code: private int defaultThreadID() {
          long tid = Thread.currentThread().getId();
          if (tid < MIN_SEQUENTIAL_THREAD_ID) {
              return (int) tid;
          } else {
              Integer id = threadIds.get();
              if (id == null) {
                  id = nextThreadId.getAndIncrement();
                  threadIds.set(id);
              }
              return id;
          }
      }
docs: Returns the default value for a new LogRecord's threadID.