docs/html/training/testing/unit-testing/local-unit-tests.jd

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

@jd:body

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<div id="tb">
  <h2>Dependencies and Prerequisites</h2>

  <ul>
    <li><a href="{@docRoot}tools/studio/index.html">Android Studio (latest version)</a>.</li>
  </ul>

  <h2>This lesson teaches you to</h2>

  <ol>
    <li><a href="#setup">Set Up Your Testing Environment</a></li>
    <li><a href="#build">Create a Local Unit Test Class</a></li>
    <li><a href="#run">Run Local Unit Tests</a></li>
  </ol>

  <h2>Try it out</h2>

  <ul>
    <li>
<a href="https://github.com/googlesamples/android-testing/tree/master/unit/BasicSample"
class="external-link">Local Unit Tests Code Samples</a></li>
    <li><a href="https://codelabs.developers.google.com/codelabs/android-testing/index.html?index=..%2F..%2Findex#0"
class="external-link">Android Testing Codelab</a></li>
  </ul>
</div>
</div>

<p>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
<a href="https://github.com/mockito/mockito" class="external-link">Mockito</a>, to fulfill any
dependency relationships.</p>

<h2 id="setup">Set Up Your Testing Environment</h2>
<p>Before building your local unit test, make sure to configure your test source code location and
project dependencies, as described in
<a href="{@docRoot}training/testing/start/index.html#config-local-tests">
Getting Started with Testing</a>.</p>


<h2 id="build">Create a Local Unit Test Class</h2>
<p>Your local unit test class should be written as a JUnit 4 test class.
<a href="http://junit.org/" class="external-link">JUnit</a> 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.</p>

<p>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 &#64;Test} annotation and contains the code to exercise
and verify a single functionality in the component that you want to test.</p>

<p>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.</p>

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

public class EmailValidatorTest {

    &#64;Test
    public void emailValidator_CorrectEmailSimple_ReturnsTrue() {
        assertThat(EmailValidator.isValidEmail("name&#64;email.com"), is(true));
    }
    ...
}
</pre>

<p>To test that components in your app return the expected results, use the
<a href="http://junit.org/javadoc/latest/org/junit/Assert.html" class="external-link">
junit.Assert</a> methods to perform validation checks (or <em>assertions</em>) to compare the state
of the component under test against some expected value. To make tests more readable, you
can use <a href="https://github.com/hamcrest" class="external-link">
Hamcrest matchers</a> (such as the {@code is()} and {@code equalTo()} methods) to match the
returned result against the expected result.</p>

<h3 id="mocking-dependencies">Mocking Android dependencies</h3>
<p>
By default, the <a href="{@docRoot}tools/building/plugin-for-gradle.html">
Android Plug-in for Gradle</a> 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.
</p>
<p>
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
<a href="https://github.com/mockito/mockito" class="external-link">Mockito</a> 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.</p>

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

<ol>
<li>
Include the Mockito library dependency in your {@code build.gradle} file, as described in
<a href="#setup">Set Up Your Testing Environment</a>.
</li>
<li>At the beginning of your unit test class definition, add the
{@code &#64;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.
</li>
<li>To create a mock object for an Android dependency, add the {@code &#64;Mock} annotation before
the field declaration.</li>
<li>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.
</li>
</ol>

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

<pre>
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;

&#64;RunWith(MockitoJUnitRunner.class)
public class UnitTestSample {

    private static final String FAKE_STRING = "HELLO WORLD";

    &#64;Mock
    Context mMockContext;

    &#64;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));
    }
}
</pre>

<p>
To learn more about using the Mockito framework, see the
<a href="http://site.mockito.org/mockito/docs/current/org/mockito/Mockito.html"
class="external-link">Mockito API reference</a> and the
{@code SharedPreferencesHelperTest} class in the
<a href="https://github.com/googlesamples/android-testing/tree/master/unit/BasicSample"
class="external-link">sample code</a>.
</p>

<h2 id="run">Run Local Unit Tests</h2>
<p>
To run your tests, follow the steps for running local unit tests
described in <a href="{@docRoot}training/testing/start/index.html#run-local-tests">
Getting Started with Testing</a>.
</p>