Class to read the property files
A couple points:


Encapsulate the resource bundle in an instance of the class instead of using static methods.
If the key doesn't exist, don't return the empty string. If this situation is acceptable, return null or change the return type to Optional&lt;String&gt; and return Optional.none(). If this situation indicates an error, throw an exception.
Don't declare MalformedURLException. That is an implementation detail. Catch and throw a more appropriate exception.