code: public synchronized Set<ProbeListener> getInterestedByClass(Class<?> clazz) {
          Set<ProbeListener> listeners = listenersByClass.get(clazz);
          if (listeners == null) {
              listeners = Collections.emptySet();
          }
          // Make copy to allow caller to iterate over contents
          return new HashSet<ProbeListener>(listeners);
      }
docs: Get the set of listeners with a {@link ProbeFilter} that matches
  the specified class. The class transformer will call this during
  class transformation to get the subset of listeners that need to be
  processed.
  
  @param clazz the candidate probe source
  
  @return the subset of listeners with filters that match {@code clazz}