How to get list of all modules programmatically in Java 9?

Question

How can I get list of all modules in current JVM instance via Java code? Is it possible? If yes, then how?

Answer 1

ModuleLayer.boot().modules().stream()
                .map(Module::getName)
                .forEach(System.out::println);

Answer 2

Using a jar or directory of modules as an input for your application, you can possibly use a ModuleFinder to start off with and further making use of the findAll to find the set of all module references that this finder can locate.

Path dir1, dir2, dir3;
ModuleFinder finder = ModuleFinder.of(dir1, dir2, dir3);
Set<ModuleReference> moduleReferences = finder.findAll();

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This is easily possible with the command line option to list the modules as:

java -p <jarfile> --list-modules

which should be sufficient though, if you do not want to intentionally get into tweaking things with the ModuleLayer and the Configuration of the java.lang.module as pointed out precisely by @Alan's answer.