exporting a Map to XML causes a DOMException

[stephengold]

The jme3-plugins sub-project provides an XMLExporter class to export savables to XML files.

However, if the Savable includes a non-empty Map, invoking the save() method causes CoreDocumentImpl.createAttribute() to throw a DOMException.

Test code:

        Map<String, String> map = new HashMap<>();
        map.put("key", "value");
        rootNode.setUserData("map", map);

        String xmlPath = "TestExport.xml";
        File xmlFile = new File(xmlPath);
        JmeExporter exporter = XMLExporter.getInstance();
        try {
            exporter.save(rootNode, xmlFile);
        } catch (IOException exception) {
            logger.log(Level.SEVERE, exception.getMessage(), exception);
        }

Typical result:

Aug 29, 2023 5:24:53 PM com.jme3.app.LegacyApplication handleError
SEVERE: Uncaught exception thrown in Thread[jME3 Main,5,main]
org.w3c.dom.DOMException: INVALID_CHARACTER_ERR: An invalid or illegal XML character is specified.
    at java.xml/com.sun.org.apache.xerces.internal.dom.CoreDocumentImpl.createAttribute(CoreDocumentImpl.java:573)
    at java.xml/com.sun.org.apache.xerces.internal.dom.ElementImpl.setAttribute(ElementImpl.java:503)
    at com.jme3.export.xml.DOMOutputCapsule.write(DOMOutputCapsule.java:165)
    at com.jme3.scene.UserData.writeList(UserData.java:258)
    at com.jme3.scene.UserData.write(UserData.java:172)
    at com.jme3.export.xml.DOMOutputCapsule.write(DOMOutputCapsule.java:555)
    at com.jme3.export.xml.DOMOutputCapsule.writeStringSavableMap(DOMOutputCapsule.java:728)
    at com.jme3.scene.Spatial.write(Spatial.java:1626)
    at com.jme3.scene.Node.write(Node.java:757)
    at com.jme3.export.xml.DOMOutputCapsule.write(DOMOutputCapsule.java:555)
    at com.jme3.export.xml.XMLExporter.save(XMLExporter.java:70)
    at com.jme3.export.xml.XMLExporter.save(XMLExporter.java:88)
    at com.jme3.export.JmeExporter.save(JmeExporter.java:62)

Note that the BinaryExporter class seems to handle maps just fine.

[stephengold]

added TestIssue2068 to master-branch jme3-examples at b5623697acf2597f7d69cad6ca48f8e11899dcc8

[JosiahGoeman]

Some additional info: It seems to happen because UserData.writeList() writes the size of the list under a key starting with the listName argument. https://github.com/jMonkeyEngine/jmonkeyengine/blob/230f1575bf9935b47cb8ecdcd8ddc7379c56e75b/jme3-core/src/main/java/com/jme3/scene/UserData.java#L258

UserData.write() calls UserData.writeList() with numbers for the listName when writing lists, arrays, and maps (all affected by this bug.) XML attribute names (or any names I think...) cannot begin with a number.

[codex128]

This problem could be fixed by substituting "0" and "1" for "A" and "B" in the read and write methods, except that would break backwards compatibility for binary files. I noticed the xml includes format_version property on the root node... could that be utilized to keep backwrds compatibility?

[riccardobl]

I noticed the xml includes format_version property on the root node... could that be utilized to keep backwrds compatibility?

Why not? But i think a more solid approach would be to prefix every key with "jme-" in the xml importer/exporter