运行改造API单元测试时出现IllegalArgumentException

Question

尝试运行单元测试时，出现以下IllegalArgumentException：

java.lang.IllegalArgumentException: in == null

at okio.Okio.source(Okio.java:128)
at okio.Okio.source(Okio.java:124)
at com.example.SearchApiTest.enqueueResponse(SearchApiTest.kt:57)
at com.example.SearchApiTest.enqueueResponse$default(SearchApiTest.kt:54)
at com.example.SearchApiTest.searchLocations(SearchApiTest.kt:48)

错误消息似乎表明该错误发生在行上：

val source = Okio.buffer(Okio.source(inputStream))

这是我下面的单元测试：

@RunWith(JUnit4::class)
class SearchApiTest {
    @Rule
    @JvmField
    val instantExecutorRule = InstantTaskExecutorRule()

    private lateinit var api: SearchApi

    private lateinit var mockWebServer: MockWebServer

    @Before
    fun createService() {
        mockWebServer = MockWebServer()
        api = Retrofit.Builder()
                .baseUrl(mockWebServer.url("/"))
                .addConverterFactory(GsonConverterFactory.create())
                .addCallAdapterFactory(LiveDataCallAdapterFactory())
                .build()
                .create(SearchApi::class.java)
    }

    @After
    fun stopService() {
        mockWebServer.shutdown()
    }

    @Test
    fun searchLocations() {
        enqueueResponse("searchLocations.json")
        val value = getValue(api.searchLocations("usa"))
        val locations = (value as ApiSuccessResponse).body
        assertThat(locations.totalResults, `is`(20))
    }

    private fun enqueueResponse(fileName: String, headers: Map<String, String> = emptyMap()) {
        val inputStream = javaClass.classLoader
                .getResourceAsStream("api-response/$fileName")
        val source = Okio.buffer(Okio.source(inputStream))
        val mockResponse = MockResponse()
        for ((key, value) in headers) {
            mockResponse.addHeader(key, value)
        }
        mockWebServer.enqueue(
                mockResponse
                        .setBody(source.readString(Charsets.UTF_8))
        )
    }
}