Question

I’m having a little trouble with a simple Java server, client application.
Basically the topics says it all: when I do a writeUTF on the server side it only sends every 2nd time it’s being executed.
For an example:

Server:

public class Server {

    /**
     * @param args
     */
    public static void main(String[] args) {
        try {
            ServerSocket server     = new ServerSocket(7777);
            Socket client           = server.accept();
            DataOutputStream out    = new DataOutputStream(new BufferedOutputStream(client.getOutputStream()));
            DataInputStream in      = new DataInputStream(new BufferedInputStream(client.getInputStream()));
            while(true) {
                for(int i = 0; i < 100; i++) {
                    out.writeUTF("Test" + i);
                    out.flush();
                }
            }
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

And client:

public class Client {

    /**
     * @param args
     */
    public static void main(String[] args) {
        try {
            Socket client           = new Socket("localhost", 7777);
            DataInputStream in      = new DataInputStream(new BufferedInputStream(client.getInputStream()));
            DataOutputStream out    = new DataOutputStream(new BufferedOutputStream(client.getOutputStream()));

            while(in.readUTF() != null) {
                System.out.println(in.readUTF());
            }
        } catch (UnknownHostException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

And the output in the console looks like this:

Test1
Test3
Test5
Test7
Test9
Test11

What is causing this behavior?

Answer 1

It’s because you’re discarding data on your client.

When you check while(in.readUTF() != null), you’re reading an entry from the stream every time to see if it’s non-null. Then you discard this value, and read a new one within the loop. (As well as losing values, this has the bug that with an odd number of total values, the while condition will evaluate to true but the next call to readUTF() would return null within the loop.)

To fix this, you should read the value to a variable, and test this before using it – something like the following:

String value;
while((value = in.readUTF()) != null) {
     System.out.println(value);
}

If you don’t like the assignment and nullity check as a single expression, you can instead assign value = in.readUTF() initially and at the end of every loop, but personally I find this more error-prone.