1. build.gradle文件添加依赖
android {
    defaultConfig {
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    testImplementation 'junit:junit:4.+'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.0'
    androidTestImplementation "org.mockito:mockito-core:2.8.9"
    androidTestImplementation "org.mockito:mockito-android:2.8.9"
    androidTestImplementation 'com.android.support.test:rules:1.0.2'
}

2. AndroidManifest文件添加权限
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.xx.xxx" >
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
</manifest>

3. 通过GrantPermissionRule申请权限
@RunWith(AndroidJUnit4.class)
public class EmptyTest {

    @Rule
    public GrantPermissionRule mRuntimePermissionRule = GrantPermissionRule.grant(
            Manifest.permission.WRITE_EXTERNAL_STORAGE,
        	Manifest.permission.INTERNET);

    @Test
    public void testEmpty() {
        String text = "";
        Assert.assertTrue(TextUtils.isEmpty(text));
    }
}