code: private static synchronized boolean isProviderPresent() {
      
      if (!SPI_CHECKED) {
        SPI_CHECKED = true;
        
        ServiceLocatorGenerator generator = lookupSPI();
        if (generator instanceof GuiceServiceLocatorGeneratorStub) {
          SPI_PRESENT = true;
          
        }
        
        if (!SPI_PRESENT) {
          LOG.warn("It appears jersey2-guice-spi is either not present or in conflict with some other Jar: {}", generator);
        }
      }
      
      return SPI_PRESENT;
    }
docs: Returns {@code true} if jersey2-guice SPI is present.