1.Android 单元测试

AndroidX Test 的 JUnit4 规则
可以查看官方文档，Test怎么写。

2.AndroidTest获取context 为何为空

2.1 context可以获取到，但是为空

//这样可以获取context
Context context = ApplicationProvider.getApplicationContext();

2.2 ApplicationProvider 源码

查看源码，跟网上的一样，InstrumentationRegistry.getInstrumentation().getTargetContext() 。

/**
 * Provides ability to retrieve the current application {@link Context} in tests.
 *
 * <p>This can be useful if you need to access the application assets (eg
 * <i>getApplicationContext().getAssets()</i>), preferences (eg
 * <i>getApplicationContext().getSharedPreferences()</i>), file system (eg
 * <i>getApplicationContext().getDir()</i>) or one of the many other context APIs in test.
 */
public final class ApplicationProvider {

  private ApplicationProvider() {}

  /**
   * Returns the application {@link Context} for the application under test.
   *
   * @see {@link Context#getApplicationContext()}
   */
  @SuppressWarnings("unchecked")
  public static <T extends Context> T getApplicationContext() {
    return (T)
        InstrumentationRegistry.getInstrumentation().getTargetContext().getApplicationContext();
  }
}

2.3 Instrumentation源码

我们查看Instrumentation 的源码，发现他mAppContext 也没初始化，获取当然就是空。

public class Instrumentation {
...
    /*package*/ final void init(ActivityThread thread,
            Context instrContext, Context appContext, ComponentName component, 
            IInstrumentationWatcher watcher, IUiAutomationConnection uiAutomationConnection) {
        mThread = thread;
        mMessageQueue = mThread.getLooper().myQueue();
        mInstrContext = instrContext;
        mAppContext = appContext;
        mComponent = component;
        mWatcher = watcher;
        mUiAutomationConnection = uiAutomationConnection;
    }

	//直接返回mAppContext
    public Context getTargetContext() {
        return mAppContext;
    }
...
}

3.正确获取方法

3.1 查看官网文档

查看 AndroidJUnitRunner 文档。
原来是需要在应用中 创建了 Application 的自定义子类，才可以用此方法获取context。

3.2 创建application

在AndroidTest目录下创建Application。

public class TestApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
    }
}

这时候获取就不为空啦。

@RunWith(AndroidJUnit4.class)
public class MyTest   {
    @Test
    public void test() {
        Context context = ApplicationProvider.getApplicationContext();
    }
}