Difference between @Before, @BeforeClass, @BeforeEach, and @BeforeAll

java

test-toolbox

unit-testing

junit-5

byNikita Barsukov·Oct 7, 2024

@Before/@BeforeEach: Executes before each test, handy for setting up test-specific conditions.

@BeforeClass/@BeforeAll: Runs once at the start, good for initializing shared resources.

Example:

public class SampleTest {

    @BeforeAll // Runs once, like a director yelling "Action!" on the movie set
    static void initAll() {
        // Expensive setup code here
    }

    @BeforeEach // Like a make-up artist touching up actor's makeup before each scene
    void init() {
        // Per-test setup here
    }

    @Test
    void aTest() {
        // The actual movie scene with A-list actors (test code)
    }
}

Why use different setup annotations?

Each annotation serves a unique purpose with varying scope and lifecycle within a test class.

Understanding Scope and Efficiency in Unit Testing

@BeforeClass/@BeforeAll: Perfect for class-level initializations. Similar to a building of a movie set.
@Before/@BeforeEach: Ideal for per-test scenarios — equivalent to setting up fresh props for each scene.

Save Time & Resources - Choose Wisely!

Use @BeforeClass or @BeforeAll for expensive operations, like opening a database connection or loading a massive amount of data. However, make sure these shareable resources remain constant during the lifecycle of all tests.

The Hitchhiker's Guide to Moving from JUnit 4 to JUnit 5

The @BeforeAll methods in JUnit 5 need to be static. So, if you are migrating from JUnit 4, don't forget to adjust the method signatures.

The Art of Arranging Your Setup Tasks

Organizing setup tasks ensures efficient, readable code and reduces the chances of tests interfering with each other.

Your Test Toolbox - Using Annotations Correctly

Tackling External Dependencies

When tests demand external dependencies, do this:

Start a server once with @BeforeClass/@BeforeAll.
Open a database for each test with @Before/@BeforeEach.

Don't Ask Hulk to do Thumb Wrestling - Use Annotations for Heavy Lifting

Offload heavy computations or data loading to setup annotations when required:

Static data can be loaded once using @BeforeClass/@BeforeAll.
Complex calculations could be done in a static block with @BeforeAll.

Clean Up Your Mess – Using @After and @AfterEach

After you've had your fun testing, make sure not to leave behind a mess. Use @After/@AfterEach for cleanup activities.

"With great power comes great responsibility"

Improper annotation usage can result in flaky tests. Avoid pitfalls like using @BeforeAll for mutable state setup without proper reset in @AfterEach.

explain-codes / Java / Difference between @Before, @BeforeClass, @BeforeEach, and @BeforeAll

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Why use different setup annotations?Your Test Toolbox - Using Annotations Correctly

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