Why extensions customize JUnit behavior - Automation Benefits in Action

Verify custom JUnit extension modifies test behavior

Preconditions (2)

Step 1: Create a test class using the custom extension with @ExtendWith annotation

Step 2: Write a test method that relies on the extension's behavior (e.g., modifies a value or logs info)

Step 3: Run the test class

Step 4: Observe the test output or state changes caused by the extension

✅ Expected Result: The test method runs with the custom behavior applied by the extension, verified by assertions or output changes

Automation Requirements - JUnit 5

Assertions Needed:

Verify the extension modifies the test state or output as expected

Confirm the test passes with the extension applied

Best Practices:

Use @ExtendWith annotation to apply the extension

Implement BeforeEachCallback or other extension interfaces properly

Use assertions to verify extension effects

Keep test code clean and focused on verifying extension behavior

Automated Solution

JUnit

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.junit.jupiter.api.extension.BeforeEachCallback;
import org.junit.jupiter.api.extension.ExtensionContext;
import static org.junit.jupiter.api.Assertions.assertEquals;

// Custom extension that sets a value before each test
class CustomValueExtension implements BeforeEachCallback {
    @Override
    public void beforeEach(ExtensionContext context) {
        context.getStore(ExtensionContext.Namespace.GLOBAL).put("value", 42);
    }
}

@ExtendWith(CustomValueExtension.class)
public class CustomExtensionTest {

    @Test
    void testValueSetByExtension(ExtensionContext context) {
        Integer value = context.getStore(ExtensionContext.Namespace.GLOBAL).get("value", Integer.class);
        assertEquals(42, value, "Extension should set value to 42 before each test");
    }

    @Test
    void testAnotherValueCheck(ExtensionContext context) {
        Integer value = context.getStore(ExtensionContext.Namespace.GLOBAL).get("value", Integer.class);
        assertEquals(42, value);
    }
}

This code defines a custom JUnit 5 extension CustomValueExtension that implements BeforeEachCallback. It sets a value of 42 in the global store before each test runs.

The test class CustomExtensionTest uses @ExtendWith(CustomValueExtension.class) to apply the extension. Each test method accesses the stored value from the extension context and asserts it equals 42.

This shows how extensions customize JUnit behavior by injecting or modifying test state before execution. Assertions verify the extension's effect, ensuring the test passes only if the extension works correctly.

Common Mistakes - 3 Pitfalls

{'mistake': 'Not using @ExtendWith annotation to apply the extension', 'why_bad': "The extension will not be applied and tests won't have the custom behavior", 'correct_approach': 'Always annotate the test class or method with @ExtendWith to activate the extension'}

Trying to access extension context parameters incorrectly in test methods

Modifying static or shared state without cleanup

Bonus Challenge

Now add data-driven testing with 3 different values set by the extension

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