code: @Override
      public boolean incrementAndCheckState(final Long increment) throws CircuitBreakingException {
          if (threshold == 0) {
              open();
          }
  
          final long used = this.used.addAndGet(increment);
          if (used > threshold) {
              open();
          }
  
          return checkState();
      }
docs: {@inheritDoc}
  
  <p>If the threshold is zero, the circuit breaker will be in a permanent <em>open</em> state.</p>