page.title=Building Local Unit Tests page.tags=testing,androidjunitrunner,junit,unit test,mock trainingnavtop=true @jd:body

Dependencies and Prerequisites

This lesson teaches you to

  1. Set Up Your Testing Environment
  2. Create a Local Unit Test Class
  3. Run Local Unit Tests

Try it out

If your unit test has no dependencies or only has simple dependencies on Android, you should run your test on a local development machine. This testing approach is efficient because it helps you avoid the overhead of loading the target app and unit test code onto a physical device or emulator every time your test is run. Consequently, the execution time for running your unit test is greatly reduced. With this approach, you normally use a mocking framework, like Mockito, to fulfill any dependency relationships.

Android Plug-in for Gradle version 1.1.0 and higher allows you to create a source directory ({@code src/test/java}) in your project to store JUnit tests that you want to run on a local machine. This feature improves your project organization by letting you group your unit tests together into a single source set. You can run the tests from Android Studio or the command-line, and the plugin executes them on the local Java Virtual Machine (JVM) on your development machine.

Set Up Your Testing Environment

Before building local unit tests, you must:

Create a Local Unit Test Class

Your local unit test class should be written as a JUnit 4 test class. JUnit is the most popular and widely-used unit testing framework for Java. The latest version of this framework, JUnit 4, allows you to write tests in a cleaner and more flexible way than its predecessor versions. Unlike the previous approach to Android unit testing based on JUnit 3, with JUnit 4, you do not need to extend the {@code junit.framework.TestCase} class. You also do not need to prefix your test method name with the {@code ‘test’} keyword, or use any classes in the {@code junit.framework} or {@code junit.extensions} package.

To create a basic JUnit 4 test class, create a Java class that contains one or more test methods. A test method begins with the {@code @Test} annotation and contains the code to exercise and verify a single functionality in the component that you want to test.

The following example shows how you might implement a local unit test class. The test method {@code emailValidator_CorrectEmailSimple_ReturnsTrue} verifies that the {@code isValidEmail()} method in the app under test returns the correct result.

import org.junit.Test;
import java.util.regex.Pattern;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

public class EmailValidatorTest {

    @Test
    public void emailValidator_CorrectEmailSimple_ReturnsTrue() {
        assertThat(EmailValidator.isValidEmail("name@email.com"), is(true));
    }
    ...
}

To test that components in your app return the expected results, use the junit.Assert methods to perform validation checks (or assertions) to compare the state of the component under test against some expected value. To make tests more readable, you can use Hamcrest matchers (such as the {@code is()} and {@code equalTo()} methods) to match the returned result against the expected result.

In your JUnit 4 test class, you can use annotations to call out sections in your test code for special processing, such as:

Mocking Android dependencies

By default, the Android Plug-in for Gradle executes your local unit tests against a modified version of the {@code android.jar} library, which does not contain any actual code. Instead, method calls to Android classes from your unit test throw an exception.

You can use a mocking framework to stub out external dependencies in your code, to easily test that your component interacts with a dependency in an expected way. By substituting Android dependencies with mock objects, you can isolate your unit test from the rest of the Android system while verifying that the correct methods in those dependencies are called. The Mockito mocking framework for Java (version 1.9.5 and higher) offers compatibility with Android unit testing. With Mockito, you can configure mock objects to return some specific value when invoked.

To add a mock object to your local unit test using this framework, follow this programming model:

  1. Include the Mockito library dependency in your {@code build.gradle} file, as described in Set Up Your Testing Environment.
  2. At the beginning of your unit test class definition, add the {@code @RunWith(MockitoJUnitRunner.class)} annotation. This annotation tells the Mockito test runner to validate that your usage of the framework is correct and simplifies the initialization of your mock objects.
  3. To create a mock object for an Android dependency, add the {@code @Mock} annotation before the field declaration.
  4. To stub the behavior of the dependency, you can specify a condition and return value when the condition is met by using the {@code when()} and {@code thenReturn()} methods.

The following example shows how you might create a unit test that uses a mock {@link android.content.Context} object. 

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.CoreMatchers.*;
import static org.mockito.Mockito.*;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.runners.MockitoJUnitRunner;
import android.content.SharedPreferences;

@RunWith(MockitoJUnitRunner.class)
public class UnitTestSample {

    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_word))
                .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));
    }
}

To learn more about using the Mockito framework, see the Mockito API reference and the {@code SharedPreferencesHelperTest} class in the sample code.

Run Local Unit Tests

The Android Plug-in for Gradle provides a default directory ({@code src/test/java}) for you to store unit test classes that you want to run on a local JVM. The plug-in compiles the test code in that directory and then executes the test app locally using the default test runner class.

As with production code, you can create unit tests for a specific flavor or build type. You should keep unit tests in a test source tree location that corresponds to your production source tree, such as:
Path to Production Class Path to Local Unit Test Class
{@code src/main/java/Foo.java} {@code src/test/java/FooTest.java}
{@code src/debug/java/Foo.java} {@code src/testDebug/java/FooTest.java}
{@code src/myFlavor/java/Foo.java} {@code src/testMyFlavor/java/FooTest.java}

Running local unit tests from Android Studio

To run local unit tests in your Gradle project from Android Studio:

  1. In the Project window, right click on the project and synchronize your project.
  2. Open the Build Variants window by clicking the left-hand tab, then change the test artifact to Unit Tests.
  3. In the Project window, drill down to your unit test class or method, then right-click and run it.

Android Studio displays the results of the unit test execution in the Run window.

Running local unit tests from the command-line

To run local unit tests in your Gradle project from the command-line, call the {@code test} task command with the {@code --continue} option.

./gradlew test --continue

If there are failing tests, the command will display links to HTML reports (one per build variant). You can find the generated HTML test result reports in the {@code <path_to_your_project>/app/build/reports/tests/} directory, and the corresponding XML files in the {@code <path_to_your_project>/app/build/test-results/} directory.