The JVM fails to correctly defined a module in the exploded build if the module is for the boot loader and the module system has already been initialized. (The module does get correctly defined in the normal build.)

The problem is caused by this code at the end of Modules::define_module():

  // If the module is defined to the boot loader and an exploded build is being
  // used, prepend <java.home>/modules/modules_name, if it exists, to the system boot class path.
  if (loader == NULL &&
      !Universe::is_module_initialized() &&
      !ClassLoader::has_jrt_entry()) {
    ClassLoader::add_to_exploded_build_list(module_symbol, CHECK);
  }