code: @Override
      public <T> T onlyOne(Class<T> serviceClass) {
          Collection<T> all = all(serviceClass);
          if (all.size() == 1) {
              return all.iterator().next();
          }
          if (all.size() > 1) {
              throw new IllegalStateException(
                  "Multiple service implementations found for " + serviceClass + ": " + toClassString(all));
          }
          return null;
      }
docs: /* (non-Javadoc)
  @see org.jboss.arquillian.core.spi.ServiceLoader#onlyOne(java.lang.Class)