Accessing Java Resources (a.k.a. getResource vs getSystemResource)

This isn’t a tough article, but one that I always forget and have to look up. This is useful for everyone who has to touch Java for the first time or experienced users who just forget (like me).

Adding a resource, especially when using a build tool Maven, is quite easy. You just include it in the jar (which in maven, is by putting it in ${projectdir}/src/main/resources). Getting it back out is a little trickier, but not too bad.

The easiest way to access this is to use the getResource instead of getSystemResource, which are on the Java Class Loader, to use a resource specific to a given classloader instead of the system. For example, try any of the following:

URL resource = getClass().getClassLoader().getResource("R.txt");
URL resource = Foo.class.getClassLoader().getResource("R.txt");
URL resource = getClass().getResource("/R.txt");
URL resource = Foo.class.getResource("/R.txt");

Note the leading slash when calling Class.getResource() instead of ClassLoader.getResource(). Class.getResource  is relative to the package containing the class unless you have a leading slash, whereas ClassLoader.getResource()  is always absolute.

Also, as a side-note, if you are trying to access resources from a dependency, don’t. You are doing it wrong. This is possible, but bad design and hard to implement.