By configuring resource-config.json, GraalVM can package resources in to the application and resources become available in the class loader. Accessing resources such as reading files can be achieved by using Class.getResource() or Class.getResourceAsStream() in the Java code.
Vert.x makes it easy for us by providing vertx-config library to read files from the classloader. You can see an example in MainVerticle clas where we are reading the json/sample.json file in the loadFile method.
In this method, we first create ConfigStoreOptions to specify the location, format and type of the file to read. We then create ConfigRetriever, configure a listener and register a handler to respond to file change.
Once file is read in, the logging output shows the content of the file (for illustration purpose).
Although reading files is possible, these files are, however, bundled together with the application. There will be times when we like to modify logging level in logback.xml file or modify other json config files on the fly when application is running. It would be very useful to be able to override configuration using external files.
See Override with External Files for details.