BufferedReader readLine method hangs and block program

Question

I'm trying to learn sockets using Java and I sucessfully sent data to a ServerSocket running in my own machine. When I try to read from this socket using readline (so I can just echo my own sent message) my program hangs and won't return.

Here's the code:

public static void main(String[] args) throws UnknownHostException, IOException {

    TCPClient cli = new TCPClient("127.0.0.1", "15000");
    try {
        cli.ostream.writeUTF("Teste");
        String echo = cli.istream.readLine(); //it hangs in this line
        System.out.println(echo);
    }

TCPClient is a class I defined so I can test my program on a simpler interface before using swing on my homwework. here's the code:

public class TCPClient {

public DataOutputStream ostream = null;
public BufferedReader istream = null;

public TCPClient(String host, String port) throws UnknownHostException {
    InetAddress ip = InetAddress.getByName(host);

    try {
        Socket socket = new Socket(host, Integer.parseInt(port));

        ostream = new DataOutputStream(socket.getOutputStream());
        istream = new BufferedReader(new InputStreamReader(socket.getInputStream()));


    } catch (IOException ex) {
        Logger.getLogger(TCPClient.class.getName()).log(Level.SEVERE, null, ex);
    }

}

My server is pretty simple. After the connection is estabilished, it enters in this loop and stays here until I close the client (because of the infinite loop). Afterwards, some exception handling returns it to the point before the connection started.

            while(true){
                String msg = istream.readLine();
                System.out.println("Arrived on server: " + msg); //just works on debug
                ostream.writeUTF("ACK: " + msg);
                ostream.flush();
            }

I don't see what am I missing.

PS: the wierd stuff is that if I debug the server, I can see the message arriving there (I can print it, for example), but this isn't possible if I just run this code. Does this have some concurrency relation I'm overlooking?

thx

What does the server do? If it doesn't answer anything to your message, or doesn't flush its write, then of course the client will wait indefinitely until a line is sent from the server — JB Nizet, Oct 21 '11 at 22:11
I'll edit the post with the server's infinite loop while echoing messages — Thiago Moraes, Oct 21 '11 at 22:13

score 11 · Accepted Answer · answered Oct 21 '11 at 22:18

11

The problem is that readLine tries reading a line. It will not return the line until it's sure that the end of line has been reached. This means that it expects either a newline character, or the end of the communication. Since the server doesn't send any newline char and doesn't close the stream, the client waits indefinitely.

answered Oct 21 '11 at 22:18

JB Nizet

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That's it! Putting a '\n' in ostream.writeUTF("ACK: " + msg); worked. The only doubt now is that 2 non-printable characters appeared on my client. What can they be? – Thiago Moraes Oct 21 '11 at 22:22
1

You write in UTF at one side, and read using the platform's default encoding on the other side. Always use a Writer to write characters. Always wrap the output stream with an OutputStreamWriter which specifies a specific, explicit encoding. Do the same to read : always use a Reader, and wrap the input stream with an InputStreamReader which specifies the same specific encoding. – JB Nizet Oct 21 '11 at 22:25
@Thiago Moraes They are the length word prefix put there by writeUTF() as documented in the Javadoc. See also my answer. – user207421 Oct 21 '11 at 22:26
Now it works properly. I used OutputStreamWriter and BufferedReader. But I still have to send "\n" from the client and from the server so that everything can work. On the brightside, I didn't have problems with encoding anymore. – Thiago Moraes Oct 21 '11 at 22:42
@ThiagoMoraes see my answer. It's all in there. Curious that you marked this one as correct really. – user207421 Oct 21 '11 at 22:53

score 1 · Answer 2 · edited Oct 21 '11 at 22:20

1

cli.ostream.writeUTF("Teste");

Shouldn't this be containing a new line? Otherwise read method will be waiting for new line I think.

Also as suggested you can try flushing the ostream after writing to it.

edited Oct 21 '11 at 22:20

Mob

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answered Oct 21 '11 at 22:15

Ashwinee K Jha

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ostream is the output stream. The OP said that the server receives this message correctly. The problem is that the client doesn't receive the answer from the server. – JB Nizet Oct 21 '11 at 22:17
I flushed ostream and added a '\n' to my string now, I wasn't doing this before. But the problem continues the same=/ – Thiago Moraes Oct 21 '11 at 22:19

user207421 · Answer 3 · 2011-10-21T22:29:45.770

1

writeUTF() doesn't write a line. See the Javadoc. writeUTF() is for use with readUTF(). And Readers are for use with Writers. So change the DataOutputStream to a BufferedWriter and call write() and then newline().

edited Oct 21 '11 at 22:29

answered Oct 21 '11 at 22:17

user207421

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I believe I understand what you said, but consider that I'm not a native english speaker so I need Unicode support. What would be the combination of classes that would be recommended in my case? – Thiago Moraes Oct 21 '11 at 22:28
@ThiagoMoraes I had a typo. Use Readers and Writers and specify the charset. – user207421 Oct 21 '11 at 22:30

BufferedReader readLine method hangs and block program

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