Proving it works: Mockito & slice tests

package com.example.taskflow.service;

import com.example.taskflow.model.Task;
import com.example.taskflow.repository.TaskRepository;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

import java.util.Optional;

import static org.assertj.core.api.Assertions.*;
import static org.mockito.Mockito.*;

@ExtendWith(MockitoExtension.class)        // turns on Mockito for this test class
class TaskServiceTest {

    @Mock
    private TaskRepository repository;      // a fake repository Mockito creates

    @InjectMocks
    private TaskService service;             // real service, with the mock injected in

    @Test
    void create_savesANotDoneTask() {
        // arrange: when the service calls save(...), echo the argument back
        when(repository.save(any(Task.class))).thenAnswer(inv -> inv.getArgument(0));

        // act
        Task result = service.create("Buy milk");

        // assert: the service applied its rule (done starts false)
        assertThat(result.getTitle()).isEqualTo("Buy milk");
        assertThat(result.isDone()).isFalse();

        // and it actually asked the repository to save exactly once
        verify(repository, times(1)).save(any(Task.class));
    }

    @Test
    void getById_whenMissing_throws404Exception() {
        // arrange: the fake DB has nothing for id 99
        when(repository.findById(99L)).thenReturn(Optional.empty());

        // act + assert: the service's "must exist" rule fires
        assertThatThrownBy(() -> service.getById(99L)).isInstanceOf(TaskNotFoundException.class);
    }
}

package com.example.taskflow.controller;

import com.example.taskflow.model.Task;
import com.example.taskflow.service.TaskService;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.test.context.bean.override.mockito.MockitoBean;
import org.springframework.test.web.servlet.MockMvc;

import java.util.List;

import static org.mockito.Mockito.when;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;

@WebMvcTest(TaskController.class)     // boot ONLY this controller + the web plumbing
class TaskControllerTest {

    @Autowired
    private MockMvc mockMvc;           // a fake HTTP client to fire requests at the controller

    @MockitoBean
    private TaskService service;       // the controller's dependency, mocked

    @Test
    void getAll_returnsJsonArray() throws Exception {
        when(service.getAll(null)).thenReturn(List.of(new Task(1L, "Buy milk", false)));

        mockMvc.perform(get("/tasks")).andExpect(status().isOk()).andExpect(jsonPath("$[0].title").value("Buy milk")).andExpect(jsonPath("$[0].done").value(false));
    }

    @Test
    void getOne_missing_returns404() throws Exception {
        when(service.getById(99L)).thenThrow(new com.example.taskflow.service.TaskNotFoundException(99L));

        mockMvc.perform(get("/tasks/99")).andExpect(status().isNotFound());
    }
}