ClassNotFoundException after loading the classfile

I tried to load classes of a jar-file by using URLClassloader.
But I have one problem: If I create a new instance from a class I loaded, I am getting a ClassNotFoundException and I don't know where the mistake is.
Here is my code to load the classes:

private void loadClasses() throws IOException, ClassNotFoundException {
	JarFile[] jarFile = {
		new JarFile("lib/core-api-bukkit-0.0.1-SNAPSHOT.jar"),
		new JarFile("lib/core-api-common-0.0.1-SNAPSHOT.jar"),
		new JarFile("lib/core-plugin-common-0.0.1-SNAPSHOT.jar"),
		new JarFile("lib/core-db-common-0.0.1-SNAPSHOT.jar")
	};

	for (JarFile currentFile : jarFile) {
		try {
			System.out.println("Current File: " + currentFile.getName());
			Enumeration<JarEntry> e = currentFile.entries();

			URL[] urls = { new URL("jar:file:" + currentFile.getName() + "!/") };
			URLClassLoader cl = URLClassLoader.newInstance(urls, getClassLoader());

			while (e.hasMoreElements()) {
			    JarEntry je = e.nextElement();
			    if (je.isDirectory() || !je.getName().endsWith(".class")) {
			        continue;
			    }
			    String className = je.getName().substring(0,je.getName().length() - 6);
			    className = className.replace('/', '.');
			    Class<?> c = cl.loadClass(className);
			    System.out.println("Load class " + c.getName());
			}
		} finally {
			currentFile.close();
		}
	}
}

Here is the console output:

java.lang.NoClassDefFoundError: at/cybercraft/core/plugin/common/player/Player
	at at.cybercraft.core.plugin.bukkit.BukkitCorePlugin.registerClasses(BukkitCorePlugin.java:28) ~[?:?]
	at at.cybercraft.core.plugin.bukkit.BukkitCorePlugin.onEnable(BukkitCorePlugin.java:22) ~[?:?]
	at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:321) ~[spigot.jar:git-Spigot-db6de12-18fbb24]
	at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:340) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:405) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at org.bukkit.craftbukkit.v1_8_R3.CraftServer.loadPlugin(CraftServer.java:357) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at org.bukkit.craftbukkit.v1_8_R3.CraftServer.enablePlugins(CraftServer.java:317) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at org.bukkit.craftbukkit.v1_8_R3.CraftServer.reload(CraftServer.java:741) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at org.bukkit.Bukkit.reload(Bukkit.java:535) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at org.bukkit.command.defaults.ReloadCommand.execute(ReloadCommand.java:25) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at org.bukkit.craftbukkit.v1_8_R3.CraftServer.dispatchCommand(CraftServer.java:641) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at net.minecraft.server.v1_8_R3.PlayerConnection.handleCommand(PlayerConnection.java:1162) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at net.minecraft.server.v1_8_R3.PlayerConnection.a(PlayerConnection.java:997) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:45) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:1) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at net.minecraft.server.v1_8_R3.PlayerConnectionUtils$1.run(SourceFile:13) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_242]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_242]
	at net.minecraft.server.v1_8_R3.SystemUtils.a(SourceFile:44) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:715) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [spigot.jar:git-Spigot-db6de12-18fbb24]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_242]
Caused by: java.lang.ClassNotFoundException: at.cybercraft.core.plugin.common.player.Player
	at java.net.URLClassLoader.findClass(URLClassLoader.java:382) ~[?:1.8.0_242]
	at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:101) ~[spigot.jar:git-Spigot-db6de12-18fbb24]
	at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:86) ~[spigot.jar:git-Spigot-db6de12-18fbb24]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:419) ~[?:1.8.0_242]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:352) ~[?:1.8.0_242]
	... 25 more

Yours sincerely
Daniel

You get the message Error occurred while enabling BukkitCorePlugin in the stack trace, together with a NoClassDefFoundError.

That looks like that your code loads a class and when it is initialised, that fails because a required class cannot be loaded by the respective class loader.

That could be because you create a new instance of ClassLoader for each jar, and those classloader do not depend from each other.

Usually, when a classloader cannot find a class itself, it delegates to its parent (in fact, it asks the parent first, and only when the parent does not come up with the requested class, it looks itself). Your classloaders do have only a common parent, but are not related to each other. So when a class from lib/core-api-common-0.0.1-SNAPSHOT.jar refers to a class that is located in lib/core-api-bukkit-0.0.1-SNAPSHOT.jar, that could not be found, although it is already loaded.