How to serialize Java 2D Shape objects as XML?

Unfortunately, naive encoding/decoding of a Shape to XML using XMLEncoder/Decoder often destroys all the vital information of the Shape!

So to do this, still using the above mentioned classes, we serialize and restore properly constructed beans that represent the parts of the shape as obtained from a PathIterator. These beans are:

• PathBean which stores the collection of PathSegment objects that form the shape of the Java-2D Shape.
• PathSegment which stores the details of a particular part of the path (segment type, winding rule & coords).

SerializeShapes GUI

A GUI to demonstrate storing and restoring shapes.

• Click the Ellipse (Ellipse2D), Rectangle (Rectangle2D) or Face (Area) buttons a couple of times.
• Exit the GUI. The shapes will be serialized to disk.
• Restart the GUI. The randomly drawn shapes from last time will be restored from disk & reappear in the GUI.

The selected shape will be filled in green, other shapes in red.

package serialize2d;

import java.awt.*;
import java.awt.event.*;
import java.awt.font.FontRenderContext;
import java.awt.geom.AffineTransform;
import java.awt.geom.*;
import java.awt.image.BufferedImage;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.Random;
import java.util.Vector;
import javax.swing.*;
import javax.swing.border.EmptyBorder;
import javax.swing.event.*;


/** A GUI to make it easy to add/remove shapes from a canvas. 
 It should persist the shapes between runs.  */
public class SerializeShapes {

    JPanel ui;
    JPanel shapePanel;
    Random rand;
    JPanel shapeCanvas;
    DefaultListModel<Shape> allShapesModel;
    ListSelectionModel shapeSelectionModel;
    RenderingHints renderingHints;

    SerializeShapes() {
        initUI();
    }

    public void initUI() {
        if (ui != null) {
            return;
        }
        renderingHints = new RenderingHints(RenderingHints.KEY_DITHERING,
                RenderingHints.VALUE_DITHER_ENABLE);
        renderingHints.put(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
        renderingHints.put(RenderingHints.KEY_ALPHA_INTERPOLATION,
                RenderingHints.VALUE_ALPHA_INTERPOLATION_QUALITY);
        renderingHints.put(RenderingHints.KEY_COLOR_RENDERING,
                RenderingHints.VALUE_COLOR_RENDER_QUALITY);
        renderingHints.put(RenderingHints.KEY_RENDERING,
                RenderingHints.VALUE_RENDER_QUALITY);
        renderingHints.put(RenderingHints.KEY_STROKE_CONTROL,
                RenderingHints.VALUE_STROKE_NORMALIZE);
        ui = new JPanel(new BorderLayout(4, 4));
        ui.setBorder(new EmptyBorder(4, 4, 4, 4));

        JPanel controls = new JPanel(new FlowLayout(FlowLayout.CENTER, 4, 4));
        ui.add(controls, BorderLayout.PAGE_START);
        shapeCanvas = new ShapeCanvas();
        ui.add(shapeCanvas);
        rand = new Random();

        allShapesModel = new DefaultListModel<Shape>();
        JList<Shape> allShapes = new JList<Shape>(allShapesModel);
        allShapes.setCellRenderer(new ShapeListCellRenderer());
        shapeSelectionModel = allShapes.getSelectionModel();
        shapeSelectionModel.setSelectionMode(
                ListSelectionModel.SINGLE_SELECTION);
        ListSelectionListener shapesSelectionListener
                = new ListSelectionListener() {

                    @Override
                    public void valueChanged(ListSelectionEvent e) {
                        shapeCanvas.repaint();
                    }
                };
        allShapes.addListSelectionListener(shapesSelectionListener);

        JScrollPane shapesScroll = new JScrollPane(
                allShapes,
                JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
                JScrollPane.HORIZONTAL_SCROLLBAR_NEVER
        );
        // TODO fix this hack..
        shapesScroll.getViewport().setPreferredSize(new Dimension(60, 200));
        ui.add(shapesScroll, BorderLayout.LINE_START);

