when to create objects using new operator or use auto-wire when testing a class?

Question

The class that I want to test is called UserService with sendEmail method, which sends an email to user. To accomplish this task it depends on EmailService. Now when writing a testcase to test this - should I create UserService userService = new UserService() and mock Email service or create context file define UserService bean there and @Autowired UserService in my test class and mock EmailService? What is the differebce between both approaches and when should i use one over the other?

Answer 1

I would say that for

unit testing purposes you could just create UserService using new and inject mock. Using spring ioc container in this case will not make any difference except tests will be slower, since they will not only create single class, but start spring container.

However if your application uses spring you need to test it somehow as well, and for integration testing approach with spinning spring context works perfectly. In this kind of testing you will test that spring context can start and that beans were injected correctly. However, usually in such kind of testing people try to replace real services with fake endpoints, changing property files accordingly. E.g. :

Sending message to some queue - run your own queue in docker and use it for testing.

Saving something to DB - run your own db in docker OR run inmemory one.

Hitting some HTTP endpoint - run wiremock in docker and mock any kind of responses, simulating connection failures, etc.