多线程环境中使用 Mockito 来 Mock 静态方法

calendar Oct 10, 2024   ---   · 3 min read · mockito JUnit5  ·
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回看三年前的一篇日志 Mockito 3.4.0 开始可 Mock  静态方法，最后对 Mockito 产生的缺憾是它无法用来 Mock 非测试线程(主线程)中的静态方法调用。其实这也是可以变通的，下面慢慢道来。


首先回顾一下 Mockito  的静态方法 Mock 的使用方法，随着 Mockito 版本的升级，引入依赖的方式也发生了些许的变化，以 Maven 项目为例，如果在 JUnit 5 下用 Mockito 的 pom.xml 依赖中为

1<dependency>
2    <groupId>org.mockito</groupId>
3    <artifactId>mockito-junit-jupiter</artifactId>
4    <version>5.14.1</version>
5    <scope>test</scope>
6</dependency>

由它引入的全部相关依赖
如今不再使用 mockito-inline 依赖了，以及它的 mockito-extensions/org.mockito.plugins.MockMaker(mock-make-inline) 和 mockito-extensions/org.mockito.plugins.MemberAccessor(memer-accessor-module)。

测试代码
 1public class MockitoDemoTest {
 2
 3    @Test
 4    public void testMockitoMockStatic() {
 5        LocalDate yearOf1970 = LocalDate.of(1970, 1, 1);
 6        try (MockedStatic<LocalDate> theMock = Mockito.mockStatic(LocalDate.class)) {
 7            theMock.when(() -> LocalDate.of(anyInt(), anyInt(), anyInt())).thenReturn(yearOf1970);
 8
 9            LocalDate localDate = LocalDate.of(2024, 10, 10);
10            assertEquals(1970, localDate.getYear());   // Ok
11        }
12    }
13}

Mock 了静态方法 LocalDate.of(year, month, day), 所以在 try(MockedStatic) 块中无论传入什么参数给 LocalDate.of() 方法返回的都是 1970-01-01

try(MockedStatic) 块只对它所在的线程起作用，如果 LocalDate.of() 是在其他线程中调用就无能为力了。

比如用下方的代码演示非主线程(测试线程)中调用 LocalDate.of()时不能被 Mock 的效果
 1public class MockitoDemoTest {
 2
 3    @Test
 4    public void testMockitoMockStatic() {
 5        LocalDate yearOf1970 = LocalDate.of(1970, 1, 1);
 6        try (MockedStatic<LocalDate> theMock = Mockito.mockStatic(LocalDate.class)) {
 7            theMock.when(() -> LocalDate.of(anyInt(), anyInt(), anyInt())).thenReturn(yearOf1970);
 8
 9            AtomicReference<LocalDate> localDateRef = new AtomicReference<>();
10            Thread thread = new Thread(() -> localDateRef.set(LocalDate.of(2024, 10, 10)));
11            thread.start();
12            thread.join();
13
14            assertEquals(1970, localDateRef.get().getYear());  // Failed
15        } catch (InterruptedException e) {
16            throw new RuntimeException(e);
17        }
18    }
19}

这个测试就会失败了
MockitoDemoTest.testMockitoMockStatic:49 expected: <1970> but was: <2024>
不过实际应用中我们多用线程池，不是说 try(MockedStatic) 只对它所在的线程起作用吗，那么我们可以对线程池来个腾龙换鸟，把主线程作为线程池中的唯一线程，这样所有的代码都会在 try(MockedStatic) 所在的线程(主线程)中执行了。

为演示全过程，先创建一个待测试的类 MockitoDemo
 1public class MockitoDemo {
 2
 3    private ExecutorService threadPool;
 4
 5    public MockitoDemo(ExecutorService threadPool) {
 6        this.threadPool = threadPool;
 7    }
 8
 9    public LocalDate getLocalDate() {
10        Future<LocalDate> future = threadPool.submit(() -> LocalDate.of(2024, 10, 10));
11        try {
12            return future.get();
13        } catch (Exception e) {
14            throw new RuntimeException(e);
15        }
16    }
17}

