Netty Server throwing Exception when a simple Java Nio Client sends a message

Question

I have a very simple Server designed in Java using Netty. I have also written a simple client using Java NIO Package. I can connect the Netty Server with the client but when the message is sent to the server I am getting the following exceptions:

My Output:

New client connected: /127.0.0.1:1125 java.io.IOException: An existing connection was forcibly closed by the remote host at sun.nio.ch.SocketDispatcher.read0(Native Method) at sun.nio.ch.SocketDispatcher.read(SocketDispatcher.java:43) at sun.nio.ch.IOUtil.readIntoNativeBuffer(IOUtil.java:223) at sun.nio.ch.IOUtil.read(IOUtil.java:192) at sun.nio.ch.SocketChannelImpl.read(SocketChannelImpl.java:380) at io.netty.buffer.PooledUnsafeDirectByteBuf.setBytes(PooledUnsafeDirectByteBuf.java:288) at io.netty.buffer.AbstractByteBuf.writeBytes(AbstractByteBuf.java:1100) at io.netty.channel.socket.nio.NioSocketChannel.doReadBytes(NioSocketChannel.java:372) at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:123) at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:644) at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:579) at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:496) at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:458) at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858) at io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:138) at java.lang.Thread.run(Thread.java:745)

My Server Program:

public class EchoServer{

    private final int port;
    public EchoServer(int port) {
        this.port = port;
    }

    public void start() throws Exception {
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(group)
                    .channel(NioServerSocketChannel.class)
                    .localAddress(new InetSocketAddress(port))
                    .childHandler(new ChannelInitializer<SocketChannel>() {

            @Override
            public void initChannel(SocketChannel ch) throws Exception { 
            System.out.println("New client connected: " + ch.localAddress()); 

           ch.pipeline().addLast(newLengthFieldBasedFrameDecoder(Integer.MAX_VALUE,0,4,0,4))            
      .addLast(new LengthFieldPrepender(4))
      .addLast(new EchoServerHandler());
    } 
      });
       ChannelFuture f = b.bind().sync();
       f.channel().closeFuture().sync();
       }
      finally {
            group.shutdownGracefully().sync();
        }
    }

    public static void main (String [] args) throws Exception {
        new EchoServer(1125).start();
    }
}

My ServerHandler:

public class EchoServerHandler extends ChannelInboundHandlerAdapter{

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
         ByteBuf in = (ByteBuf) msg;
         System.out.println(in.toString(CharsetUtil.UTF_8));        

    }

      @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        cause.printStackTrace();
        ctx.close();
    }
}

My NIO Java Client:

public class NIOJavaClient {

    public static void main(String[] args) throws IOException, InterruptedException {

        InetSocketAddress addr = new InetSocketAddress("localhost", 1125);
        SocketChannel client = SocketChannel.open(addr);

        System.out.println("Connecting to Server on port 1125...");

        int i=0;
         while(i<10){

            byte[] message = new String("Hi Server\n").getBytes();
            ByteBuffer buffer = ByteBuffer.wrap(message);
            client.write(buffer);
            buffer.clear();
                        i++;
    }
        client.close();
    }

}

Now My required things are:

1) I can't send message from my client to server.Present I am using a Java NIO Client (not Netty). In future I may send message in bytes from a C# client to the netty server. Is it possible and if so how to do it?

2) I am using the LengthFieldBasedFrameDecoder and LengthPrepender to eliminate half strings (the strings that are sent from NETTY client are cut to a size everytime). I am successfully receiving the message when I use a Netty Client but when I use a different Client designed in Mina or C# I am getting an Exception: length frame exceeded. How can I eliminate this?

In a brief manner I want to connect a client designed in any language which sends bytes to the Netty Server and also eliminate the half strings. This is a critical situation for me. Any help would be really appreciable.

Ferrybig Ferrybig · Accepted Answer · 2017-05-27T16:15:02

Since you are using the LengthFieldBasedFrameDecoder in your code, this means that you need to send your packets from simple program in the format this handler accepts, namely, prepended by a 4 byte value in network order that gives the length of the upcoming data.

Example for your NIO client

byte[] message = new String("Hi Server\n").getBytes();
byte[] fullMessage = new byte[message.length + 4];
fullMessage[0] = (byte)(message.length >> 24);
fullMessage[1] = (byte)(message.length >> 16);
fullMessage[2] = (byte)(message.length >> 8);
fullMessage[3] = (byte)message.length;
System.arraycopy(message, 0, messageFull, 4, message.length);
ByteBuffer buffer = ByteBuffer.wrap(fullMessage);
client.write(buffer);
buffer.clear();

There seems to be a better way, since your protocol work on lines ended with a newline (but this may also be caused by your attempts to debug the issue), instead of prefixing the packets with a length, just wait for a newline character using the DelimiterBasedFrameDecoder handler, use it like this:

ch.pipeline.addLast(new DelimiterBasedFrameDecoder(Delimiters.lineDelimiter()))
// Don't forget to remove your old 2 handlers

Netty Server throwing Exception when a simple Java Nio Client sends a message

1 Answers