java - Copy STDOUT to file without stopping it showing onscreen

Question

The program I am making is designed to be run unattended, because of this I have redirected the stdout and stderr streams to a log file. While this works without any problems, while I am still making and debugging the software I would like it to show on the screen as well. Is this possible?

To redirect the streams I have used

System.setErr(logWriter);
System.setOut(logWriter);

Thanks.

Answer 1

Yes. Logging frameworks (i.e. log4j) are best, isDebugMode is handy in a development environment but if you really need to "tee" your stdout you can like so.

import java.io.PrintStream;
import java.io.File;
public class TeeStream extends PrintStream {
    PrintStream out;
    public TeeStream(PrintStream out1, PrintStream out2) {
        super(out1);
        this.out = out2;
    }
    public void write(byte buf[], int off, int len) {
        try {
            super.write(buf, off, len);
            out.write(buf, off, len);
        } catch (Exception e) {
        }
    }
    public void flush() {
        super.flush();
        out.flush();
    }
}

http://www.exampledepot.com/egs/java.lang/Redirect.html

// import java.io.FileOutputStream;
String dateString = new SimpleDateFormat("yyyyMMdd").format(new Date());
File logFile = new File("mylogfile_" + dateString +".log");
PrintStream logOut = new PrintStream(new FileOutputStream(logFile, true));

PrintStream teeStdOut = new TeeStream(System.out, logOut);
PrintStream teeStdErr = new TeeStream(System.err, logOut);

System.setOut(teeStdOut);
System.setErr(teeStdErr);

You'll soon find LOG.somelevel(msg) much more manageable than System.out.println(msg). It is great to raise the log level when things are working well and lower the level when they aren't without deploying a debug build.