如何为含System.getenv的构造函数实现JUnit5全测试覆盖

问题背景 #

需要为包含System.getenv()调用的Java代码实现100%JUnit5测试覆盖，但原代码中环境变量读取逻辑在构造函数内，导致测试配置无法提前生效，无法覆盖envWord == null ? "default" : envWord的两个分支。

原代码实现 #

ServiceToTest类 #

@Service
public class ServiceToTest {

    private final CustomContainer customContainer;

    public ServiceToTest() {
        Object configuration = new Object();
        String envWord = System.getenv("envword");
        this.customContainer = new CustomContainer(configuration, envWord == null ? "default" : envWord);

    }

    public String getContainerName() {
        return customContainer.getContainerName();
    }

}

CustomContainer类 #

public class CustomContainer {
    @Getter
    String containerName;
    Object configuration;

    public CustomContainer(Object configuration, String containerName) {
        this.configuration = configuration;
        this.containerName = containerName;
    }

}

尝试过的无效方案 #

曾使用ReflectionTestUtils、@SystemStubsExtension、Spy等方式，但由于构造函数执行时机早于测试配置，无法控制System.getenv()的返回值，导致无法覆盖空值分支。

重构方案（实现100%覆盖） #

通过引入抽象接口解耦环境变量读取和对象创建逻辑，实现测试时的Mock控制。

1. CustomContainerFactory接口及实现 #

CustomContainerFactory接口

public interface CustomContainerFactory {
    CustomContainer create(Object configuration, String name);
}

CustomContainerFactoryImpl实现类

@Service
public class CustomContainerFactoryImpl implements CustomContainerFactory {

    @Override
    public CustomContainer create(Object configuration, String name) {
        return new CustomContainer(configuration, name);
    }

}

2. EnvironmentAccessor接口及实现 #

EnvironmentAccessor接口

public interface EnvironmentAccessor {
    String getEnv(String name);
}

EnvironmentAccessorImpl实现类

@Service
public class EnvironmentAccessorImpl implements EnvironmentAccessor {

    @Override
    public String getEnv(String name) {
        return System.getenv(name);
    }

}

3. 重构后的ServiceToTest类 #

@Service
public class ServiceToTest {

    private final CustomContainer customContainer;
    
    public ServiceToTest(EnvironmentAccessor environmentAccessor, CustomContainerFactory customContainerFactory) {
        Object configuration = new Object();
        String envWord = environmentAccessor.getEnv("anything");
        this.customContainer = customContainerFactory.create(configuration, envWord == null ? "default" : envWord);

    }

    public String getContainerName() {
        return customContainer.getContainerName();
    }

}

优化后的测试类 #

@ExtendWith(MockitoExtension.class)
class TestService {

    private static CustomContainer mockCustomContainer = mock(CustomContainer.class);
    private static CustomContainerFactory customContainerFactoryMock = mock(CustomContainerFactoryImpl.class);
    private static EnvironmentAccessor environmentAccessorMock = mock(EnvironmentAccessorImpl.class);

    private static ServiceToTest serviceToTest;

    @BeforeAll
    static void setup() {
        when(environmentAccessorMock.getEnv(anyString())).thenReturn("hi");
        when(customContainerFactoryMock.create(any(), anyString())).thenReturn(mockCustomContainer);
        serviceToTest = new ServiceToTest(environmentAccessorMock, customContainerFactoryMock);

    }

    @Test
    void testGetContainerNameNotNull() {
        assertNotNull(serviceToTest.getContainerName());
    }

    @Test
    void coverNullReturnFromGetEnv() {
        when(environmentAccessorMock.getEnv(anyString())).thenReturn(null);
        assertAll(() -> new ServiceToTest(environmentAccessorMock, customContainerFactoryMock));
    }

}

重构后通过Mock抽象接口，可分别控制环境变量的返回值，完整覆盖了分支逻辑，实现100%测试覆盖。

内容的提问来源于stack exchange，提问作者BugsOverflow