Selecting tests by tags and classes in JUnit - Test Execution Trace

Test Overview

This test suite demonstrates how to select and run specific tests in JUnit 5 by using tags and test classes. It verifies that only tests with the specified tag or in the specified class are executed.

Test Code - JUnit 5

JUnit

import org.junit.jupiter.api.Tag;
import org.junit.jupiter.api.Test;
import org.junit.platform.launcher.Launcher;
import org.junit.platform.launcher.LauncherDiscoveryRequest;
import org.junit.platform.launcher.LauncherDiscoveryRequestBuilder;
import org.junit.platform.launcher.core.LauncherFactory;
import org.junit.platform.launcher.listeners.SummaryGeneratingListener;
import org.junit.platform.engine.discovery.DiscoverySelectors;
import org.junit.platform.engine.discovery.TagFilter;

import static org.junit.jupiter.api.Assertions.assertEquals;

public class TagAndClassSelectionTest {

    @Tag("fast")
    @Test
    void fastTest() {
        assertEquals(2, 1 + 1);
    }

    @Tag("slow")
    @Test
    void slowTest() {
        assertEquals(4, 2 * 2);
    }

    @Test
    void untaggedTest() {
        assertEquals(3, 1 + 2);
    }

    public static void main(String[] args) {
        SummaryGeneratingListener listener = new SummaryGeneratingListener();

        // Build request to select tests tagged with 'fast'
        LauncherDiscoveryRequest requestByTag = LauncherDiscoveryRequestBuilder.request()
                .selectors(DiscoverySelectors.selectClass(TagAndClassSelectionTest.class))
                .filters(TagFilter.includeTags("fast"))
                .build();

        Launcher launcher = LauncherFactory.create();
        launcher.registerTestExecutionListeners(listener);
        launcher.execute(requestByTag);

        long testsFoundByTag = listener.getSummary().getTestsFoundCount();
        long testsSucceededByTag = listener.getSummary().getTestsSucceededCount();

        System.out.println("Tests found with tag 'fast': " + testsFoundByTag);
        System.out.println("Tests succeeded with tag 'fast': " + testsSucceededByTag);

        // Reset listener for next run
        listener = new SummaryGeneratingListener();

        // Build request to select tests by class only (all tests in this class)
        LauncherDiscoveryRequest requestByClass = LauncherDiscoveryRequestBuilder.request()
                .selectors(DiscoverySelectors.selectClass(TagAndClassSelectionTest.class))
                .build();

        launcher.registerTestExecutionListeners(listener);
        launcher.execute(requestByClass);

        long testsFoundByClass = listener.getSummary().getTestsFoundCount();
        long testsSucceededByClass = listener.getSummary().getTestsSucceededCount();

        System.out.println("Tests found by class: " + testsFoundByClass);
        System.out.println("Tests succeeded by class: " + testsSucceededByClass);

        // Assertions to verify behavior
        assertEquals(1, testsFoundByTag, "Should find exactly 1 test with tag 'fast'");
        assertEquals(1, testsSucceededByTag, "The tagged test should pass");
        assertEquals(3, testsFoundByClass, "Should find all 3 tests in the class");
        assertEquals(3, testsSucceededByClass, "All tests in the class should pass");
    }
}

Execution Trace - 8 Steps

Step	Action	System State	Assertion	Result
1	Test starts - JUnit launcher is created	No tests executed yet	—	PASS
2	LauncherDiscoveryRequest built to select tests tagged with 'fast' in TagAndClassSelectionTest class	Request ready to find tests with tag 'fast'	—	PASS
3	Launcher executes tests matching the 'fast' tag	Only the test method 'fastTest' is found and executed	Verify test 'fastTest' passes	PASS
4	Summary shows 1 test found and passed for tag 'fast'	Test summary reports 1 test found and succeeded	Assert testsFoundByTag == 1 and testsSucceededByTag == 1	PASS
5	LauncherDiscoveryRequest built to select all tests in TagAndClassSelectionTest class	Request ready to find all tests in the class	—	PASS
6	Launcher executes all tests in the class	All three test methods (fastTest, slowTest, untaggedTest) are found and executed	Verify all tests pass	PASS
7	Summary shows 3 tests found and passed for class selection	Test summary reports 3 tests found and succeeded	Assert testsFoundByClass == 3 and testsSucceededByClass == 3	PASS
8	Test ends successfully with all assertions passing	All tests executed as expected	—	PASS

Failure Scenario

Failing Condition: No tests found with the specified tag due to typo or missing tag annotation

Execution Trace Quiz - 3 Questions

Which test method is executed when selecting tests by tag 'fast'?

