Swing HTMLEditorKit / JEditorPane doesn't handle
and empty lines correctly

Question

When a JEditorPane backed by an HTMLEditorKit contains a <br> tag followed by an empty line, that line is not rendered correctly and the caret is not handled correctly. Consider this sample code:

import java.awt.*;
import java.io.*;

import javax.swing.*;
import javax.swing.text.*;
import javax.swing.text.html.*;

public class HTMLEditorTest {

    public static void main(String[] args) throws IOException, BadLocationException {
        JFrame frame = new JFrame();

        Reader stringReader = new StringReader("test<br><p>a");
        HTMLEditorKit htmlKit = new HTMLEditorKit();
        HTMLDocument htmlDoc = (HTMLDocument) htmlKit.createDefaultDocument();
        htmlKit.read(stringReader, htmlDoc, 0);
        JEditorPane editorPane = new JEditorPane();
        editorPane.setEditorKit(htmlKit);
        editorPane.setDocument(htmlDoc);

        frame.getContentPane().add(BorderLayout.CENTER, new JScrollPane(editorPane));
        frame.setBounds(100, 100, 500, 400);
        frame.setVisible(true);
    }
}

The empty line after the <br> tag is not rendered. When the caret is positioned left of the 'a' char and the arrow up key is pressed, the caret disappears:

Before pressing 'up':

After pressing 'up':

Note that the distance between 'test' and 'a' is too small, and the caret has disappeared.

When you then enter text, the missing empty line becomes visible:

The problem seems to be that the empty line is rendered with a height of 0px, and thus is not visible, including the caret if it is on that line. Once the line has content, that content forces a non-zero line height.

Do you know a simple workaround / fix for this problem? I reckon in the worst case, I have to write my own editor kit (see also here and here for custom line wrapping in JEditorPane) and/or custom tag (also here).

Thanks, but that's not the problem. Since I am using explicitly an HTMLEditorKit, the API interprets the string as HTML already. I left out the `...` for simplicity. It works the same way if these tags are included, though. — Markus Fischer, Oct 18 '16 at 09:07

score 2 · Accepted Answer · answered Oct 19 '16 at 11:06

Found a solution, using a custom editor kit:

public class MyEditorKit extends HTMLEditorKit {

    private static final int MIN_HEIGHT_VIEWS = 10;

    @Override
    public ViewFactory getViewFactory() {

        return new HTMLFactory() {

            @Override
            public View create(Element e) {
                View v = super.create(e);
                // Test for BRView must use String comparison, as the class is package-visible and not available to us
                if ((v instanceof InlineView) && !v.getClass().getSimpleName().equals("BRView")) {

                    View v2 = new InlineView(e) {

                        @Override
                        public float getMaximumSpan(int axis) {
                            float result = super.getMaximumSpan(axis);
                            if (axis == Y_AXIS) {
                                result = Math.max(result, MIN_HEIGHT_VIEWS);
                            }
                            return result;
                        }

                        @Override
                        public float getMinimumSpan(int axis) {
                            float result = super.getMinimumSpan(axis);
                            if (axis == Y_AXIS) {
                                result = Math.max(result, MIN_HEIGHT_VIEWS);
                            }
                            return result;
                        }

                        @Override
                        public float getPreferredSpan(int axis) {
                            float result = super.getPreferredSpan(axis);
                            if (axis == Y_AXIS) {
                                result= Math.max(result, MIN_HEIGHT_VIEWS);
                            }

                            return result;
                        }
                    };

                    v = v2;
                }

                return v;
            }
        };
    }
}

The editor kit returns a custom HTMLFactory. This factory creates custom InlineView objects for leaf elements, where the InlineView cannot have a height of 0. It will always have at least a MIN_HEIGHT_VIEW, which I set to 10 pixels (works reasonably well with default font sizes). The original implementation makes sense when rendering HTML just for viewing, as an empty line after a <br> tag should indeed be ignored. But for editing, users will expect to see the caret on the next line after inserting a linebreak.

Swing HTMLEditorKit / JEditorPane doesn't handle and empty lines correctly

1 Answers1

Swing HTMLEditorKit / JEditorPane doesn't handle
and empty lines correctly