It appears to be an undocumented “feature" that URLClassLoader can accept base jar-scheme URLs such as:

 jar:file/….!/
 jar:http/….!/

by virtue of those URLs being passed to URLClassPath, which is contrary to what is stated on URLClassLoader:

"Any URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be opened as needed.”

The above only reliably applies to file-scheme URLs.