What's wrong with URLClassLoader when working with directories?

This is my code:

File folder = /* the folder with a.txt file */
final URL[] urls = new URL[] {
  folder.toURI().toURL()
};
ClassLoader loader = new URLClassLoader(urls);
assertThat(loader.getResource("/a.txt"), is(notNullValue()));
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Doesn't work. getResource() returns NULL. Why?

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Because the resource names are always relative to the classpath you constructed the class loader with. You can't use an absolute name (that is, one that starts with /).