Question

For my homework assignment, I have a network of Nodes that are passing messages to each other. Each Node is connected to a set amount of other Nodes (I’m using 4 for testing). Each Link has a weight, and all the Nodes have computed the shortest path for how they want their messages sent. Every Packet that is sent is composed of the message protocol (a hard-coded int), an integer that tells how many messages have passed through the sending Node, and the routing path for the Packet.

Every Node has a Thread for each of its Links. There is an active Socket in each Link. The Packets are sent by adding a 4-byte int to the beginning of the message telling the message’s length.

Everything works fine until I stress the network. For my test, there are 10 Nodes, and I get 5 of them to send 10000 packets in a simple while() loop with no Thread.sleep(). Without exception, there is always an error at some point during execution at the if(a!=len) statement.

Please let me know if I can clarify anything. Thanks in advance! Here is the code (from the Link Thread; send() and forward() are called from the Node itself):

protected void listen(){
    byte[] b;
    int len;
    try{
        DataInputStream in = new DataInputStream(sock.getInputStream());
        while(true){
            len = in.readInt();
            b = new byte[len];
            int a = in.read(b,0,len);
            if(a!=len){
System.out.println("ERROR: " + a + "!=" + len);
                throw new SocketException(); //may have to fix...this will happen when message is corrupt/incomplete
            }
            Message m = new Message(b);
            int p = m.getProtocol();
            switch (p){
                case CDNP.PACKET:
                    owner.incrementTracker();
                    System.out.print("\n# INCOMMING TRACKER: " + m.getTracker() + "\n>>> ");
                    owner.forward(m);
            }
        }
    }catch (IOException e){
e.printStackTrace();
    }
}

public void send(int tracker){
    String[] message = { Conv.is(CDNP.PACKET), Conv.is(tracker), owner.getMST().toString() };
    Message m = new Message(message);
    forward(m);
}

public synchronized void forward(Message m){
    try{
        OutputStream out = sock.getOutputStream();
        //convert length to byte array of length 4
        ByteBuffer bb = ByteBuffer.allocate(4+m.getLength());
        bb.putInt(m.getLength());
        bb.put(m.getBytes());
        out.write(bb.array());
        out.flush();
    }catch (UnknownHostException e){
System.out.println("ERROR: Could not send to Router at " + sock.getRemoteSocketAddress().toString());
        return;
    }catch (IOException e1){

    }
}

Answer 1

int a = in.read(b,0,len);
if(a!=len){

That won’t work. The InputStream may not read all the bytes you want, it may read only what is available right now, and return that much without blocking.

To quote the Javadocs (emphasis mine):

Reads up to len bytes of data from the input stream into an array of bytes. An attempt is made to read as many as len bytes, but a smaller number may be read, possibly zero. The number of bytes actually read is returned as an integer.

You need to continue reading in a loop until you have all the data you want (or the stream is finished).

Or, since you are using a DataInputStream, you can also use

in.readFully(b, 0, len);

which always reads exactly len bytes (blocking until those have arrived, throwing an exception when there is not enough data).