Generic logic to walk both jar and filesystem isn't simple...

FooBar.kt

package com.example

import org.junit.Test
import java.net.URI
import java.net.URL
import java.nio.file.FileSystem
import java.nio.file.FileSystemNotFoundException
import java.nio.file.FileSystems
import java.nio.file.Files
import java.nio.file.Paths

class FooBar {
    @Test
    fun `test walk`() {
        fun dump(fs: FileSystem, uri: URI) = Files.walk(fs.provider().getPath(uri)).forEach(::println)

        // Filesystem walking...
        run {
            val cwd = Paths.get("")
            val uri = cwd.toUri()
            val fs = cwd.fileSystem

            println(uri.path)
            dump(fs, uri)
        }

        // You might imagine the URI above could be used in the logic below but for some reason the path
        // component of the URI passed to FileSystems.getFileSystem(uri) must be "/" if the scheme is "file"
        // but not if it's "jar". So it seems you can't easily write a scheme agnostic solution :(

        // Jar walking...
        run {
            val pkg = URL("jar:file:/usr/lib/jvm/java-8-oracle/jre/lib/rt.jar!/java/lang")
            val uri = pkg.toURI()
            val fs = try {
                FileSystems.getFileSystem(uri)
            } catch (e: FileSystemNotFoundException) {
                FileSystems.newFileSystem(uri, mapOf<String, Any>())
            }

            println(uri.path)
            dump(fs, uri)
        }
        
        // Note that there's no easy way to get a URL for a package rather than a class.
        println(this.javaClass.getResource("/java/lang/String.class"))
        // The above is fine but the following all return null:
        println(this.javaClass.getResource("/java/lang/"))
        println(this.javaClass.getResource("/java/lang"))
        println(this.javaClass.getResource("java/lang/"))
        println(this.javaClass.getResource("java/lang"))
        println(this.javaClass.classLoader.getResource("/java/lang/"))
        println(this.javaClass.classLoader.getResource("/java/lang"))
        println(this.javaClass.classLoader.getResource("java/lang/"))
        println(this.javaClass.classLoader.getResource("java/lang"))
    }
}