        Action addEllipse = new AbstractAction("Ellipse") {

            @Override
            public void actionPerformed(ActionEvent e) {
                int w = rand.nextInt(100) + 10;
                int h = rand.nextInt(100) + 10;
                int x = rand.nextInt(shapeCanvas.getWidth() - w);
                int y = rand.nextInt(shapeCanvas.getHeight() - h);
                Ellipse2D ellipse = new Ellipse2D.Double(x, y, w, h);
                addShape(ellipse);
            }
        };
        addEllipse.putValue(Action.MNEMONIC_KEY, KeyEvent.VK_E);

        Action addRectangle = new AbstractAction("Rectangle") {

            @Override
            public void actionPerformed(ActionEvent e) {
                int w = rand.nextInt(100) + 10;
                int h = rand.nextInt(100) + 10;
                int x = rand.nextInt(shapeCanvas.getWidth() - w);
                int y = rand.nextInt(shapeCanvas.getHeight() - h);
                Rectangle2D rectangle = new Rectangle2D.Double(x, y, w, h);
                addShape(rectangle);
            }
        };
        addRectangle.putValue(Action.MNEMONIC_KEY, KeyEvent.VK_R);

        final int faceStart = 128513;
        final int faceEnd = 128528;
        final int diff = faceEnd - faceStart;
        StringBuilder sb = new StringBuilder();
        for (int count = faceStart; count <= faceEnd; count++) {
            sb.append(Character.toChars(count));
        }
        final String s = sb.toString();
        Vector<Font> compatibleFontList = new Vector<Font>();
        GraphicsEnvironment ge
                = GraphicsEnvironment.getLocalGraphicsEnvironment();
        Font[] fonts = ge.getAllFonts();
        for (Font font : fonts) {
            if (font.canDisplayUpTo(s) < 0) {
                compatibleFontList.add(font);
            }
        }
        JComboBox fontChooser = new JComboBox(compatibleFontList);
        ListCellRenderer fontRenderer = new DefaultListCellRenderer() {

            @Override
            public Component getListCellRendererComponent(
                    JList list, Object value, int index,
                    boolean isSelected, boolean cellHasFocus) {
                Component c = super.getListCellRendererComponent(
                        list, value, index,
                        isSelected, cellHasFocus);
                JLabel l = (JLabel) c;
                Font font = (Font) value;
                l.setText(font.getName());
                return l;
            }
        };
        fontChooser.setRenderer(fontRenderer);
        final ComboBoxModel<Font> fontModel = fontChooser.getModel();

        BufferedImage bi = new BufferedImage(1, 1, BufferedImage.TYPE_INT_RGB);
        Graphics2D g = bi.createGraphics();
        final FontRenderContext fontRenderContext = g.getFontRenderContext();

        Action addFace = new AbstractAction("Face") {

            @Override
            public void actionPerformed(ActionEvent e) {
                int codepoint = faceStart + rand.nextInt(diff);
                String text = new String(Character.toChars(codepoint));

                Font font = (Font) fontModel.getSelectedItem();
                Area area = new Area(
                        font.deriveFont(80f).
                        createGlyphVector(fontRenderContext, text).
                        getOutline());
                Rectangle bounds = area.getBounds();
                float x = rand.nextInt(
                        shapeCanvas.getWidth() - bounds.width) - bounds.x;
                float y = rand.nextInt(
                        shapeCanvas.getHeight() - bounds.height) - bounds.y;
                AffineTransform move = AffineTransform.
                        getTranslateInstance(x, y);
                area.transform(move);
                addShape(area);
            }
        };
        addFace.putValue(Action.MNEMONIC_KEY, KeyEvent.VK_F);

