Kotlin - reading resource bundle from the XML file(s) instead of '.properties' file(s)

props.xml

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">
<properties>
  <comment>default locales related file</comment>
  <entry key="app_name">My App</entry>
  <entry key="prop1">Example 1</entry>
</properties>

props_pl.xml

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">
<properties>
  <comment>polish locales related file</comment>
  <entry key="app_name">Moja apka</entry>
  <entry key="prop1">Przykład 1</entry>
</properties>

UsageExample.kt

import utils.XMLResourceBundle
import utils.XMLResourceBundleControl
import java.util.*

fun main() {
    val resBndl1 = ResourceBundle.getBundle("props", Locale.forLanguageTag("pl"), XMLResourceBundleControl())
    println(resBndl1.getString("app_name")    // prints: "Moja apka"
    
    val resBndl2 = XMLResourceBundle.getBundle("props")   // gets default Locale
    println(resBndl2.getString("prop1")       // prints: "Example 1"
} // fun main()

XMLResourceBundle.kt

package utils

import java.io.InputStream
import java.util.*

// see also:
// https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/util/ResourceBundle.Control.html
// https://gist.github.com/asicfr/1b76ea60029264d7be15d019a866e1a4

/**
 * Represents ResourceBundle read from the XML file instead of the '.properties' file
 */
class XMLResourceBundle(stream: InputStream?) : ResourceBundle() {
    private val props: Properties = Properties()
    private val selfKeys: Set<String>
        get() = props.keys.map { it.toString() }.toSet()

    init { props.loadFromXML(stream) }

    public override fun handleKeySet(): Set<String> = selfKeys

    // implementation of abstract methods of the parent class ResourceBundle
    override fun handleGetObject(key: String): Any? = props.getProperty(key)

    override fun getKeys(): Enumeration<String> =
        Collections.enumeration(selfKeys union if (parent != null) parent.keySet() else emptySet())

    companion object {
        fun getBundle(
            baseName: String,
            locale: Locale = Locale.getDefault(),
            loader: ClassLoader? = null
        ): XMLResourceBundle = (
                if (loader == null)
                    ResourceBundle.getBundle(baseName, locale, XMLResourceBundleControl())
                else
                    ResourceBundle.getBundle(baseName, locale, loader, XMLResourceBundleControl())
                ) as XMLResourceBundle
    } // companion object
} // class XMLResourceBundle(stream:InputStream?)

XMLResourceBundleControl.kt

package utils

import java.io.BufferedInputStream
import java.io.IOException
import java.util.*

// see also:
// https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/util/ResourceBundle.Control.html
// https://gist.github.com/asicfr/1b76ea60029264d7be15d019a866e1a4


class XMLResourceBundleControl : ResourceBundle.Control() {
    @Throws(IllegalAccessException::class, InstantiationException::class, IOException::class)
    override fun newBundle(
        baseName: String?,
        locale: Locale?,
        format: String?,
        loader: ClassLoader?,
        reload: Boolean
    ): ResourceBundle? {
        require(!(baseName == null || locale == null || format == null || loader == null)) {
            "baseName, locale, format and loader cannot be all null"
        }
        require(format == XML) { "format must be xml" }

        val bundleName: String = toBundleName(baseName, locale)
        val resourceName = toResourceName(bundleName, format)
        val url = loader.getResource(resourceName) ?: return null
        val urlConn = url.openConnection() ?: return null
        if (reload) { urlConn.useCaches = false }
        return urlConn.getInputStream()?.use { stream -> BufferedInputStream(stream).use (::XMLResourceBundle) }
    } // override fun newBundle(...): ResourceBundle?

    override fun getFormats(baseName: String?): List<String> = SINGLETON_LIST

    companion object {
        private const val XML = "xml"
        private val SINGLETON_LIST = listOf(XML)
    }
} // class XMLResourceBundleControl