Removing access to System.out in java

Question

I maintain an application which acts as a container for multiple individual programs. These programs have their own dedicated logging facility, i.e. everything they log does to a special log file.

Nevertheless, application developers seem to love to throw System.out.println and e.printStackTrace calls all over, making it impossible to maintain a clean console when running the container.

How can I prevent these applications from polluting System.out and System.err?

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Implementation notes:

• the applications use Log4j for logging;
• the container also uses the console for logging, but it is strictly reserved for lifecycle events and problems, so I still need the console;
• the applications are loaded using a custom classloader, but no security checks are applied.

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Update:

Simply redirecting System.out would not work since it redirects all output, so something like this fails:

    System.setOut(new PrintStream(new OutputStream() {

        @Override
        public void write(int b) {

            throw new Error("Not on my watch you don't");

        }
    }));

    Logger logger = Logger.getLogger(Runner.class);
    logger.info("My log message");

This should succeed.

Update 2:

The applications are loaded and configured using code similar to

App app = new UrlClassLoader(...).loadClass(className)).newInstance();
app.setLogger(loggerForClass(app));

Log4j is loaded from the system class loader.

Answer 1

Assuming that you can control your containers output you can do the following:

import java.io.*;
public class SysOut {
    public static void main(String[] args) throws Exception {
            PrintStream pw = new PrintStream(new FileOutputStream("a.txt"));
            PrintStream realout = System.out;
            System.setOut(pw);
            System.out.println("junk");
            realout.print("useful");
}
}

$ java SysOut 
useful
$ cat a.txt 
junk