In an attempt to fix this Spring Boot issue, I have implemented a custom URLResource subclass that can list resources from a jar. As permitted by the javadoc of Resource, it returns null for both getFile() and getFilePath() as it does not map to a file but to a particular entry within a jar file. Unfortunately, when processing a resource that returns null from getFilePath(), ServletContextImpl.getResourcePaths(String) ignores the resource:

for (Resource res : resource.list()) {
    Path file = res.getFilePath();
    if (file != null) {
        Path base = res.getResourceManagerRootPath();
        if (base == null) {
            resources.add(file.toString()); //not much else we can do here
        } else {
            String filePath = file.toAbsolutePath().toString().substring(base.toAbsolutePath().toString().length());
            filePath = filePath.replace('\\', '/'); //for windows systems
            if (Files.isDirectory(file)) {
                filePath = filePath + "/";
            }
            resources.add(filePath);
        }
    }
}

I considered a workaround when I return a dummy Path along with a null resource manager root path so that the path's toString() is used, but I'm concerned that there may be unexpected side-effects of doing do if getFilePath() is called on the Resource elsewhere.