Communication via standard input / output in Java

Question

I am trying to send messages between two JVMs: a server starts a second process. The second process is then to send a message to the server which prints the message to the console. The code is as follows:

public class Server
{
    public static void main(String[] args)
    {
        Process client=null;
        BufferedReader clientInput=null;

        try
        {
            client=Runtime.getRuntime().exec("java Client");
            clientInput=new BufferedReader(new InputStreamReader(client.getInputStream()));
        }
        catch(IOException e){}

        System.out.println("Waiting for the client to connect...");

        try
        {
            String msg=clientInput.readLine();
            System.out.println(msg);
        }
        catch(IOException e){}

        client.destroy();
    }
}

and

public class Client 
{
    public static void main(String[] args) 
    {   
        BufferedWriter out = new BufferedWriter( new OutputStreamWriter(System.out));

        try 
        {
            out.write("Ready\n");
            out.flush();
        } 
        catch (Exception e){}
    }
}

If I run this, I get as output from the server null. In the end, the communication should be both ways. Any help greatly appreciated.

EDIT: I don't get any errors (just removed the print statements from the catch blocks to save space).

Alberto Gutierrez · Accepted Answer · 2012-02-15 18:23:44Z

1

I think you need to add a while loop:

while ((s = in.readLine()) != null && s.length() != 0)
    System.out.println(s);
}

answered Feb 15, 2012 at 18:23

Alberto Gutierrez

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Comments

Daniel · Accepted Answer · 2012-02-16 08:16:59Z

1

You receive null at the end of the stream. The client correctly starts, sends Ready, and the the ends, so the stream ends.

Totally correct behaviour. If the client would end itself (but instead do something else like reading server messages on stdin), the server would never receive a null.

Edit: NEVER EVER (!!!!!) do this:

catch(IOException e){}

At least write:

catch(IOException e){ e.printStackTrace() }

This will show you your error!

In my company, this is one of the elementary rules of code style!

edited Feb 16, 2012 at 8:16

answered Feb 15, 2012 at 16:39

Daniel

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2 Comments

coderino Over a year ago

The problem is that all I receive is null - the "Ready" is never received though.

coderino Over a year ago

I have the e.printStackTrace() in my code. As I said above, I just removed it so not to clutter up too much space. As I said, no exceptions are thrown.

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