code: public void registerCoreExtensions() {
      if (extensionsRegistered) {
        throw H2O.fail("Extensions already registered");
      }
  
      long before = System.currentTimeMillis();
      ServiceLoader<AbstractH2OExtension> extensionsLoader = ServiceLoader.load(AbstractH2OExtension.class);
      for (AbstractH2OExtension ext : extensionsLoader) {
        if (isEnabled(ext)) {
          ext.init();
          coreExtensions.put(ext.getExtensionName(), ext);
        }
      }
      extensionsRegistered = true;
      registerCoreExtensionsMillis = System.currentTimeMillis() - before;
    }
docs: Register H2O extensions.
  <p/>
  Use SPI to find all classes that extends water.AbstractH2OExtension
  and call H2O.addCoreExtension() for each.