The addClassLoaders(ClassLoader ... classloaders) was trying to concatenate ClassLoader.class to the stream of classloaders, and the toArray() method would fail given that ClassLoader.class is not of type ClassLoader.

Notice that I could not understand why there is interest in adding the ClassLoader.class so my solution was to remove it.

If what we are seeking is that the class loader of the ClassLoader is to be added, then probably we are missing a ClassLoader.class.getClassLoader(). But doing so would be inconsistent as the following would not produce same output:

ConfigurationBuilder configurationBuilder = new ConfigurationBuilder().addClassLoader( foo ).addClassLoader( bar );

ConfigurationBuilder configurationBuilder = new ConfigurationBuilder().addClassLoader( foo, bar );

In the second case, the ClassLoader.class would be ignored, because the ClassLoader[] classLoaders field of the ConfigurationBuilder is null and then classLoaders would simply match the passed { foo, bar } array.