Android测试：Dagger注入ViewModel的Fragment集成测试报空指针异常

现有实现 #

Fragment中获取ViewModel的代码 #

Fragment通过lazy委托从导航图的ViewModelStore中获取实例：

private val myViewModel: MyViewModel by lazy {
        ViewModelProvider(findNavController().getBackStackEntry(R.id.nav_graph), Factory(this) { stateHandle ->
            (activity as MainActivity).myComponent.myViewModel()
                .create(stateHandle)}).get(MyViewModel::class.java)
    }

ViewModel的实现 #

ViewModel使用@AssistedInject和@AssistedFactory处理SavedStateHandle：

class MyViewModel @AssistedInject constructor(
    private val myRepository: MyRepository,
    @Assisted val savedStateHandle: SavedStateHandle
    ) : ViewModel() {

    @AssistedFactory
    interface MyViewModelFactory : ViewModelProvider.Factory {
        fun create(savedStateHandle: SavedStateHandle) : MyViewModel
    }

    // 其他ViewModel代码
}

测试报错信息 #

应用运行正常，但集成测试时抛出空指针异常：

java.lang.NullPointerException: Parameter specified as non-null is null: method androidx.lifecycle.ViewModelProvider.<init>, parameter owner
at androidx.lifecycle.ViewModelProvider.<init>(Unknown Source:2)

原因是findNavController().getBackStackEntry(R.id.nav_graph)返回null，导致ViewModelProvider的owner参数为空。

当前测试代码 #

@RunWith(AndroidJUnit4::class)
class MyFragmentTest {

    @get:Rule
    var activityRule = ActivityScenarioRule(MainActivity::class.java)

    private lateinit var scenario: FragmentScenario<MyFragment>
    private lateinit var binding: FragmentMyBinding
    private lateinit var mockNavController : NavController
    
    @Test
    fun setUp() {
        mockNavController = mock(NavController::class.java)
        scenario = launchFragmentInContainer<MyFragment> {
            MyFragment().also { fragment ->
                fragment.viewLifecycleOwnerLiveData.observeForever { viewLifecycleOwner ->
                    if (viewLifecycleOwner != null) {
                        // The fragment’s view has just been created
                        mockNavController.setGraph(R.navigation.nav_graph)
                        Navigation.setViewNavController(fragment.requireView(), mockNavController)
                    }
                }
            }
        }
    }

解决方案 #

1. 修正测试方法的生命周期注解 #

setUp是JUnit的初始化方法，不能用@Test标注，应该替换为@Before，并单独编写测试方法。

2. 模拟有效的BackStackEntry #

当前mock的NavController没有返回有效的BackStackEntry，这是导致NPE的核心原因。需要mockSavedStateViewModelStoreOwner（BackStackEntry的父接口），并让NavController返回该实例。

3. 注入Mock ViewModel #

测试中无需依赖真实的Dagger组件，直接提供mock的ViewModel实例，隔离底层依赖。

修改后的测试代码示例：

@RunWith(AndroidJUnit4::class)
class MyFragmentTest {

    private lateinit var mockNavController: NavController
    private lateinit var mockBackStackEntry: SavedStateViewModelStoreOwner
    private lateinit var mockViewModel: MyViewModel

    @Before
    fun setUp() {
        // 初始化所有mock对象
        mockNavController = mock(NavController::class.java)
        mockBackStackEntry = mock(SavedStateViewModelStoreOwner::class.java)
        mockViewModel = mock(MyViewModel::class.java)

        // 让NavController返回mock的BackStackEntry
        `when`(mockNavController.getBackStackEntry(R.id.nav_graph)).thenReturn(mockBackStackEntry)

        // 为mock的BackStackEntry提供有效的ViewModelStore
        val viewModelStore = ViewModelStore()
        `when`(mockBackStackEntry.viewModelStore).thenReturn(viewModelStore)
    }

    @Test
    fun testFragmentUIElements() {
        // 启动Fragment并设置导航控制器
        launchFragmentInContainer<MyFragment> {
            viewLifecycleOwnerLiveData.observeForever { viewLifecycleOwner ->
                if (viewLifecycleOwner != null) {
                    Navigation.setViewNavController(requireView(), mockNavController)
                    // 将mock的ViewModel注入到导航栈的ViewModelStore中
                    ViewModelProvider(mockBackStackEntry, object : ViewModelProvider.Factory {
                        override fun <T : ViewModel> create(modelClass: Class<T>): T {
                            return mockViewModel as T
                        }
                    }).get(MyViewModel::class.java)
                }
            }
        }

        // 示例：验证UI控件是否正常显示
        onView(withId(R.id.your_ui_element_id)).check(matches(isDisplayed()))
    }
}

关键说明 #

• 生命周期修正：@Before注解的方法会在每个测试用例执行前初始化mock对象，确保测试环境一致。
• BackStackEntry模拟：必须让getBackStackEntry返回非null实例，否则ViewModelProvider无法初始化。这里通过mockSavedStateViewModelStoreOwner并提供ViewModelStore解决。
• Mock ViewModel注入：直接使用mock的ViewModel可以隔离Repository等依赖，让测试专注于UI逻辑验证，无需启动完整的Dagger组件。

内容的提问来源于stack exchange，提问作者MajinKenn