单元测试入门：JUnit 5 + Mockito 打桩与断言

<dependency>    <groupId>org.springframework.boot</groupId>    <artifactId>spring-boot-starter-test</artifactId>    <scope>test</scope></dependency>

class OrderServiceTest {    private OrderService orderService;    private OrderRepository repository;    @BeforeAll                  // 整个类执行一次：适合建昂贵资源    static void initAll() {        System.out.println("== 类级初始化 ==");    }    @BeforeEach                 // 每个测试方法前执行：把被测对象恢复成干净状态    void setUp() {        repository = mock(OrderRepository.class);        orderService = new OrderService(repository);    }    @AfterEach                  // 每个测试方法后执行：清理临时状态    void tearDown() {        // 单元测试里通常无需做什么；此处演示结构    }    @Test    void should_return_order_when_id_exists() {        // Given（准备）        Order order = new Order(1L, new BigDecimal("99.00"));        when(repository.findById(1L)).thenReturn(Optional.of(order));        // When（执行）        Order result = orderService.getById(1L);        // Then（断言）        assertThat(result.getAmount()).isEqualByComparingTo("99.00");    }}

// JUnit 5 原生断言assertEquals(5, list.size());assertThrows(IllegalArgumentException.class, () -> service.withdraw(-1));assertAll("订单校验",        () -> assertEquals("A001", order.getNo()),        () -> assertEquals(Status.CREATED, order.getStatus()));assertTimeout(Duration.ofMillis(200), () -> service.quickTask());

// AssertJ 流式断言：链式组合，失败信息更直观List<User> users = userService.findActive();assertThat(users)        .hasSize(3)        .extracting(User::getUsername)        .containsExactlyInAnyOrder("alice", "bob", "carol");assertThatThrownBy(() -> service.withdraw(-1))        .isInstanceOf(IllegalArgumentException.class)        .hasMessageContaining("金额");assertThat(order.getAmount()).isEqualByComparingTo("99.00");   // BigDecimal 用它，别用 equals

@ExtendWith(MockitoExtension.class)          // 没有它，@Mock 不会生效（新手第一坑）class UserServiceTest {    @Mock    private UserRepository userRepository;    // 假的仓库，不碰数据库    @InjectMocks    private UserService userService;          // 被测对象，Mockito 自动把上面的 @Mock 注入进去    @Test    void should_return_user_when_found() {        // Given：规定 findById(1) 返回一个用户        User user = new User(1L, "alice");        when(userRepository.findById(1L)).thenReturn(Optional.of(user));        // When        User result = userService.getById(1L);        // Then        assertThat(result.getUsername()).isEqualTo("alice");        verify(userRepository, times(1)).findById(1L);   // 验证确实只调了一次    }    @Test    void should_throw_when_not_found() {        when(userRepository.findById(9L)).thenReturn(Optional.empty());        assertThatThrownBy(() -> userService.getById(9L))                .isInstanceOf(NoSuchElementException.class);    }    @Test    void should_pass_saved_user_to_repository() {        // ArgumentCaptor：把传给仓库的参数「抓」出来检查        ArgumentCaptor<User> captor = ArgumentCaptor.forClass(User.class);        userService.register("bob", "bob@example.com");        verify(userRepository).save(captor.capture());        assertThat(captor.getValue().getUsername()).isEqualTo("bob");        assertThat(captor.getValue().getEmail()).isEqualTo("bob@example.com");    }}

@ParameterizedTest@ValueSource(ints = {0, -1, -100})void should_reject_non_positive_amount(int amount) {    assertThatThrownBy(() -> service.withdraw(amount))            .isInstanceOf(IllegalArgumentException.class);}@ParameterizedTest@CsvSource({        "1, alice, 10.0",        "2, bob,   0.0",        "3, carol, -5.5"})void should_parse_order_line(long id, String name, BigDecimal amount) {    Order order = parser.parse(id + "," + name + "," + amount);    assertThat(order.getId()).isEqualTo(id);}@ParameterizedTest@MethodSource("provideBoundaryCases")void should_classify_boundary(Order order, Level expected) {    assertThat(classifier.classify(order)).isEqualTo(expected);}static Stream<Arguments> provideBoundaryCases() {    return Stream.of(            Arguments.of(new Order(1L, new BigDecimal("0")), Level.LOW),            Arguments.of(new Order(2L, new BigDecimal("100")), Level.MID),            Arguments.of(new Order(3L, new BigDecimal("9999")), Level.HIGH)    );}

