

看源码：

public class FixedLengthFrameDecoder extends ByteToMessageDecoder {
    
        private final int frameLength;
    
        /**
         * Creates a new instance.
         *
         * @param frameLength the length of the frame
         */
        public FixedLengthFrameDecoder(int frameLength) {
            if (frameLength <= 0) {
                throw new IllegalArgumentException(
                        "frameLength must be a positive integer: " + 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);
            }
        }
    }

在构造函数里面吧frameLength放进去，表示固定的长度有多长。最重要的方法就是decode方法，上篇文章将结果，主要是实现基类ByteToMessageDecoder的decode方法：

@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);
            }
        }

如果解析到数据，就放到out里面，解析方法在decode里面：

protected Object decode(
                @SuppressWarnings("UnusedParameters") ChannelHandlerContext ctx, ByteBuf in) throws Exception {
            if (in.readableBytes() < frameLength) {
                return null;
            } else {
                return in.readRetainedSlice(frameLength);
            }
        }

可读的小于frameLength，说明不能读出来，那就跳出，out的size不会变，上一篇文章讲过，他们会来到这里：

if (outSize == out.size()) {
                        if (oldInputLength == in.readableBytes()) {
                            break;
                        } else {
                            continue;
                        }
                    }

回来，如果可读的大于或者等于frameLength，那就读数：

in.readRetainedSlice(frameLength);

挺简单。