test method

 @Test
    public void saveClientTest() {
        final Client newClient = new Client();
        newClient.setId(3);
        newClient.setFirstName("Dmitry");
        newClient.setLastName("Zub");

        ClientService clientService = Mockito.mock(ClientService.class);
        Mockito.when(clientService.save(newClient)).thenAnswer(new Answer<Client>() {
                @Override
                public Client answer(InvocationOnMock invocationOnMock) throws Throwable {
                    clients.add(newClient);
                    return newClient;
                }
            });

        ClientRestController clientRestController = new ClientRestController();
        ReflectionTestUtils.setField(clientRestController, "clientService", clientService);

        Client client = clientRestController.saveClient(newClient);

        Assert.assertEquals(Integer.valueOf(3), client.getId());
        Assert.assertEquals("Dmitry", client.getFirstName());
        Assert.assertEquals("Zub", client.getLastName());
        Assert.assertEquals(3, clients.size());
    }