Question

I have just started learning java. I modified the client side code for a server/client communication program, by creating two threads for the client side, main thread for receiving user’s input, and inputThread for receiving server’s response. I am sure that server has sent the response to client, however, no response message is obtain at client.

Here is my code. Can anyone help me to figure it out? Thanks

package clientnio;

import java.net.*; 
import java.nio.*; 
import java.io.*;
import java.nio.channels.*; 
import java.util.Scanner; 

public class ClientNIO {
public static int bufferLen = 50;
public static SocketChannel client;
public static ByteBuffer writeBuffer;
public static ByteBuffer readBuffer;

public static void main(String[] args) { 
    writeBuffer = ByteBuffer.allocate(bufferLen);
    readBuffer = ByteBuffer.allocate(bufferLen);

    try { 
        SocketAddress address = new InetSocketAddress("localhost",5505); 
        System.out.println("Local address: "+ address);
        client=SocketChannel.open(address);
        client.configureBlocking(false); 

        //readBuffer.flip();
        new inputThread(readBuffer);

        /*
        String a="asdasdasdasddffasfas"; 
        writeBuffer.put(a.getBytes()); 
        writeBuffer.clear();  
        int d=client.write(writeBuffer);
        writeBuffer.flip();
        */

        while (true) {
            InputStream inStream = System.in;
            Scanner scan = new Scanner(inStream);
            if (scan.hasNext()==true) {
                String inputLine = scan.nextLine();
                writeBuffer.put(inputLine.getBytes()); 
                //writeBuffer.clear();
                System.out.println(writeBuffer.remaining());
                client.write(writeBuffer);
                System.out.println("Sending data: "+new String(writeBuffer.array()));
                writeBuffer.flip(); 
                Thread.sleep(300);
            }
        }                 
    } 
    catch(Exception e) { 
        System.out.println(e);
    } 

}
}

class inputThread extends Thread {
private ByteBuffer readBuffer;
public inputThread(ByteBuffer readBuffer1) {  
    System.out.println("Receiving thread starts.");
    this.readBuffer = readBuffer1;
    start();
}

@Override
public void run() {
    try {
        while (true) {
            readBuffer.flip();
            int i=ClientNIO.client.read(readBuffer); 
            if(i>0) { 
                byte[] b=readBuffer.array(); 
                System.out.println("Receiving data: "+new String(b)); 
                    //client.close(); 
                    //System.out.println("Connection closed."); 
                    //break; 
            }
            Thread.sleep(100); 
        }
    }
    catch (Exception e) {
       System.out.println(e);
    }
  }
}

Answer 1

Disclaimer: I’m not an active user of Java. (I only used it in school.)

---

Advice: I think it will greatly simplify the debugging process if you use blocking mode, at least until your code example is working correctly. (Currently your code does not seem to benefit from the non-blocking mode.)

---

I have identified two issues, culminating into four possible lines of code that may require changing:

1. When a ByteBuffer allocates its backing array, it sets itself ready to write by setting position to zero and limit to the capacity of that array. Your two uses of ByteBuffer.flip() (in the writing loop and the reading loop respectively) seem to be contrary to the convention.
2. Calling the ByteBuffer.array() method always returns the whole backing array, thus it always has size bufferLen. Because of this, a String constructed from the full-size array may contain junk from a previous transmission.
   • Typically, the array needs to be trimmed to the transmission size, and the conversion between a String and a byte array must use the same encoding as the server.

---

My suggested changes for first issue:
(Note: I don’t know how to fix the array trimming and encoding issue.)

writeBuffer.put(inputLine.getBytes()); 
writeBuffer.flip();                       // <--here
client.write(writeBuffer);
...
writeBuffer.clear();                      // <-- should be clear() instead of flip()
Thread.sleep(300);

---

// readBuffer.flip();                      // <-- remove this line
int i=ClientNIO.client.read(readBuffer); 
if(i>0) { 
    readBuffer.flip();                     // <-- move it here
    byte[] b=readBuffer.array(); 
    System.out.println("Receiving data: "+new String(b)); 
    ...
}

---

References

• http://docs.oracle.com/javase/1.4.2/docs/api/java/nio/ByteBuffer.html
• http://docs.oracle.com/javase/1.4.2/docs/api/java/nio/channels/SocketChannel.html
• Socketchannel always null
• http://www.exampledepot.com/egs/java.nio.charset/ConvertChar.html