Netty decoder method called multiple times

Question

I'm a new Netty user and I'm having some difficulty after reading the user guide and looking at the factorial example. For my project I need to set up a one-to-many server-client connection, which will be used for sending data from electronic passports for remote verification. To accomplish this I have chosen to use version 4.0.23 of Netty as it is the currently highest stable version.

I'm having some difficulty with my adaptation of the above factorial example. When I send a message and encode and decode it, somehow the decode method is called three times, which leads to exceptions after reading from the buffer.

My minimal working example is below. Please note that SSL is not enabled, since I am not providing additional arguments upon launch.

The Server initializer

public class VerifyServerInitializer extends ChannelInitializer {
    private final SslContext sslCtx;

    public VerifyServerInitializer(SslContext sslCtx) {
        this.sslCtx = sslCtx;
    }

    @Override
    public void initChannel(SocketChannel ch) {
        ChannelPipeline pipeline = ch.pipeline();

        // Convert Strings to ByteBufs and vice versa
        pipeline.addLast(new MessageDecoder());
        pipeline.addLast(new MessageEncoder());

        // Add the business logic.
        pipeline.addLast(new VerifyServerHandler());
    }
}

The Server itself

public class VerifyServer {
    static final boolean SSL = System.getProperty("ssl") != null;
    static final int PORT = Integer.parseInt(System.getProperty("port", "8322"));

    public static void main(String[] args) throws Exception {
        // Configure SSL.
        final SslContext sslCtx;

        if (SSL) {
            // TODO: Change to a trusted certificate
            SelfSignedCertificate ssc = new SelfSignedCertificate();
            sslCtx = SslContext.newServerContext(ssc.certificate(), ssc.privateKey());
        } else {
            sslCtx = null;
        }

        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .handler(new LoggingHandler(LogLevel.INFO))
                    .childHandler(new VerifyServerInitializer(sslCtx));

            b.bind(PORT).sync().channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

The Server handler

public class VerifyServerHandler extends SimpleChannelInboundHandler {

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
        System.out.println("Incoming data: " + msg);
    }
}

The Encoder

public class MessageEncoder extends MessageToByteEncoder {
    @Override
    protected void encode(ChannelHandlerContext ctx, String msg, ByteBuf out) throws Exception {
        System.out.printf("Encoding '%s' into a byte array.
", msg);
        byte[] encoded = msg.getBytes(Charset.forName("UTF-8"));

        System.out.println("Encoding result: " + new String(encoded));
        System.out.printf("Length of the byte array: %d.
", encoded.length);

        //Write a message.
        out.writeByte((byte) 'F');      // magic number
        out.writeInt(encoded.length);   // data length
        out.writeBytes(encoded);        // data
    }
}

The Decoder

public class MessageDecoder extends ByteToMessageDecoder {
    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List

Geert Smelt · Accepted Answer

I have found the solution and will post it here for future reference should anyone else run into this issue.

In the JavaDoc of the decode method there is the following:

Decode the from one ByteBuf to an other. This method will be called till either the input ByteBuf has nothing to read when return from this method or till nothing was read from the input ByteBuf.

All in all it would appear that because I did not add anything to the out List, the method will keep decoding until no more data is found in the ByteBuf. Furthermore, because I updated the reader index if a correct magic number is found, the rest of the data is read from beyond that magic number. Since there is only one of these, the next iteration will not find one and throw a CorruptedFrameException which will cause execution to halt, because I overrode the exceptionCaught method to close the context.

Netty decoder method called multiple times

Answers (2)

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