Detecting write to standard error stream

Question

Is there no way (regardless of how "hacky" it is) to detect when Java's System.err has been written to in order to be able to execute logic if and when this happens? — I'm currently working with a custom subclass of Thread (let's call it SwallowingThread) which swallows a number of exceptions in its implementation of Thread.run() in a manner similar to the following code:

public final class SwallowingThread extends Thread {

    ...

    @Override
    public void run() {
        ServerSocket socket = new ServerSocket(80, 100);
        try {
            Socket connected = socket.accept();
            // Do stuff with socket here
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

In my code, however, I want to be able to handle cases of UnknownHostException and IOException which occur while using an instance of SwallowingThread; Is there no way to detect that this catching and printing to standard error after it has occured? — I had originally tried writing a UncaughtExceptionHandler to do this only to find out that it isn't catching the exceptions because they were swallowed rather than simply being e.g. wrapped in a RuntimeException and thrown onward.

Of course, a "better" way of solving this problem is to re-write the logic for the class, but is there no quick-and-dirty way of solving this issue without having to touch SwallowingThread?

You could create a `PrintStream` that wraps `System.err` maybe? `System.setErr(new MyDelegatingPrintStream(System.err));` — Sean Bright, Jan 25 '17 at 16:56
Possible duplicate of [Redirect System.out and System.err](http://stackoverflow.com/questions/14715748/redirect-system-out-and-system-err) — Murat Karagöz, Jan 25 '17 at 16:59
@MuratK, no, I do not want to simply put the data which was entered into stderr into another place: I want to be able to execute some logic once/if this data is written to stderr. — errantlinguist, Jan 25 '17 at 17:03

Christoph Bimminger · Accepted Answer · 2017-01-25T17:12:08.923

You can implement your own class (I call it DelegatingErrorPrintStream) derived from PrintStream which notifies you if there's new output, and then delegates to System.err And now you can set YOUR DelegatingErrorPrintStream as output stream for System.err using System.setErr(err);

Full example including usage:

import java.io.FileNotFoundException;
import java.io.PrintStream;
import java.io.UnsupportedEncodingException;

public class ErrorNotifierExample {
private static final class ErrorDelegatingPrintStream extends PrintStream {
    public ErrorDelegatingPrintStream(PrintStream defaultErr)
            throws FileNotFoundException, UnsupportedEncodingException {
        super(defaultErr);
    }

    @Override
    public void print(boolean b) {
        super.print(b);
        notifyListener(b);
    }

    @Override
    public void print(char c) {
        super.print(c);
        notifyListener(c);
    }

    @Override
    public void print(int i) {
        super.print(i);
        notifyListener(i);
    }

    @Override
    public void print(long l) {
        super.print(l);
        notifyListener(l);
    }

    @Override
    public void print(float f) {
        super.print(f);
        notifyListener(f);
    }

    @Override
    public void print(double d) {
        super.print(d);
        notifyListener(d);
    }

    @Override
    public void print(char[] s) {
        super.print(s);
        notifyListener(s);
    }

    @Override
    public void print(String s) {
        super.print(s);
        notifyListener(s);

    }

    @Override
    public void print(Object obj) {
        super.print(obj);
        notifyListener(obj);

    }

    @Override
    public PrintStream append(CharSequence csq, int start, int end) {
        notifyListener(csq); // TODO will need some special handling
        return super.append(csq, start, end);
    }

    private void notifyListener(Object string) {
        // TODO implement your handling here. System.out printing is just an
        // example.
        System.out.println(String.valueOf(string));
    }
}

public static void main(String[] args) throws FileNotFoundException, UnsupportedEncodingException {
    ErrorDelegatingPrintStream errReplacement = new ErrorDelegatingPrintStream(System.err);
    System.setErr(errReplacement);

    System.err.println("TEST01");
    throw new RuntimeException("just a test output for ERROR handling");
}
}

All of these answers were excellent, but this was nearly exactly what I ended up doing (specifically by passing a `Consumer — errantlinguist, Jan 25 '17 at 17:29

Arnaud · Answer 2 · 2017-01-25T18:29:00.573

As several persons already suggested, you may go with a custom PrintStream.

It will replace the standard error stream but also encapsulate it and call it when needed.

Since the exceptions and stack traces are what you are interested in, overriding print(String) should be enough (println(String) already calls this method + newLine()).

The stream could look like that :

import java.io.PrintStream;

public class CustomPrintStream extends PrintStream {


    public CustomPrintStream(final PrintStream out) {

        super(out);

    }

    @Override
    public void print(final String s) {

        super.print(s);
        // some String is written to the error stream, Do something !

    }

}

You would use it that way, at the beginning of your application, just call

System.setErr(new CustomPrintStream(System.err));

Detecting write to standard error stream

2 Answers2