Realm Unit Test on Android

Question

Trying to unit test a class that does some calls to Realm (0.87.4), the test setup fails with

java.lang.NoClassDefFoundError: rx/Observable$OnSubscribe
at io.realm.RealmConfiguration$Builder.<init>(RealmConfiguration.java:279)
at org.testapp.db.MyClassTest.setUp(MyClassTest.java:34)
... 
Caused by: java.lang.ClassNotFoundException: rx.Observable$OnSubscribe
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)

My test class starts with:

@RunWith(MockitoJUnitRunner.class)
public class MyClassTest extends TestCase {

@Rule
public TemporaryFolder testFolder = new TemporaryFolder();

Realm realm;

@Before
public void setUp() throws Exception {
    File tempFolder = testFolder.newFolder("realmdata");
    RealmConfiguration config = new RealmConfiguration.Builder(tempFolder).build();

    realm = Realm.getInstance(config);
}
...

My gradle has:

testCompile 'junit:junit:4.12'
testCompile "org.mockito:mockito-core:1.10.19"
testCompile "org.robolectric:robolectric:3.0"
compile 'io.realm:realm-android:0.87.4'

How to solve this?

=== EDIT 1 ===

I added to my gradle:

testCompile 'io.reactivex:rxjava:1.1.0'

and

android {
  // ...
  testOptions { 
    unitTests.returnDefaultValues = true
  }
}

the new error is

java.lang.UnsatisfiedLinkError: no realm-jni in java.library.path
at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1886)
at java.lang.Runtime.loadLibrary0(Runtime.java:849)
at java.lang.System.loadLibrary(System.java:1088)
at io.realm.internal.RealmCore.loadLibrary(RealmCore.java:117)

Answer 1

Unit tests are hard or impossible when using Realm in the class that you are testing (thanks Dmitry for mentioning). What I can do is run the tests as instrumental tests (thanks Dmitry, Christian).

And that is quite easy, I won't have to change anything to the test methods...

A. Move the test class into an "androidTest" folder, instead of "test". (Since Android Studio 1.1 you should put your Unit tests in /src/test and Android Instrumentation Tests in /src/androidTest)

B. Add the dependencies for instrumental tests in the gradle build file, use "androidTest" because they're instrumental:

androidTestCompile 'junit:junit:4.12'
androidTestCompile 'io.reactivex:rxjava:1.1.0'
androidTestCompile 'com.android.support.test:runner:0.4.1'
androidTestCompile 'com.android.support.test:rules:0.4.1'
androidTestCompile 'com.android.support:support-annotations:23.1.1'

C. In the test class, replace the runner at the top with AndroidJUnit4:

@RunWith(AndroidJUnit4.class)
public class MyClassTest extends TestCase {
...

Create an Android run configuration of type "Android Tests", run it and voila, it will test the same methods fine now, but on a device. Makes me very happy.

Answer 2

Since Realm 0.87 you also need to include RxJava to your dependencies:

compile 'io.reactivex:rxjava:1.1.0'