Feb 22

We’ve all got code riddled with System.out.println() messages. We should use a logging framework, but often we’re in a rush and we end up writing to stdout because its simpler at the time.

If there are too many System.out.println() calls and we want to stop them going to the console then we can redirect stdout and stderr to a different stream using the setOut() and setErr() methods provided by the System class.

If we want to instead have them written to a Swing text component such as a JTextArea or JTextPane then what we need is an OutputStream subclass that will write to the text component.

The following code shows how that can achieved, resulting in System.out.println() call writing to your designated JTextComponent.

import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintStream;

import javax.swing.SwingUtilities;
import javax.swing.text.BadLocationException;
import javax.swing.text.Document;
import javax.swing.text.JTextComponent;

public class RedirectOutput {

	private JTextComponent textComponent;
	public static void sendTo(JTextComponent textComponent) {
		new RedirectOutput(textComponent).redirectSystemStreams();
	private RedirectOutput(JTextComponent textComponent) {
		this.textComponent = textComponent;
	private void updateTextComponent(final String text) {
		SwingUtilities.invokeLater(new Runnable() {
			public void run() {
				Document doc = textComponent.getDocument();
				try {
					doc.insertString(doc.getLength(), text, null);
				} catch (BadLocationException e) {
					throw new RuntimeException(e);
				textComponent.setCaretPosition(doc.getLength() - 1);

	private void redirectSystemStreams() {
		OutputStream out = new OutputStream() {
			public void write(final int b) throws IOException {
				updateTextComponent(String.valueOf((char) b));

			public void write(byte[] b, int off, int len) throws IOException {
				updateTextComponent(new String(b, off, len));

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

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


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One Response to “Redirect system out to text component”

  1. Boris Says:

    Excellent and clear solution, just need to add JFrame class which will show this OutputStream.
    Thanks a lot!

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