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.

Set Up Your Testing Environment

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In your Android Studio project, you must store the source files for local unit tests at module-name/src/test/java/. This directory already exists when you create a new project.

You also need to configure the testing dependencies for your project to use the standard APIs provided by the JUnit 4 framework. If your test needs to interact with Android dependencies, include theMockito library to simplify your local unit tests. To learn more about using mock objects in your local unit tests, see Mocking Android dependencies.

In your app's top-level build.gradle file, you need to specify these libraries as dependencies:

dependencies {
    // Required -- JUnit 4 framework
    testCompile 'junit:junit:4.12'
    // Optional -- Mockito framework
    testCompile 'org.mockito:mockito-core:1.10.19'
}

Create a Local Unit Test Class

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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 junit.framework.TestCase class. You also do not need to prefix your test method name with the ‘test’ keyword, or use any classes in the junit.framework or junit.extensions package.

//JUnit是最流行的和广泛使用的Java测试框架。最近的版本Junit4比之前的版本都更加简洁和流畅。你也不需要继承TestCase类，你也不需要强制让测试方法的前缀为test

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 @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 emailValidator_CorrectEmailSimple_ReturnsTrueverifies that the 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 is() and equalTo() methods) to match the returned result against the expected result.

Mock Android dependencies

By default, the Android Plug-in for Gradle executes your local unit tests against a modified version of the 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 build.gradle file, as described in Set Up Your Testing Environment.
2. At the beginning of your unit test class definition, add the @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 @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 when() andthenReturn() methods.

The following example shows how you might create a unit test that uses a mock 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 SharedPreferencesHelperTest class in the sample code.

Run Local Unit Tests

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To run your local unit tests, follow these steps:

1. Be sure your project is synchronized with Gradle by clicking Sync Project  in the toolbar.
2. Run your test in one of the following ways:
   • To run a single test, open the Project window, and then right-click a test and click Run .
   • To test all methods in a class, right-click a class or method in the test file and click Run .
   • To run all tests in a directory, right-click on the directory and select Run tests .

The Android Plugin for Gradle compiles the local unit test code located in the default directory (src/test/java/), builds a test app, and executes it locally using the default test runner class. Android Studio then displays the results in the Run window.