Java Nio通道数据未正确读取

Question

I send a stream of data through a java nio channel, which is basically build like this:

<int:size of packet><int:packet id><packet data>

packet data is filled with different data types in a specific order (the packet id tells how to parse it). When I try to send some data locally it works just fine, but as soon as I try to run it on a Windows Server 2012 it reads an invalid packet size value such as negative values or far too big values:

Client output:

Sending packet: network.PacketChunkRequest (size: 24)
Sending packet: network.PacketChunkRequest (size: 24)
Sending packet: network.PacketChunkRequest (size: 24)
Sending packet: network.PacketChunkRequest (size: 24)
Sending packet: network.PacketChunkRequest (size: 24)
Sending packet: network.PacketChunkRequest (size: 24)
Sending packet: network.PacketChunkRequest (size: 24)
Sending packet: network.PacketChunkRequest (size: 24)
Sending packet: network.PacketChunkRequest (size: 24)
2988
-2032198748
java.lang.IllegalArgumentException
    at java.nio.ByteBuffer.allocate(Unknown Source)
    at network.ClientSocket.run(ClientSocket.java:66)
    at java.lang.Thread.run(Unknown Source)

The numbers at the end are the read package sizes, where you can notice that one ridiculous number which leads to an exception while trying to prepare a buffer for it.

Server output:

Sending packet: network.PacketLoginAck (size: 16)
Sending packet: network.PacketPlayerData (size: 951)
Sending packet: network.PacketWorldInfo (size: 33)
Received packet: network.PacketChunkRequest (size: 24)
Sending packet: network.PacketChunkData (size: 2988)     // This is the first package that still worked
Received packet: network.PacketChunkRequest (size: 24)
Received packet: network.PacketChunkRequest (size: 24)
Received packet: network.PacketChunkRequest (size: 24)
Received packet: network.PacketChunkRequest (size: 24)
Received packet: network.PacketChunkRequest (size: 24)
Received packet: network.PacketChunkRequest (size: 24)
Received packet: network.PacketChunkRequest (size: 24)
Received packet: network.PacketChunkRequest (size: 24)
Sending packet: network.PacketChunkData (size: 2518)
Sending packet: network.PacketChunkData (size: 2741)
Sending packet: network.PacketChunkData (size: 2966)
Sending packet: network.PacketChunkData (size: 2449)
Sending packet: network.PacketChunkData (size: 2769)
Sending packet: network.PacketChunkData (size: 1862)
Sending packet: network.PacketChunkData (size: 2526)
Sending packet: network.PacketChunkData (size: 2353)

a PacketChunkRequest contains two ints, two coordinates, while the PacketChunkData also contains these two ints, plus the binary data, which is a int describing the length of the data followed by the actual data.

I send my data like this:

for(Packet p : packets) {
    System.out.println("Sending packet: "+p.getClass().getName()+" (size: "+p.length()+")");

    ByteBuffer b = p.getBuffer();
    while (b.hasRemaining()) {
        clientChannel.write(b);
    }
    b.clear();

    sentPackages.add(p);
}

This is the code for reading the packets:

List<ByteBuffer> packets = new ArrayList<ByteBuffer>();

ByteBuffer bin = null;

int packetLength = 0;

while((bytesRead = channel.read(buffer)) > 0) {
    buffer.flip();
    while(buffer.remaining() > 0) {
        if(packetLength == 0) {
            if(buffer.remaining() < 4) break;

            packetLength = buffer.getInt();
            System.out.println(packetLength);        // This is the output of the length
            bin = ByteBuffer.allocate(packetLength); // This is where the error happens
        }  

        int readSize = Math.min(packetLength, buffer.remaining());

        buffer.limit(buffer.position() + readSize);

        bin.put(buffer);

        buffer.limit(bytesRead);

        packetLength -= readSize;

        if(packetLength == 0) {
            bin.flip();

            packets.add(bin);
        }
    }

    byte[] remaining = new byte[buffer.remaining()];
    for(int i = 0; buffer.remaining() > 0; i++) remaining[i] = buffer.get(); 

    buffer.clear();
    for(byte b : remaining) buffer.put(b);
}

This code tries to reconstruct the individual packages, that were sent together, by reading as much bytes into single buffers as the packet size (the first int of a package).

This error isn't always reproducable but keeps coming, but not locally (at least I never encountered it)

Answer 1

You're getting out of sync with the sender. I haven't analysed your receiving code in detail, but clearly it doesn't handle every possible case and correctly read a packet length as an integer. You're reading part of a pacet as the length word. You need to reconsider your logic.

I would question why you're using NIO for this at all. Try it with a DataInputStream , using successively readInt() and readFully() until EOFException is thrown.