Java NIO 问题/对 isReadable 工作原理的误解

Question

I've found that the NIO is poorly documented at best except for the simplistic case. Even so, I've been through the tutorials and several refactors and ultimately pushed back to the simplest case and I'm still occasionally having isReadable firing off with a 0 byte SocketChannel read. It's not happening every execution.

I used to be calling the read from the attached object in a separate thread and thought it might be race conditions but I've since gone to having the read occur in the selector's thread and still the problem persists. I imagine it could be my test client, but I'm not sure what would be triggering it inconsistently as the client socket shouldn't be closing until it receives a response from the server.

So in the code included, the "hello" message sent by this snippet makes it across fine every single time as I would expect

 out.write("hello".getBytes()); out.write(EOT); out.flush();

It is after this that I will occasionally get a 0 length socket channel. And sometimes get the proper response from this snippet:

 out.write(dataServerCredentials.getBytes()); out.write(EOT); out.flush();

Any insight into this would be appreciated, it's killing my slowly. I've already tried finding answers here and the one question that seemed relevant didn't really shed much light onto my problems.

Thanks in advance!

Code snippets below:

Selection method:

public void execute()
{
    initializeServerSocket();

    for (;;)
    {
        try
        {
            System.out.println("Waiting for socket activity");

            selector.select();

            Iterator<SelectionKey> selectedKeys = 
                this.selector.selectedKeys().iterator();
            while(selectedKeys.hasNext())
            {
                SelectionKey key = selectedKeys.next();
                selectedKeys.remove();

                if (!key.isValid()) 
                {
                    continue;
                }

                if (key.isAcceptable())
                {   // New connection
                    // TODO: Create helper method for this that configures user info?
                    System.out.println("Accepting connection");

                    ServerSocketChannel serverSocketChannel =
                        (ServerSocketChannel)key.channel();
                    SocketChannel socketChannel =
                        serverSocketChannel.accept();

                    socketChannel.socket().setSoTimeout(0);
                    socketChannel.configureBlocking(false);
                    SelectionKey newKey = 
                        socketChannel.register(selector, SelectionKey.OP_READ);

                    // Create and attach an AuthProcessor to drive the states of this
                    // new Authentication request
                    newKey.attach(new AuthenticationRequestProcessor(newKey));

                }
                else if (key.isReadable())
                {   // Socket has incoming communication
                    AuthenticationRequestProcessor authProcessor =
                        (AuthenticationRequestProcessor)key.attachment();

                    if (authProcessor == null)
                    {   // Cancel Key
                        key.channel().close();
                        key.cancel();
                        System.err.print("Cancelling Key - No Attachment");
                    }
                    else
                    {   
                        if (authProcessor.getState() ==
                            AuthenticationRequestProcessor.TERMINATE_STATE)
                        {   // Cancel Key
                            key.channel().close();
                            key.cancel();
                        }
                        else
                        {   // Process new socket data
                            authProcessor.process(readStringFromKey(key));
                        }
                    }
                }                    
            }
        }
        catch (IOException e)
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

Read Method (Ignore some of the stupidities in here, this was yanked from another thread)

protected String readStringFromKey(SelectionKey key)
{
    SocketChannel socketChannel = (SocketChannel)key.channel();

    readBuffer.clear();

    String message = null;

    try
    {
        final int bytesRead = socketChannel.read(readBuffer);

        if (-1 == bytesRead)
        {   // Empty/Closed Channel
            System.err.println("Error - No bytes to read on selected channel");
        }
        else
        {   // Convert ByteBuffer into a String
            System.out.println("Bytes Read: " + bytesRead);
            readBuffer.flip();
            message = byteBufferToString(readBuffer, bytesRead);
            readBuffer.clear();
        }
    }
    catch (IOException e)
    {
        // TODO Auto-generated catch block

        e.printStackTrace();
    }

    // Trim EOT off the end of the message
    return message.trim();
}

Client snippets:

    public void connect()
{
    boolean connectionStatus = false;
    String connectionHost = null;
    int connectionPort = 0;
    String connectionAuthKey = null;

    try
    {   // Login
        authenticationSocket = new Socket(AUTH_HOST, AUTH_PORT);
        out = authenticationSocket.getOutputStream();
        in = new BufferedInputStream(authenticationSocket.getInputStream());

        out.write("hello".getBytes());
        out.write(EOT);
        out.flush();


        StringBuilder helloResponse = new StringBuilder();

        // Read response off socket
        int currentByte = in.read();

        while (currentByte > -1 && currentByte != EOT)
        {
            helloResponse.append((char)currentByte);
            currentByte = in.read();
        }

        outgoingResponses.offer(Plist.fromXml(helloResponse.toString()));
        System.out.println("\n" + helloResponse.toString());

        out.write(credentials.getBytes());
        out.write(EOT);
        out.flush();

        // Read status
        int byteRead;

        StringBuilder command = new StringBuilder();

        do 
        {
            byteRead = in.read();
            if (0 < byteRead) 
            {
                if (EOT == byteRead)
                {
                    Logger.logData(command.toString());

                    Map<String, Object> plist = Plist.fromXml(command.toString());
                    outgoingResponses.offer(plist);

                    // Connection info for Data Port
                    connectionStatus = (Boolean)plist.get(STATUS_KEY);
                    connectionHost = (String)plist.get(SERVER_KEY);
                    connectionPort = (Integer)plist.get(PORT_KEY);
                    connectionAuthKey = (String)plist.get(AUTH_KEY);

                    Logger.logData("Server =>" + plist.get("server"));

                    command = new StringBuilder();

                }
                else
                {
                    command.append((char)byteRead);
                }
            }
        } 
        while (EOT != byteRead);
    }
    catch (UnknownHostException e)
    {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    catch (IOException e)
    {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    catch (XmlParseException e)
    {
        Logger.logData("Invalid Plist format");
        e.printStackTrace();
    }
    finally
    {   // Clean up handles
        try
        {
            authenticationSocket.close();
            out.close();
            in.close();
        }
        catch (IOException e)
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    System.out.println("Connection status =>" + connectionStatus);
    System.out.println("Connection host =>" + connectionHost);
    System.out.println("Connection port =>" + connectionPort);

    if (connectionStatus)
    {
        dataServerHost = connectionHost;
        dataServerPort = connectionPort;
        dataServerAuthKey = connectionAuthKey;
        System.out.println("Connecting to data server @: " + dataServerHost + ":" + dataServerPort);
        connectToDataServer();
    }
}

Answer 1

I recall that spurious selector wakeup is possible.

While it's funny that there's nothing to read when you are just told there's something to read, it is usually not a problem for programs. A program typically should expect arbitrary number of bytes when reading a TCP stream; and the case of 0 byte usually doesn't need special handling.

Your program is theoretically wrong. You cannot expect that you can read the entire message at once. One read may return only part of it. Could be just 1 byte. There's no guarantee.

The "righteous" way is to accumulate all bytes read in a buffer. Look for EOT in the buffer. If message is fragmented, several reads may be needed before a whole message arrives.

loop 
  select();
  if readable
     bytes = read()
     buffer.append(bytes)
     while( buffer has EOT at position i)
       msg = buffer[0-i]
       left shift buffer by i

You can see in this flow, it doesn't matter if read() reads 0 bytes. And it is really not about NIO. Even in traditional blocking TCP IO, this strategy must be done to be theoretically correct.

But, practically, if you do observe that the whole message always come in one piece, you don't need such complication; your original code is practically correct in your environment.

Now you observed that sometimes 0 byte is read. Then your previous practical assumption must be revised. You may add some special branch to ignore the 0 byte chunk.