Extract any and all class references from a jar, and its nested jars recursively with the beauty of Kotlin. Standalone version from https://github.com/jensim/refleKt/blob/master/src/main/kotlin/se/jensim/reflekt/internal/JarFileClassLocator.kt

JarFileClassExtractor.kt

import java.io.File
import java.util.zip.ZipEntry
import java.util.zip.ZipFile
import java.util.zip.ZipInputStream

object JarFileClassLocator {

    private val classRegexp = Regex("^.*/([A-Z]+[A-Za-z0-9]${"$"})*[A-Z]+[A-Za-z0-9]*\\.class$")
    private fun String.fileToClassRef() = dropLast(6).replace("/", ".").replace("$", ".")

    val classFiles: Set<String> by lazy {
        JarFileClassLocator::class.java.protectionDomain.codeSource
                ?.let { ZipFile(File(it.location.toURI())) }
                ?.let { getClasses(it) }
                .orEmpty()
    }

    fun getClasses(jarFile: ZipFile): Set<String> =
            getClassFiles(jarFile, emptySet(), jarFile.entries()?.toList().orEmpty())
                    .map { it.fileToClassRef() }.toSet()

    private tailrec fun getClassFiles(zipFile: ZipFile, foundClassFiles: Set<String>, jarFileEntries: Collection<ZipEntry>): Set<String> {
        if (jarFileEntries.isEmpty()) {
            return foundClassFiles
        }

        val newClasses = jarFileEntries.map { it.name }.filter { it.matches(classRegexp) }
        val jars = jarFileEntries.filter { !it.isDirectory && it.name.endsWith(".jar") }
                .map { ZipInputStream(zipFile.getInputStream(it)) }
                .flatMap { it.getEntries() }

        return getClassFiles(zipFile, foundClassFiles + newClasses, jars)
    }
    
    private fun ZipInputStream.getEntries():List<ZipEntry> = try{
        use {
            generateSequence { it.nextEntry }.toList()
        }
    }catch (e:Exception){
        emptyList()
    }
}

JarFileClassLocatorTest.kt

import org.junit.Assert.assertEquals
import org.junit.Rule
import org.junit.Test
import org.junit.rules.TemporaryFolder
import java.io.File
import java.io.FileOutputStream
import java.util.zip.ZipEntry
import java.util.zip.ZipFile
import java.util.zip.ZipOutputStream

class JarFileClassLocatorTest {

    @Rule
    @JvmField
    val tmpDir: TemporaryFolder = TemporaryFolder()

    @Test
    fun getClassFileFromNestedJar() {
        val nestedJarPath = "${tmpDir.root}/nested.jar"
        val findMeClass = "FindMe.class"
        writeToZip(findMeClass, nestedJarPath, "i am a class".toByteArray())
        val nestedJarData = File(tmpDir.root, "nested.jar").readBytes()
        writeToZip("nested.jar", "${tmpDir.root}/outer.jar", nestedJarData)

        val classes = JarFileClassLocator.getClasses(ZipFile(File("${tmpDir.root}/outer.jar")))

        assertEquals(classes, setOf("com.example.FindMe"))
    }

    private fun writeToZip(entryName: String, zipFile: String, data: ByteArray) {
        ZipOutputStream(FileOutputStream(File(zipFile))).use {
            it.putNextEntry(ZipEntry("com/example/$entryName"))
            it.write(data)
        }
    }
}