code: private <T extends Assignable> InputStream findExtensionImpl(final Class<T> extensionClass) {
          try {
              // Add all extension impls found in all CLs
              for (final ClassLoader cl : this.getClassLoaders()) {
                  final InputStream stream = cl.getResourceAsStream("META-INF/services/" + extensionClass.getName());
                  if (stream != null) {
                      return stream;
                  }
              }
  
              // None found
              throw new RuntimeException("No extension implementation found for " + extensionClass.getName()
                  + ", please verify classpath or add a extensionOverride");
          } catch (Exception e) {
              throw UnknownExtensionTypeExceptionDelegator.newExceptionInstance(extensionClass);
          }
      }
docs: Iterates through the classloaders to load the provider-configuration file for <code>extensionClass</code> in
  META-INF/services/ using its binary name.
  
  @param <T>
  @param extensionClass
  SPI type for which the configuration file is looked for
  @return An {@link InputStream} representing <code>extensionClass</code>'s configuration file
  @throws RuntimeException
  if it doesn't find a provider-configuration file for <code>extensionClass</code>
  @throws UnknownExtensionTypeExceptionDelegator