SocketChannel read returns 0 in blocking mode

Question

I have simple client and server, the client is based on NIO where as the server is a simple old style program.

I am using the client in its default mode which is blocking. In the program i try to write from the client side, server reads it. Then server replies and the client reads it.

I am able to write into the server with no issues, but the read from the Server in the client is proving to be problematic. As it is in blocking mode, i expect that it never returns 0 according to the documentation. But thats not what is happening, i always see the return from client_channel.read as 0.

*******************************SERVER*******************************************

class MyBlockingServer extends Thread
{
    private int M_PortNumber;
    private ServerSocket M_ServerSocket;

    MyBlockingServer(int PortNumber)
    {
        M_PortNumber = PortNumber;
        try {
            M_ServerSocket = new ServerSocket(M_PortNumber);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public void run()
    {
        int my_number = 0;
        while(true)
        {
            try {
                Socket ClientServerTuple = M_ServerSocket.accept();
                //System.out.println("Server address is "+ ClientServerTuple.getLocalAddress() + "Server Port is " + ClientServerTuple.getLocalPort());
                //System.out.println("Client address is " + ClientServerTuple.getRemoteSocketAddress() + "Client address is" + ClientServerTuple.getPort());


                DataInputStream inputStream = new DataInputStream(ClientServerTuple.getInputStream());

                byte b[] = new byte[48];
                inputStream.read(b);


                System.out.println("[SERVER]" + new String(b));


                DataOutputStream outputStream = new DataOutputStream(ClientServerTuple.getOutputStream());


                byte c[] = new byte[100];
                String output= new String("Thanks for connection, you suck tata" + " "+ my_number);

                c = output.getBytes();
                outputStream.write(c);

                my_number++;
                System.out.println("write done");

                ClientServerTuple.close();

            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

    void socket_close()
    {
        try {
            M_ServerSocket.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

public class JavaBlocking
{

    public static void main(String []args)
    {
        MyBlockingServer Server = new MyBlockingServer(8000);
        try {
            Server.start();
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }
}

*******************************CLIENT*******************************************

public class JavaChannels 
{

    public static void main(String []args)
    {
        SocketChannel client_channel = null;


        try {
            client_channel = SocketChannel.open();

        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        System.out.println("[Async Client] Socket channel open");

        try {
            client_channel.connect(new InetSocketAddress("127.0.0.1",8000));

        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        System.out.println("[Async Client] Socket channel connected");

        ByteBuffer my_buffer = ByteBuffer.allocate(248);



        try {
            my_buffer.put("seven77".getBytes("UTF-8"));
        } catch (UnsupportedEncodingException e2) {
            // TODO Auto-generated catch block
            e2.printStackTrace();
        }

        my_buffer.flip();

        try {
            int bytes_written = client_channel.write(my_buffer);

            while(my_buffer.hasRemaining())
            {
                bytes_written = client_channel.write(my_buffer);
            }

            System.out.println("[Async Client] Wrote "+ bytes_written +" bytes");
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        System.out.println("[Async Client] Socket channel write finished");

        my_buffer.clear();
        my_buffer.flip();


        try {
            int read_length = client_channel.read(my_buffer);
            System.out.println("Initial read is " + read_length + " bytes");
            while(read_length !=-1)
            {
                read_length = client_channel.read(my_buffer);
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                System.out.println("Reading the buffer." +"Read "+read_length +"bytes");
            }
        } catch (IOException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        }

        System.out.println("[Async Client] server says" + new String(my_buffer.array()));

        try {
            client_channel.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }


}

The output i am seeing from the client side is as follows

Initial read is 0 bytes
Reading the buffer.Read 0bytes
Reading the buffer.Read 0bytes
Reading the buffer.Read 0bytes

Answer 1

I think that this is wrong:

    System.out.println("[Async Client] Socket channel write finished");
    my_buffer.clear();
    my_buffer.flip();

The clear prepares the buffer for reading by setting the position to zero and the limit to the capacity.

But the flip then sets the limit to the position; ie zero. That means than when you attempt to read into the buffer, there is space for zero bytes.

Get rid of that flip call.

---

As it is in blocking mode, i expect that it never returns 0 according to the documentation.

Which documentation? The javadocs for SocketChannel.read(ByteBuffer) says:

"It is guaranteed, however, that if a channel is in blocking mode and there is at least one byte remaining in the buffer then this method will block until at least one byte is read. "

In this case, the highlighted condition is false.