JTextArea in other JFrame display realtime console output

Question

I have a "ConsoleFrame" which should display my console output in real-time to a JTextArea.

I redirected the output streams:

private void redirectSystemStreams() {
    OutputStream out = new OutputStream() {
        @Override
        public void write(int b) throws IOException {
            updateTextArea(String.valueOf((char) b));
        }

        @Override
        public void write(byte[] b, int off, int len) throws IOException {
            updateTextArea(new String(b, off, len));
        }

        @Override
        public void write(byte[] b) throws IOException {
            write(b, 0, b.length);
        }
    };

    System.setOut(new PrintStream(out, true));
    System.setErr(new PrintStream(out, true));
}

and call the SwingUtilities.invokeAndWait method to append the new text, which works fine

private void updateTextArea(final String text) {
    try {
        SwingUtilities.invokeAndWait(new Runnable() {
            @Override
            public void run() {
                txt_console.append(text);
            }
        });
    } catch (InterruptedException ex) {
    } catch (InvocationTargetException ex) {
    }
}

but it shows me in my new ConsoleFrame this error: java.lang.Error: Cannot call invokeAndWait from the event dispatcher thread and I get that because of the EDT - but why does it work and how can I adapt my code to make it work properly?

Answer 1

• invokeAndWait must be called out of EDT, otherwise caused am exceptions,

• carefully with invokeAndWait , because can freeze whole Swing GUI, locked by exceptions from RepaintManager (not in all cases only GUI is created, relayout, refreshed some of methods), then applications required restart,

• for invokeAndWait is required to test if (EventQueue.isDispatchThread()) { / if (SwingUtilities.isEventDispatchThread()) { on true you can to setText("") / append("" ) without any side effects, output is done on EDT, but is about good practicies to wrap inside invokeLater

• use SwingWorker , there are implemented methods process , publish , setProcess and done , all mentioned methods notified EDT by default ,

• SwingWorker is designated to run only once time, for repeatly (on some period) to use Executor for SwingWorker or Runnable#Thread as most simple, clear and without any side issues, effects

Answer 2

You can use SwingUtilities.invokeLater() from any thread, and for error stream it is likely best to use it even when you are already in EDT:

private void updateTextArea(final String text) {
    SwingUtilities.invokeLater(new Runnable() {
        @Override
        public void run() {
            txt_console.append(text);
        }
    });
}

That particular error you get comes from outside the EDT, and then invokeAndWait() may be called so you get the output to your console.