netty源码阅读之解码之基于固定长度解码器分析

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 {

如果数据包是A|BC|DEFG|HI以3为长度将会解析成ABC|DEF|GHI，理解起来可以说是很简单了。继承自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。