Mockito の Mock・Stub・Spy を業務テストで使いこなす方法

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

import java.util.List;
import java.util.Optional;

import static org.junit.jupiter.api.Assertions.*;
import static org.mockito.Mockito.*;

@ExtendWith(MockitoExtension.class)
class UserServiceTest {

    record User(String id, String name, String email) {}

    interface UserRepository {
        Optional<User> findById(String id);
        List<User> findAll();
        void save(User user);
    }

    static class UserService {
        private final UserRepository repository;
        UserService(UserRepository repository) { this.repository = repository; }

        User findById(String id) {
            return repository.findById(id)
                .orElseThrow(() -> new IllegalArgumentException("ユーザーが見つかりません: " + id));
        }

        List<User> findActiveUsers() {
            return repository.findAll().stream()
                .filter(user -> user.email() != null && !user.email().isBlank())
                .toList();
        }

        void register(String id, String name, String email) {
            if (repository.findById(id).isPresent()) {
                throw new IllegalStateException("ユーザーID が重複しています: " + id);
            }
            repository.save(new User(id, name, email));
        }
    }

    @Mock UserRepository userRepository;
    @InjectMocks UserService userService;

    @Test
    @DisplayName("存在するユーザーを ID で取得できる")
    void findById_returnsUser() {
        var expected = new User("U001", "田中太郎", "[email protected]");
        when(userRepository.findById("U001")).thenReturn(Optional.of(expected));

        User actual = userService.findById("U001");

        assertEquals("田中太郎", actual.name());
        verify(userRepository, times(1)).findById("U001");
    }

    @Test
    @DisplayName("存在しないユーザーを取得すると例外が発生する")
    void findById_throwsWhenNotFound() {
        when(userRepository.findById("U999")).thenReturn(Optional.empty());
        assertThrows(IllegalArgumentException.class, () -> userService.findById("U999"));
    }

    @Test
    @DisplayName("メールアドレスを持つユーザーだけを返す")
    void findActiveUsers_filtersBlankEmail() {
        when(userRepository.findAll()).thenReturn(List.of(
            new User("U001", "田中", "[email protected]"),
            new User("U002", "鈴木", ""),
            new User("U003", "佐藤", "[email protected]")
        ));
        assertEquals(2, userService.findActiveUsers().size());
    }

    @Test
    @DisplayName("既存ユーザーの登録を試みると例外が発生する")
    void register_throwsWhenDuplicate() {
        when(userRepository.findById("U001"))
            .thenReturn(Optional.of(new User("U001", "田中", "[email protected]")));
        assertThrows(IllegalStateException.class,
            () -> userService.register("U001", "田中", "[email protected]"));
        verify(userRepository, never()).save(any());
    }

    @Test
    @DisplayName("新規ユーザーを正常に登録できる")
    void register_savesNewUser() {
        when(userRepository.findById("U100")).thenReturn(Optional.empty());
        userService.register("U100", "山田花子", "[email protected]");
        verify(userRepository).save(argThat(u -> "U100".equals(u.id()) && "山田花子".equals(u.name())));
    }
}