        Action delete = new AbstractAction("Delete") {

            @Override
            public void actionPerformed(ActionEvent e) {
                int idx = shapeSelectionModel.getMinSelectionIndex();
                if (idx < 0) {
                    JOptionPane.showMessageDialog(
                            ui,
                            "Select a shape to delete",
                            "Select a Shape",
                            JOptionPane.ERROR_MESSAGE);
                } else {
                    allShapesModel.removeElementAt(idx);
                    shapeCanvas.repaint();
                }
            }
        };
        delete.putValue(Action.MNEMONIC_KEY, KeyEvent.VK_D);

        controls.add(new JButton(addEllipse));
        controls.add(new JButton(addRectangle));
        controls.add(new JButton(addFace));
        controls.add(fontChooser);
        controls.add(new JButton(delete));

        try {
            ArrayList<Shape> shapes = deserializeShapes();
            for (Shape shape : shapes) {
                allShapesModel.addElement(shape);
            }
        } catch (Exception ex) {
            System.err.println("If first launch, this is as expected!");
            ex.printStackTrace();
        }
    }

    private void addShape(Shape shape) {
        allShapesModel.addElement(shape);
        int size = allShapesModel.getSize() - 1;
        shapeSelectionModel.addSelectionInterval(size, size);
    }

    class ShapeCanvas extends JPanel {

        ShapeCanvas() {
            setBackground(Color.WHITE);
        }

        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D) g;
            g2.setRenderingHints(renderingHints);
            Stroke stroke = new BasicStroke(1.5f);
            g2.setStroke(stroke);
            int idx = shapeSelectionModel.getMinSelectionIndex();
            Shape selectedShape = null;
            if (idx > -1) {
                selectedShape = allShapesModel.get(idx);
            }
            Enumeration en = allShapesModel.elements();
            while (en.hasMoreElements()) {
                Shape shape = (Shape) en.nextElement();
                if (shape.equals(selectedShape)) {
                    g2.setColor(new Color(0, 255, 0, 191));
                } else {
                    g2.setColor(new Color(255, 0, 0, 191));
                }
                g2.fill(shape);
                g2.setColor(new Color(0, 0, 0, 224));
                g2.draw(shape);
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(500, 300);
        }
    }

    public JComponent getUI() {
        return ui;
    }

    public static void main(String[] args) {
        Runnable r = new Runnable() {
            @Override
            public void run() {
                SerializeShapes se = new SerializeShapes();

                JFrame f = new JFrame("Serialize Shapes");
                f.addWindowListener(new SerializeWindowListener(se));
                f.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
                f.setContentPane(se.getUI());
                f.setResizable(false);
                f.pack();
                f.setLocationByPlatform(true);
                f.setVisible(true);
            }
        };
        SwingUtilities.invokeLater(r);
    }

    public void serializeShapes() throws FileNotFoundException {
        ArrayList<Shape> shapes
                = new ArrayList<Shape>();
        Enumeration en = allShapesModel.elements();
        while (en.hasMoreElements()) {
            Shape shape = (Shape) en.nextElement();
            shapes.add(shape);
        }
        ShapeIO.serializeShapes(shapes, this.getClass());
        try {
            Desktop.getDesktop().open(
                    ShapeIO.getSerializeFile(this.getClass()));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public ArrayList<Shape> deserializeShapes() throws FileNotFoundException {
        return ShapeIO.deserializeShapes(this.getClass());
    }

    class ShapeListCellRenderer extends DefaultListCellRenderer {

        @Override
        public Component getListCellRendererComponent(
                JList<? extends Object> list, Object value,
                int index, boolean isSelected, boolean cellHasFocus) {
            Component c = super.getListCellRendererComponent(list, value, index,
                    isSelected, cellHasFocus);
            JLabel l = (JLabel) c;
            Shape shape = (Shape) value;
            ShapeIcon icon = new ShapeIcon(shape, 40);
            l.setIcon(icon);
            l.setText("");

            return l;
        }
    }

    class ShapeIcon implements Icon {

        Shape shape;
        int size;

        ShapeIcon(Shape shape, int size) {
            this.shape = shape;
            this.size = size;
        }

        @Override
        public void paintIcon(Component c, Graphics g, int x, int y) {
            Graphics2D g2 = (Graphics2D) g;
            g2.setRenderingHints(renderingHints);
            Rectangle bounds = shape.getBounds();
            int xOff = -bounds.x;
            int yOff = -bounds.y;
            double xRatio = (double) bounds.width / (double) size;
            double yRatio = (double) bounds.height / (double) size;
            double ratio = xRatio > yRatio ? xRatio : yRatio;
            AffineTransform scale = AffineTransform.getScaleInstance(1 / ratio, 1 / ratio);
            AffineTransform shift = AffineTransform.getTranslateInstance(xOff, yOff);

