@Test
public void testConcurrentClientBuilding() throws Exception {

    // we use it to check instance uniqueness
    final Set<DockerCmdExecFactory> instances = Collections.synchronizedSet(new HashSet<>());
    Runnable runnable = () -> {
        DockerCmdExecFactory factory = DockerClientBuilder.getDefaultDockerCmdExecFactory();
        // factory created
        assertNotNull(factory);
        // and is unique
        assertFalse(instances.contains(factory));
        instances.add(factory);
    };
    parallel(AMOUNT, runnable);
    // set contains all required unique instances
    assertEquals(instances.size(), AMOUNT);
}