Java interprocess communication

[krats ram]

I am trying to write some code for twp java programs to communicate. Below are the two programs

    

import java.io.*;
import java.util.*;

public class Caller {

	public static void main(String [] args) throws Exception {
		
		List<String> cmd = new ArrayList<String>();
		cmd.add("java");
		cmd.add("Callee");
		
		ProcessBuilder pb = new ProcessBuilder(cmd);
		Process proc = pb.start();
		OutputStream os = proc.getOutputStream();
		InputStream is = proc.getInputStream();
		
		PrintWriter pw = new PrintWriter(new OutputStreamWriter(os));
		BufferedReader br = new BufferedReader(new InputStreamReader(is));

		String str = null;
		do {
			System.out.println("Whats up Caller ! ");
			pw.println("Whats up Caller ! ");
			pw.flush();
			System.out.println(" ... " + str);
		} while ((str = br.readLine()) != null);
		
		pw.close();
		br.close();
	}
}

and the other program

import java.io.*;

public class Callee {

	private void run() throws Exception{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String line = null;
		while ((line = br.readLine()) != null) {
			System.out.println(line + " got your msg "); 
			System.out.flush();
			System.out.println(line + " got your msg "); 
			System.out.flush();
		}
	}
	
	public static void main(String [] args) throws Exception {
		new Callee().run();
	}

}

When I run the caller, the two programs run for a while with output

Whats up Caller !
... Whats up Caller ! got your msg


and then they block. Any suggestions?

[Steve Luke]

You should read and implement this article: When Runtime.exec() won't by Michael Daconta. The vast majority of problems found with Runtime.exec() and Processes in general are addressed their (the fact that he uses Runtime.exec() and you use ProcessBuilder is only significant in that the article was written long ago - everything still applies).

My guess is that your Callee's output buffer is filling. You send 1 line from the caller, the callee sends 2 lines back, and the caller consumers just 1 leaving the other line in the buffer waiting to be consumed. Eventually the buffer fills because you add to it twice as fast as you consume from it.

The article gives a nice class for consuming and reporting all the output from the Process.

[Ulf Dittmer]

Just starting up the child process, and then using sockets for communication would probably be less brittle.