<plugin>    <groupId>org.jacoco</groupId>    <artifactId>jacoco-maven-plugin</artifactId>    <version>0.8.11</version>    <executions>        <execution>            <id>prepare-agent</id>            <goals><goal>prepare-agent</goal></goals>        </execution>        <execution>            <id>report</id>            <phase>verify</phase>            <goals><goal>report</goal></goals>        </execution>    </executions></plugin>

package com.example.demo.service;import java.math.BigDecimal;public class Order {    private final long id;    private final BigDecimal amount;    public Order(long id, String amount) { this.id = id; this.amount = new BigDecimal(amount); }    public long getId() { return id; }    public BigDecimal getAmount() { return amount; }}

package com.example.demo.service;public interface ShippingClient {    BigDecimal quote(long orderId);   // 真实场景要打外部物流系统，所以必须 Mock}

package com.example.demo.service;import java.math.BigDecimal;public class OrderService {    private static final BigDecimal FREE_THRESHOLD = new BigDecimal("100");    private final ShippingClient shippingClient;    public OrderService(ShippingClient shippingClient) { this.shippingClient = shippingClient; }    /** 满 100 免运费：直接返回 0，并且根本不去询价 */    public BigDecimal freight(Order order) {        if (order.getAmount().compareTo(FREE_THRESHOLD) >= 0) {            return BigDecimal.ZERO;        }        return shippingClient.quote(order.getId());    }}

package com.example.demo.service;import org.junit.jupiter.api.BeforeEach;import org.junit.jupiter.api.Test;import org.junit.jupiter.api.extension.ExtendWith;import org.junit.jupiter.params.ParameterizedTest;import org.junit.jupiter.params.provider.CsvSource;import org.mockito.InjectMocks;import org.mockito.Mock;import org.mockito.junit.jupiter.MockitoExtension;import java.math.BigDecimal;import static org.assertj.core.api.Assertions.assertThat;import static org.mockito.ArgumentMatchers.anyLong;import static org.mockito.Mockito.never;import static org.mockito.Mockito.verify;import static org.mockito.Mockito.when;@ExtendWith(MockitoExtension.class)class FreightRuleTest {    @Mock    private ShippingClient shippingClient;      // 外部物流系统：假扮即可    @InjectMocks    private OrderService orderService;          // 被测对象，Mockito 把上面的 @Mock 注进来    @ParameterizedTest(name = "金额 {0} 应付运费 {1}")    @CsvSource({"99.99, 8.00", "100.00, 0", "100.01, 0"})    void should_charge_freight_below_threshold_only(String amount, String expected) {        when(shippingClient.quote(1L)).thenReturn(new BigDecimal("8.00"));        BigDecimal freight = orderService.freight(new Order(1L, amount));        assertThat(freight).isEqualByComparingTo(expected);    }    @Test    void should_not_call_shipping_client_when_amount_reaches_threshold() {        BigDecimal freight = orderService.freight(new Order(7L, "100"));        assertThat(freight).isEqualByComparingTo("0");        verify(shippingClient, never()).quote(anyLong());   // 关键：证明「根本没去询价」    }}

[INFO] Tests run: 4, Failures: 0, Errors: 0, Skipped: 0FreightRuleTest ✔  金额 99.99 应付运费 8.00                  ✔  金额 100.00 应付运费 0                  ✔  金额 100.01 应付运费 0                  ✔  should_not_call_shipping_client_when_amount_reaches_threshold