            AffineTransform totalTransform = new AffineTransform();

            totalTransform.concatenate(scale);
            totalTransform.concatenate(shift);

            Area b = new Area(shape).createTransformedArea(totalTransform);
            bounds = b.getBounds();

            g2.setColor(Color.BLACK);
            g2.fill(b);
        }

        @Override
        public int getIconWidth() {
            return size;
        }

        @Override
        public int getIconHeight() {
            return size;
        }
    }
}

class SerializeWindowListener extends WindowAdapter {

    SerializeShapes serializeShapes;

    SerializeWindowListener(SerializeShapes serializeShapes) {
        this.serializeShapes = serializeShapes;
    }

    @Override
    public void windowClosing(WindowEvent e) {
        try {
            serializeShapes.serializeShapes();
        } catch (FileNotFoundException ex) {
            ex.printStackTrace();
            System.exit(1);
        }
        System.exit(0);
    }
}

ShapeIO

Performs the I/O to/from XML.

package serialize2d;

import java.awt.Shape;
import java.beans.*;
import java.io.*;
import java.util.ArrayList;

public class ShapeIO {

    /** Save the list of shapes to the file system. */
    public static void serializeShapes(
            ArrayList<Shape> shapes, Class serializeClass) 
            throws FileNotFoundException {
        File f = getSerializeFile(serializeClass);
        XMLEncoder xmle = new XMLEncoder(new FileOutputStream(f));

        ArrayList<PathBean> pathSegmentsCollection = new ArrayList<>();
        for (Shape shape : shapes) {
            ArrayList<PathSegment> pathSegments = 
                    BeanConverter.getSegmentsFromShape(shape);
            PathBean as = new PathBean(pathSegments);
            pathSegmentsCollection.add(as);
        }

        xmle.writeObject(pathSegmentsCollection);
        xmle.flush();
        xmle.close();
    }

    /** Load the list of shapes from the file system. */
    public static ArrayList<Shape> deserializeShapes(Class serializeClass) 
            throws FileNotFoundException {
        File f = getSerializeFile(serializeClass);
        XMLDecoder xmld = new XMLDecoder(new FileInputStream(f));
        ArrayList<PathBean> pathSegmentsCollection
                = (ArrayList<PathBean>) xmld.readObject();
        ArrayList<Shape> shapes = new ArrayList<Shape>();
        for (PathBean pathSegments : pathSegmentsCollection) {
            shapes.add(BeanConverter.getShapeFromSegments(pathSegments));
        }

        return shapes;
    }

    /** Provide an unique, reproducible & readable/writable path for a class. */
    public static File getSerializeFile(Class serializeClass) {
        File f = new File(System.getProperty("user.home"));
        String[] nameParts = serializeClass.getCanonicalName().split("\\.");

        f = new File(f, "java");
        for (String namePart : nameParts) {
            f = new File(f, namePart);
        }
        f.mkdirs();
        f = new File(f, nameParts[nameParts.length-1] + ".xml");

        return f;
    }
}

BeanConverter

Obtains a PathIterator from the Shape and converts it to a serializable bean. Converts the bean back into a GeneralPath.

package serialize2d;

import java.awt.Shape;
import java.awt.geom.*;
import java.util.ArrayList;

/** Utility class to convert bean to/from a Shape. */
public class BeanConverter {

    /** Convert a shape to a serializable bean.  */
    public static ArrayList<PathSegment> getSegmentsFromShape(Shape shape) {
        ArrayList<PathSegment> shapeSegments = new ArrayList<PathSegment>();
        for (
                PathIterator pi = shape.getPathIterator(null); 
                !pi.isDone(); 
                pi.next()) {
            double[] coords = new double[6];
            int pathSegmentType = pi.currentSegment(coords);
            int windingRule = pi.getWindingRule();
            PathSegment as = new PathSegment(
                    pathSegmentType, windingRule, coords);
            shapeSegments.add(as);
        }
        return shapeSegments;
    }

