assertEventualCollectionCondition ... awaiting some future condition

assertEventualCollectionCondition.java

  // >>> How to use
  assertEventualCollectionCondition(
                () -> em.createQuery("from Dog").getResultList(),
                Collection::isEmpty);
   
   // >>> 

    private <T> void assertEventualCollectionCondition(Supplier<Collection<T>> supplier, Predicate<Collection<T>> test) throws Exception {
        assertEventualCollectionCondition(30, TimeUnit.SECONDS, supplier, test);
    }

    private <T> void assertEventualCollectionCondition(int timeOut, TimeUnit timeUnit, Supplier<Collection<T>> supplier, Predicate<Collection<T>> test) throws Exception {
        final long stopTimeMillis = System.currentTimeMillis() + timeUnit.toMillis(timeOut);

        while (true) {
            Collection<T> elements = supplier.get();
            if (test.test(elements)) {
                return;
            }
            if (System.currentTimeMillis() > stopTimeMillis) {
                fail(String.format("Timed out waiting for collection condition. Resulting collection had [%s] elements. Contents: %s", (elements == null) ? null : elements.size(), elements));
            } else {
                TimeUnit.SECONDS.sleep(1);
            }
        }
    }