测试中FileInputStream实例始终为null？已用工厂类规避PowerMock

Let's break down why you're seeing f as null and how to fix it:

Root Causes #

1. Missing Mock Initialization #

You're using @RunWith(SpringRunner.class), but this runner doesn't automatically handle Mockito's @Mock and @InjectMocks annotations. Without initializing Mockito, your mocked objects stay null, so when CodesConverterService tries to use fileInputStreamFactory, it's actually operating on a null reference.

2. Unresolved Constructor Dependency #

Look at your CodesConverterService constructor:

public CodesConverterService(FileInputStreamFactory fileInputStreamFactory, YamlFactory yamlFactory) {
    this.fileInputStreamFactory = fileInputStreamFactory;
}

It requires two parameters, but your test only mocks FileInputStreamFactory. Mockito's @InjectMocks can't instantiate CodesConverterService without a mock for YamlFactory, so the service itself might not be properly initialized either.

Fixes #

Option 1: Initialize Mockito in JUnit 4 #

Add a setup method to initialize Mockito's annotations, and mock the missing YamlFactory:

@RunWith(SpringRunner.class)
public class CodesConverterServiceTest {
    @InjectMocks
    private CodesConverterService codesConverterService;
    @Mock
    private FileInputStreamFactory fileInputStreamFactory;
    @Mock
    private YamlFactory yamlFactory; // Add this missing mock
    @Mock
    private FileInputStream fileInputStream;

    @Before
    public void setUp() {
        MockitoAnnotations.openMocks(this); // Initialize Mockito annotations
    }

    @Test
    public void whenLoadMappingsFromSource_GoodPath() throws FileNotFoundException {
        // Stub the mock behavior
        when(fileInputStreamFactory.getStream("mappingFile")).thenReturn(fileInputStream);
        
        // Execute the method
        this.codesConverterService.loadMappingsFromSource();
        
        // Verify the mock was called (optional but good practice)
        verify(fileInputStreamFactory).getStream("mappingFile");
    }
}

Option 2: Use Mockito's Runner Directly #

Replace SpringRunner with MockitoJUnitRunner (this handles annotation initialization automatically):

@RunWith(MockitoJUnitRunner.class) // Switch to Mockito's runner
public class CodesConverterServiceTest {
    @InjectMocks
    private CodesConverterService codesConverterService;
    @Mock
    private FileInputStreamFactory fileInputStreamFactory;
    @Mock
    private YamlFactory yamlFactory; // Add this missing mock
    @Mock
    private FileInputStream fileInputStream;

    @Test
    public void whenLoadMappingsFromSource_GoodPath() throws FileNotFoundException {
        when(fileInputStreamFactory.getStream("mappingFile")).thenReturn(fileInputStream);
        this.codesConverterService.loadMappingsFromSource();
        
        // Verify interaction
        verify(fileInputStreamFactory).getStream("mappingFile");
    }
}

Why This Works #

• Adding YamlFactory as a mock satisfies the constructor dependency, letting @InjectMocks properly create CodesConverterService.
• Initializing Mockito ensures your mocked objects are created and injected into the service, so when fileInputStreamFactory.getStream() is called, it uses your stubbed return value instead of trying to create a real FileInputStream.

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