    /** Convert a serializable bean to a shape.  */
    public static Shape getShapeFromSegments(PathBean shapeSegments) {
        GeneralPath gp = new GeneralPath();
        for (PathSegment shapeSegment : shapeSegments.getPathSegments()) {
            double[] coords = shapeSegment.getCoords();
            int pathSegmentType = shapeSegment.getPathSegmentType();
            int windingRule = shapeSegment.getWindingRule();
            gp.setWindingRule(windingRule);
            if (pathSegmentType == PathIterator.SEG_MOVETO) {
                gp.moveTo(coords[0], coords[1]);
            } else if (pathSegmentType == PathIterator.SEG_LINETO) {
                gp.lineTo(coords[0], coords[1]);
            } else if (pathSegmentType == PathIterator.SEG_QUADTO) {
                gp.quadTo(coords[0], coords[1], coords[2], coords[3]);
            } else if (pathSegmentType == PathIterator.SEG_CUBICTO) {
                gp.curveTo(
                        coords[0], coords[1], coords[2], 
                        coords[3], coords[4], coords[5]);
            } else if (pathSegmentType == PathIterator.SEG_CLOSE) {
                gp.closePath();
            } else {
                System.err.println("Unexpected value! " + pathSegmentType);
            }
        }
        return gp;
    }
}

PathBean

Stores a collection of path segments in a seriallizable bean.

package serialize2d;

import java.awt.geom.*;
import java.util.ArrayList;

/** PathBean stores the collection of PathSegment objects
 that constitute the path of a Shape. */
public class PathBean {

    public ArrayList<PathSegment> pathSegments;

    public PathBean() {}

    public PathBean(ArrayList<PathSegment> pathSegments) {
        this.pathSegments = pathSegments;
    }

    public ArrayList<PathSegment> getPathSegments() {
        return pathSegments;
    }

    public void setPathSegments(ArrayList<PathSegment> pathSegments) {
        this.pathSegments = pathSegments;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder("{");
        for (PathSegment pathSegment : pathSegments) {
            sb.append(" \n\t");
            sb.append(pathSegment.toString());
        }
        sb.append(" \n");
        sb.append("}");
        return "PathSegments: " + sb.toString();
    }
}

PathSegment

Stores the path segment of one part of the entire path.

package serialize2d;

import java.util.Arrays;

/** PathSegment bean stores the detail on one segment of the path
 that constitutes a Shape. */
public class PathSegment {

    public int pathSegmentType;
    public int windingRule;
    public double[] coords;

    public PathSegment() {}

    public PathSegment(int pathSegmentType, int windingRule, double[] coords) {
        this.pathSegmentType = pathSegmentType;
        this.windingRule = windingRule;
        this.coords = coords;
    }

    public int getPathSegmentType() {
        return pathSegmentType;
    }

    public void setPathSegmentType(int pathSegmentType) {
        this.pathSegmentType = pathSegmentType;
    }

    public int getWindingRule() {
        return windingRule;
    }

    public void setWindingRule(int windingRule) {
        this.windingRule = windingRule;
    }

    public double[] getCoords() {
        return coords;
    }

    public void setCoords(double[] coords) {
        this.coords = coords;
    }

    @Override
    public String toString() {
        String sC = (coords != null ? "" : Arrays.toString(coords));
        String s = String.format(
                "PathSegment: Path Segment Type:- %d \t"
                + "Winding Rule:- %d \tcoords:- %s",
                getPathSegmentType(), getWindingRule(), sC);
        return s;
    }
}

Notes

This is intended as a proof of concept as opposed to a polished approach.

• XML serialized data becomes big real fast, it would normally be zipped. Zip compression might shave 30-40% off the byte size of a serialized object or a class file, but 80-95% off XML. In any case, zip works well for the next point as well.
• For the type of project where we wish to offer to serialize and restore shapes, we’ll probably also want to include more details of the shapes (e.g. fill color or texture and draw color or stroke etc.) as well as other data like images or fonts. This is also where Zip comes in handy, since we can put them all in the same archive, each with best levels of compression (e.g. standard for the XML and none for images).

A zip archive of the source files in this answer can be downloaded from my cloud drive.