Why do I have ParameterResolutionException when I don't even use parametrized test?

Question

I want to write a test for my BookService. This is that test. I don't know why I get the below error all the time:

org.junit.jupiter.api.extension.ParameterResolutionException: No ParameterResolver registered for parameter 
[com.mrfisherman.library.service.domain.BookService bookService] in constructor 
[public com.mrfisherman.library.service.domain.BookServiceTest(com.mrfisherman.library.service.domain.BookService,
com.mrfisherman.library.persistence.repository.BookRepository)].

As you can see I don't use parametrized tests here. Thank you in advance!

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT, classes = Server.class)
class BookServiceTest {

    private final BookService bookService;
    private final BookRepository bookRepository;

    public BookServiceTest(BookService bookService, BookRepository bookRepository) {
        this.bookService = bookService;
        this.bookRepository = bookRepository;
    }

    @Test
    void saveBook() {
        //given
        Book book = new Book();
        book.setTitle("Book 1");
        book.setPublishYear(1990);
        book.setType(BookFormat.REAL);
        book.setIsbn("1234567890");
        book.setDescription("Very good book");
        book.setNumberOfPages(190);
        book.setSummary("Very short summary");
        book.setCategories(Set.of(new Category("horror"), new Category("drama")));

        //when
        bookService.saveBook(book);

        //then
        Optional<Book> loaded = bookRepository.findById(book.getId());
        assertThat(loaded).isPresent();

    }
}

Answer 1

In JUnit Jupiter, a ParameterResolutionException is thrown whenever a test class constructor, lifecycle method (such as @BeforeEach), or test method declares a parameter that cannot be resolved by one of the registered ParameterResolver extensions.

Thus, a ParameterResolutionException can be thrown even when you are not using a @ParameterizedTest method.

When using @SpringBootTest, the SpringExtension is automatically registered for you. The SpringExtension implements the ParameterResolver extension API from JUnit Jupiter so that you can have beans from your ApplicationContext injected into constructors and methods in your test class.

The easiest way to solve your issue is to annotate the BookServiceTest constructor with @Autowired.

For more information and alternative approaches, check out the Dependency Injection with SpringExtension section of the Spring reference docs.