Test Spring-Boot Repository interface methods without touching the database using Mockito

"Mock" your repository method calls. Also, use @InjectMocks instead @Autowired for TransactionService. And you can also use MockitoJUnitRunner. How to mock repository calls:

when(customerRepository.save(sender)).thenReturn(someSenderInstance);

To verify that mocked method call has been invoked use:

verify(customerRepository, times(1)).save(sender);

Also, remember one thing: You are testing services! Therefore, all calls to database should be mocked.