The concurrent get code uses a timed wait for task completion:

  exec.awaitTermination(AWAIT_TERMINATION_TIMEOUT, TimeUnit.SECONDS);

However, it then uses an unconditional get():

  for (Future<HTTPSampleResult> future : retExec) {
      final HTTPSampleResult binRes = future.get();

This will wait until task completion, which may be forever.

The solution is to use a very short timeout (the code has already waited for the required time) and catch the TimeoutException.