How could I read Java Console Output into a String buffer

Ok, this was a fun problem. Dosen't seem to be an elegant way of solving it for all PrintStream methods at once. (Unfortunately there is no FilterPrintStream.)

I did write up an ugly reflection-based workaround though (not to be used in production code I suppose :)

class LoggedPrintStream extends PrintStream {

    final StringBuilder buf;
    final PrintStream underlying;

    LoggedPrintStream(StringBuilder sb, OutputStream os, PrintStream ul) {
        super(os);
        this.buf = sb;
        this.underlying = ul;
    }

    public static LoggedPrintStream create(PrintStream toLog) {
        try {
            final StringBuilder sb = new StringBuilder();
            Field f = FilterOutputStream.class.getDeclaredField("out");
            f.setAccessible(true);
            OutputStream psout = (OutputStream) f.get(toLog);
            return new LoggedPrintStream(sb, new FilterOutputStream(psout) {
                public void write(int b) throws IOException {
                    super.write(b);
                    sb.append((char) b);
                }
            }, toLog);
        } catch (NoSuchFieldException shouldNotHappen) {
        } catch (IllegalArgumentException shouldNotHappen) {
        } catch (IllegalAccessException shouldNotHappen) {
        }
        return null;
    }
}

...that can be used like this:

public class Test {
    public static void main(String[] args) {

        // Create logged PrintStreams
        LoggedPrintStream lpsOut = LoggedPrintStream.create(System.out);
        LoggedPrintStream lpsErr = LoggedPrintStream.create(System.err);

        // Set them to stdout / stderr
        System.setOut(lpsOut);
        System.setErr(lpsErr);

        // Print some stuff
        System.out.print("hello ");
        System.out.println(5);
        System.out.flush();

        System.err.println("Some error");
        System.err.flush();

        // Restore System.out / System.err
        System.setOut(lpsOut.underlying);
        System.setErr(lpsErr.underlying);

        // Print the logged output
        System.out.println("----- Log for System.out: -----\n" + lpsOut.buf);
        System.out.println("----- Log for System.err: -----\n" + lpsErr.buf);
    }
}

Resulting output:

hello 5
Some error
----- Log for System.out: -----
hello 5

----- Log for System.err: -----
Some error

(Note though, that the out field in FilterOutputStream is protected and documented, so it is part of the API :-)

You can use PipedInputStream and PipedOutputStream.

//create pairs of Piped input and output streasm for std out and std err
final PipedInputStream outPipedInputStream = new PipedInputStream();
final PrintStream outPrintStream = new PrintStream(new PipedOutputStream(
    outPipedInputStream));
final BufferedReader outReader = new BufferedReader(
    new InputStreamReader(outPipedInputStream));
final PipedInputStream errPipedInputStream = new PipedInputStream();
final PrintStream errPrintStream = new PrintStream(new PipedOutputStream(
    errPipedInputStream));
final BufferedReader errReader = new BufferedReader(
    new InputStreamReader(errPipedInputStream));
final PrintStream originalOutStream = System.out;
final PrintStream originalErrStream = System.err;
final Thread writingThread = new Thread(new Runnable() {
    @Override
    public void run() {
        try {
            System.setOut(outPrintStream);
            System.setErr(errPrintStream);
            // You could also set the System.in here using a
            // PipedInputStream
            DoSomething();
            // Even better would be to refactor DoSomething to accept
            // PrintStream objects as parameters to replace all uses of
            // System.out and System.err. DoSomething could also have 
            // an overload with DoSomething() calling: 
            DoSomething(outPrintStream, errPrintStream);
        } finally {
            // may also want to add a catch for exceptions but it is
            // essential to restore the original System output and error
            // streams since it can be very confusing to not be able to
            // find System.out output on your console
            System.setOut(originalOutStream);
            System.setErr(originalErrStream);
            //You must close the streams which will auto flush them
            outPrintStream.close();
            errPrintStream.close();
        }
    } // end run()
}); // end writing thread
//Start the code that will write into streams
writingThread.start();
String line;
final List<String> completeOutputStreamContent = new ArrayList<String>();
while ((line = outReader.readLine()) != null) {
    completeOutputStreamContent.add(line);
} // end reading output stream
final List<String> completeErrorStreamContent = new ArrayList<String>();
while ((line = errReader.readLine()) != null) {
    completeErrorStreamContent.add(line);
} // end reading output stream

You can't do that once the program is finished running. You need to do it before the program starts to write output.

See this article^{(archive.org)} for details on how to replace stdout and stderr. The core calls are System.setOut() and System.setErr().