@Override
        public ResponseData execute(final RequestData request) {
            final long now = SystemUtil.currentTimeMillis();
            if (now - previousTime < interval) {
                throw new IllegalStateException();
            }
            previousTime = now;

            count++;
            if (count <= exceptionCount) {
                throw new CrawlerSystemException("exception " + count);
            }

    }

    /**
     * Test constructor with null message and valid cause
     */
    public void test_constructor_withNullMessageAndValidCause() {
        Exception cause = new IllegalStateException("State error");
        CrawlerSystemException exception = new CrawlerSystemException(null, cause);

        assertNotNull(exception);
        assertNull(exception.getMessage());