Logback+Swing in small tool

Question

I need to hack up a small tool. It should read a couple of files and convert them. Right now that works in my IDE. For the user, I'd like to add a small UI which simply shows the log output.

Do you know of a ready-to-use Swing appender for logback? Or something which redirects System.out to a little UI with nothing more than a text field and a "Close" button?

PS: I'm not looking for Chainsaw or Jigsaw or Lilith. I want the display of the log messages in the application, please.

Answer 1

No warranty, but here's a sample that I just wrote:

/**
 * A Logback appender that appends messages to a {@link JTextArea}.
 * @author David Tombs
 */
public class JTextAreaAppender extends AppenderBase<ILoggingEvent>
{
    private final JTextArea fTextArea;
    private final PatternLayout fPatternLayout;

    public JTextAreaAppender(final Context loggerContext, final JTextArea textArea)
    {
        fTextArea = textArea;

        // Log the date, level, class name (no package), and the message.
        fPatternLayout = new PatternLayout();
        fPatternLayout.setPattern("%d{HH:mm:ss.SSS} %-5level - %msg");
        fPatternLayout.setContext(loggerContext);
        fPatternLayout.start();

        // Make sure not to call any subclass methods right now.
        super.setContext(loggerContext);
    }

    @Override
    protected void append(final ILoggingEvent eventObject)
    {
        // Actual appending must be done from the EDT.
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run()
            {
                final String logStr = fPatternLayout.doLayout(eventObject);

                // If the text area already has lines in it, append a newline first.
                if (fTextArea.getDocument().getLength() > 0)
                {
                    fTextArea.append("\n" + logStr);
                }
                else
                {
                    fTextArea.setText(logStr);
                }
            }
        });
    }    
}

Answer 2

You need to write a custom appender class like so:

public class MyConsoleAppender extends AppenderBase<ILoggingEvent> {
  private Encoder<ILoggingEvent> encoder = new EchoEncoder<ILoggingEvent>();
  private ByteArrayOutputStream  out     = new ByteArrayOutputStream();

  public MyConsoleAppender() {
     LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory();
     setContext(lc);
     start();
     lc.getLogger("ROOT").addAppender(this);
  }

  @Override
  public void start() {
     try {
        encoder.init(out);
     } catch (IOException e) {}
     super.start();
  }

  @Override
  public void append(ILoggingEvent event) {
     try {
        encoder.doEncode(event);
        out.flush();
        String line = out.toString(); // TODO: append _line_ to your JTextPane
        out.reset();
     } catch (IOException e) {}
  }
}

You can replace the EchoEncoder with a PatternLayoutEncoder (see CountingConsoleAppender example in the logback examples folder).

The encoder will write each event to a byte buffer, which you can then extract a string and write this to your JTextPane or JTextArea, or whatever you want.

Answer 3

I often rely on JTextArea#append(), as suggested in this example. Unlike most of Swing, the method happens to be thread safe.

Addendum: Console is a related example that redirects System.out and System.err to a JTextArea.