Question

Given an instance x of Callable<T>, how can I run x in a separate process such that I can redirect the standard input and output of the process? For example, is there a way to build a Process from a Callable? Is there a standard Executor that gives control over input and output?

[UPDATE] It’s not important that the Callable execute in a new process as opposed to a new thread. What I want is to put the Callable instance in a ‘harness’, so that I can control its stdin/stdout. AFAIK, this calls for a new process.

Answer 1

More generally:

Given an instance x of Callable utilizing global variables A and B, how can I run x concurrently such that x sees custom values for A and B, rather than the ‘original’ values of A and B?

And the best answer is, don’t use global variables. Dependency inject things like that. Extend Callable and add methods setStdIn, setStdOut (and setStdErr if you need it).

I know this isn’t the answer you’re looking for, but the solutions I have seen for this all require a new process, and the only way you are getting that Callable into a new process is to change the code of the Callable so it is serializable, or provides a class name, or some other hack, so instead of making changes that will give you a nasty, brittle solution, just do it right*

* ‘right’ being to use the widely accepted pattern of dependency injection to provide loose coupling. YMMV

UPDATE: In response to comment 1. Here is your new interface:

import java.io.InputStream; import java.io.PrintStream; import java.util.concurrent.Callable;   public interface MyCallable<V> extends Callable<V> {   void setStdIn(InputStream in);   void setStdOut(PrintStream out);   } 

and your tasks would look like:

import java.io.InputStream; import java.io.PrintStream;   public class CallableTask implements MyCallable<Object> {      private InputStream in = System.in;     private PrintStream out = System.out;      public void setStdIn(InputStream in) {         this.in = in;     }      public void setStdOut(PrintStream out) {         this.out = out;     }      public Object call() throws Exception {         out.write(in.read());         return null;     } 

}

No need for a process. Any solution (even one using processes) is almost certainly going to require code changes to the Callables in some way. This is the simplest (just replace System.out with this.out).