code: public FragmentBuilder getFragmentBuilder() {
          FragmentBuilder builder = builders.get();
  
          if (builder == null) {
              if (log.isLoggable(Level.FINEST)) {
                  log.finest("Creating new FragmentBuilder");
              }
              builder = new FragmentBuilder();
              builders.set(builder);
  
              int currentCount = threadCounter.incrementAndGet();
              int builderCount = builder.incrementThreadCount();
  
              if (log.isLoggable(Level.FINEST)) {
                  log.finest("Associate Thread with FragmentBuilder(1): Total Thread Count = " + currentCount
                          + " : Fragment Thread Count = " + builderCount);
                  synchronized (threadNames) {
                      threadNames.add(Thread.currentThread().getName());
                  }
              }
          }
  
          return builder;
      }
docs: This method returns the appropriate fragment builder for the current
  thread.
  
  @return The fragment builder for this thread of execution