Java execute multiple console commands on loaded program

Question

I want to load a program from the console and then execute commands on it later on. The loading works fine, but how can I execute commands on the loaded program without starting it every time I want to use it?

For example: ./randomapp command1 command2

But on Java, I have to usw ./randomapp command1 every time I want to execute something on it, so the program doesn't stay loaded.

Answer 1

Not knowing anything about your Java program or the app you'd like to execute and feed with commands, this is a shot in the dark. Also, all error handling is omitted. I'm using tee for the program to be run because it's convenient.

public class Command {
    private static OutputStream os;
    private static OutputStreamWriter osw;
    private static Process process;
    public static void start(){
        try { 
            ProcessBuilder pb = new ProcessBuilder( "/usr/bin/tee", "x.dat" );
            process = pb.start();
            os = process.getOutputStream();
            osw = new OutputStreamWriter( os );
        } catch( Exception e ){
            // error handling
        } catch( Error e ){
            // error handling
        }
    }

    public static void terminate() throws Exception {
        process.waitFor();
    }

    public static void command( String str ) throws Exception {
        String cmd = str + System.lineSeparator();
        osw.write( cmd, 0, cmd.length() );
        osw.flush();
    }

I've tested this using:

Command.start();
Command.command( "line 1" );
Thread.sleep( 2000 );
Command.command( "line 2" );
Thread.sleep( 2000 );
Command.command( "line 3" );
Command.terminate();

The lines can be found on x.dat.