如果不管不顾线程池的角色，写如下的测试代码
 1public class MockitoDemoTest {
 2
 3    @Test
 4    public void testMockitoMockStatic() {
 5        MockitoDemo mockitoDemo = new MockitoDemo(Executors.newFixedThreadPool(2));
 6
 7        LocalDate yearOf1970 = LocalDate.of(1970, 1, 1);
 8        try (MockedStatic<LocalDate> theMock = Mockito.mockStatic(LocalDate.class)) {
 9            theMock.when(() -> LocalDate.of(anyInt(), anyInt(), anyInt())).thenReturn(yearOf1970);
10            LocalDate localDate = mockitoDemo.getLocalDate();
11            assertEquals(1970, localDate.getYear());  // Failed
12        }
13    }
14}

测试也将无法通过
MockitoDemoTest.testMockitoMockStatic:46 expected: <1970> but was: <2024>
变通要来了，针对线程池下手，用当前线程作为唯一线程的线程池替代它就行，完整的实现是
 1public class MockitoDemoTest {
 2
 3    @Test
 4    public void testMockitoMockStatic() {
 5        ThreadPoolExecutor threadPool = new ThreadPoolExecutor(1, 1, 0, TimeUnit.SECONDS, new LinkedBlockingQueue<>()) {
 6            @Override
 7            public void execute(Runnable command) {
 8                command.run();
 9            }
10        };
11        MockitoDemo mockitoDemo = new MockitoDemo(threadPool);
12
13        LocalDate yearOf1970 = LocalDate.of(1970, 1, 1);
14        try (MockedStatic<LocalDate> theMock = Mockito.mockStatic(LocalDate.class)) {
15            theMock.when(() -> LocalDate.of(anyInt(), anyInt(), anyInt())).thenReturn(yearOf1970);
16            LocalDate localDate = mockitoDemo.getLocalDate();
17            assertEquals(1970, localDate.getYear());   // Ok
18        }
19    }
20}

测试通过

主要的行为就是覆盖了 execute(Runnable command) 方法，直接执行 command.run(), 所起的作用就是提交到 threadPool 的任务全部直接调用创建该 threadPool 实例的线程去执行，即实现了一个只有主线程(测试线程)的线程池，从而使得 try(MockedStatic) 与 MockitoDemo.getLocalDate() 中的 LocalDate.of(2024, 10, 10) 由同一个线程执行。

话外

Maven 中使用当前的 Mockito 5.14.1 版本时，运行 mvn test 会看到如下控制台输出

Mockito is currently self-attaching to enable the inline-mock-maker. This will no longer work in future releases of the JDK. Please add Mockito as an agent to your build what is described in Mockito's documentation: https://javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/Mockito.html#0.3
此篇测试用的 JDK 版本为 21, 参考信息所指引的链接 https://javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/Mockito.html#0.3，在 pom.xml 中加上
 1 <plugin>
 2     <groupId>org.apache.maven.plugins</groupId>
 3     <artifactId>maven-dependency-plugin</artifactId>
 4     <executions>
 5         <execution>
 6             <goals>
 7                 <goal>properties</goal>
 8             </goals>
 9         </execution>
10     </executions>
11 </plugin>
12 <plugin>
13     <groupId>org.apache.maven.plugins</groupId>
14     <artifactId>maven-surefire-plugin</artifactId>
15     <configuration>
16         <argLine>@{argLine} -javaagent:${org.mockito:mockito-core:jar}</argLine>
17     </configuration>
18 </plugin>

再次运行 mvn test, 提示信息便消失了。 永久链接 https://yanbin.blog/mockito-mock-static-method-in-multiple-threading-env/, 来自 隔叶黄莺 Yanbin's Blog
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