Android 上的 Mockito、Context.getString(id) 和 NullPointerException

Question

我刚开始学习一个 Mockito 测试框架，我按照官方教程从：developer.android.com

代码是：

private static final String FAKE_STRING = "HELLO WORLD";

@Mock
Context mMockContext;

@Test
public void readStringFromContext_LocalizedString() {
    // Given a mocked Context injected into the object under test...
    when(mMockContext.getString(R.string.hello_world))
            .thenReturn(FAKE_STRING);
    ClassUnderTest myObjectUnderTest = new ClassUnderTest(mMockContext);

    // ...when the string is returned from the object under test...
    String result = myObjectUnderTest.getHelloWorldString();

    // ...then the result should be the expected one.
    assertThat(result, is(FAKE_STRING));
}

我写了以下ClassUnderTest：

public class ClassUnderTest {

private Context context;

public ClassUnderTest(Context context)
{
    this.context=context;
}

public String getHelloWorldString()
{
    return context.getString(R.string.hello_world);
}

}

运行后我得到 NullPointerException：

    java.lang.NullPointerException
at android.content.Context.getHelloWorldString(Context.java:293)
at com.darekk.draggablelistview.unit_tests.MockitoTest.readStringFromContext_LocalizedString(MockitoTest.java:46)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.mockito.internal.runners.JUnit45AndHigherRunnerImpl.run(JUnit45AndHigherRunnerImpl.java:37)
at org.mockito.runners.MockitoJUnitRunner.run(MockitoJUnitRunner.java:62)
at org.junit.runners.Suite.runChild(Suite.java:128)
at org.junit.runners.Suite.runChild(Suite.java:27)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
at android.support.test.internal.runner.TestExecutor.execute(TestExecutor.java:59)
at android.support.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:262)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1619)

当我用我自己的类替换 android Context 时，它可以工作。

平台：IntelliJ IDEA 社区版 15.0.6

非常感谢任何帮助。

Answer 1

这段代码应该是你测试的第一行：

doReturn("Sample Hello world string")
            .when(mMockContext)
            .getString(any(Integer.class));

需要一个方法的存根

Answer 2

您似乎使用的是来自developer.android.com 的确切代码。显然他们发布了一个不完整的示例..将此方法添加到您的测试中：

@Before
public void setup(){
   MockitoAnnotations.initMocks(this);
}

更多参考结帐Mockito's docs

Answer 3

Context.getString() 是最终方法，因此默认情况下您不能使用 Mockito 模拟它。 要启用最终方法的模拟，您需要 Mockito v2 并按照此处的说明进行操作 Mock the unmockable: opt-in mocking of final classes/methods

在src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker 中创建一个包含一行的文件：

mock-maker-inline

一旦你这样做了，你就可以模拟 getString() when(mMockContext.getString(R.string.string_name)).thenReturn("Mocked String");

Answer 4

在您的 build.gradle 文件中添加以下依赖项

testImplementation "org.powermock:powermock:1.6.5"

运行上面的代码，它将开始工作。