junit-before-beforeclass-beforeeach-beforeall

Posted on: 2019-10-08 2019-11-19
Tags: JUnit, JUnit 5

@Before vs @BeforeClass vs @BeforeEach vs @BeforeAll

1. Introduction

In this short tutorial, we’re going to explain the differences between the @Before, @BeforeClass, @BeforeEach and @BeforeAll annotations in JUnit 4 and 5 – with practical examples of how to use them.

We’ll also cover briefly their @After complementary annotations.

Let’s start with JUnit 4.

2. @Before

Methods annotated with the @Before annotation are executed before each test. This is useful when we want to execute some common code before running a test.

Let’s see an example where we initialize a list and add some values:

@RunWith(JUnit4.class)
public class BeforeAndAfterAnnotationsUnitTest {

    // ...

    private List<String> list;

    @Before
    public void init() {
        LOG.info("startup");
        list = new ArrayList<>(Arrays.asList("test1", "test2"));
    }

    @After
    public void finalize() {
        LOG.info("finalize");
        list.clear();
    }
}

Notice that we also added another method annotated with @After in order to clear the list after the execution of each test.

After that, let’s add some tests to check the size of our list:

@Test
public void whenCheckingListSize_thenSizeEqualsToInit() {
    LOG.info("executing test");
    assertEquals(2, list.size());

    list.add("another test");
}

@Test
public void whenCheckingListSizeAgain_thenSizeEqualsToInit() {
    LOG.info("executing another test");
    assertEquals(2, list.size());

    list.add("yet another test");
}

In this case, it’s crucial to make sure that test environment is properly set up before running each test since the list is modified during every test execution.

If we take a look at the log output we can check that the init and finalize methods were executed once per test:

... startup
... executing another test
... finalize
... startup
... executing test
... finalize

3. @BeforeClass

When we want to execute an expensive common operation before each test, it’s preferable to execute it only once before running all tests using @BeforeClass. Some examples of common expensive operations are the creation of a database connection or the startup of a server.

Let’s create a simple test class that simulates the creation of a database connection:

@RunWith(JUnit4.class)
public class BeforeClassAndAfterClassAnnotationsUnitTest {

    // ...

    @BeforeClass
    public static void setup() {
        LOG.info("startup - creating DB connection");
    }

    @AfterClass
    public static void tearDown() {
        LOG.info("closing DB connection");
    }
}

Notice that these methods have to be static, so they’ll be executed before running the tests of the class.

As we did before, let’s also add some simple tests:

@Test
public void simpleTest() {
    LOG.info("simple test");
}

@Test
public void anotherSimpleTest() {
    LOG.info("another simple test");
}

This time, if we take a look at the log output we can check that the setup and tearDown methods were executed only once:

... startup - creating DB connection
... simple test
... another simple test
... closing DB connection

4. @BeforeEach and @BeforeAll

@BeforeEach and @BeforeAll are the JUnit 5 equivalents of @Before and @BeforeClass. These annotations were renamed with clearer names to avoid confusion.

Let’s duplicate our previous classes using these new annotations, starting with the @BeforeEach and @AfterEach annotations:

@RunWith(JUnitPlatform.class)
class BeforeEachAndAfterEachAnnotationsUnitTest {

    // ...

    private List<String> list;

    @BeforeEach
    void init() {
        LOG.info("startup");
        list = new ArrayList<>(Arrays.asList("test1", "test2"));
    }

    @AfterEach
    void finalize() {
        LOG.info("finalize");
        list.clear();
    }

    // ...
}

If we check logs, we can confirm that it works in the same way as with the @Before and @After annotations:

... startup
... executing another test
... finalize
... startup
... executing test
... finalize

Finally, let’s do the same with the other test class to see the @BeforeAll and @AfterAll annotations in action:

@RunWith(JUnitPlatform.class)
public class BeforeAllAndAfterAllAnnotationsUnitTest {

    // ...

    @BeforeAll
    public static void setup() {
        LOG.info("startup - creating DB connection");
    }

    @AfterAll
    public static void tearDown() {
        LOG.info("closing DB connection");
    }

    // ...
}

And the output is the same as with the old annotation:

... startup - creating DB connection
... simple test
... another simple test
... closing DB connection

5. Conclusion

In this article, we showed the differences between the @Before, @BeforeClass, @BeforeEach and @BeforeAll annotations in JUnit and when each of them should be used.

As always, the full source code of the examples is available over on GitHub.

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