A fastTest

B slowTest

C untaggedTest

D All tests in the class

Key Result

Using tags and class selectors in JUnit allows you to run focused subsets of tests, which helps speed up testing and organize tests by categories.

Practice

1. What is the main purpose of using @Tag in JUnit tests?

easy

A. To change the test method names automatically

B. To group tests so you can run specific sets easily

C. To skip tests permanently

D. To define the order in which tests run

Solution

1. Step 1: Understand the role of @Tag annotation

   The @Tag annotation is used to label tests with a category or group name.

2. Step 2: Identify the benefit of tagging tests

   Tagging allows you to select and run only tests with specific tags, making test runs focused and faster.

3. Final Answer:

   To group tests so you can run specific sets easily -> Option B

4. Quick Check:

   Tags group tests for selective runs [OK]

Hint: Tags group tests for selective execution [OK]

Common Mistakes:

• Thinking @Tag changes test order
• Believing @Tag skips tests permanently
• Assuming @Tag renames test methods

2. Which of the following is the correct way to tag a test method in JUnit 5?

easy

A. @Tag("fast") public void testMethod() {}

B. @Tag fast public void testMethod() {}

C. @Tag:fast public void testMethod() {}

D. @Tag = "fast" public void testMethod() {}

Solution

1. Step 1: Recall correct annotation syntax in Java

   Annotations use the format @AnnotationName("value") with parentheses and quotes for string values.

2. Step 2: Match the correct syntax for @Tag

   @Tag("fast") public void testMethod() {} uses @Tag("fast") correctly before the method declaration.

3. Final Answer:

   @Tag("fast") public void testMethod() {} -> Option A

4. Quick Check:

   Annotation syntax = @Tag("value") [OK]

Hint: Annotations need parentheses and quotes for string values [OK]

Common Mistakes:

• Omitting parentheses around tag value
• Missing quotes around the tag string
• Using colon or equals instead of parentheses

3. Given the following test class, which tests will run if you execute with the tag filter --include-tag fast?

@Tag("slow")
class ExampleTests {
  @Test
  void testA() {}

  @Test
  @Tag("fast")
  void testB() {}

  @Test
  @Tag("fast")
  void testC() {}
}

medium

A. Only testA

B. All tests: testA, testB, testC

C. Only testB and testC

D. No tests run

Solution

1. Step 1: Understand tag filtering behavior

   When running with --include-tag fast, only tests tagged with "fast" run.

2. Step 2: Analyze tags on each test

   Class has @Tag("slow"), so by default tests inherit "slow" tag. testA has no method tag, so only "slow" applies. testB and testC have @Tag("fast") overriding or adding to tags.

3. Step 3: Determine which tests match "fast" tag

   Only testB and testC have the "fast" tag, so only these run.

4. Final Answer:

   Only testB and testC -> Option C

5. Quick Check:

   Include tag "fast" runs only tests tagged "fast" [OK]

Hint: Method tags override class tags for filtering [OK]

Common Mistakes:

• Assuming class tag applies to all tests ignoring method tags
• Thinking all tests run without tag filtering
• Confusing include and exclude tag filters

4. You want to run tests only from the class LoginTests but your command mvn test -Dgroups=LoginTests runs no tests. What is the likely problem?

medium

A. The command should be mvn test -Dtest=LoginTests to run by class

B. JUnit 5 uses tags, not groups; the correct filter is by tags or class name

C. You must add @Group annotation to the class for this to work

D. The class name is case-sensitive and must be lowercase

Solution

1. Step 1: Understand Maven test filtering options

   Maven Surefire plugin uses -Dtest=ClassName to run tests by class name, not -Dgroups.

2. Step 2: Identify correct command to run tests by class

   To run tests from LoginTests, use mvn test -Dtest=LoginTests.

3. Final Answer:

   The command should be mvn test -Dtest=LoginTests to run by class -> Option A

4. Quick Check:

   Run by class uses -Dtest=ClassName [OK]

Hint: Use -Dtest=ClassName to run tests by class [OK]

Common Mistakes:

• Using -Dgroups which is for TestNG, not JUnit
• Assuming class names are lowercase
• Expecting @Group annotation in JUnit

5. You have two test classes: FastTests tagged with @Tag("fast") and SlowTests tagged with @Tag("slow"). You want to run all tests except those tagged "slow" and only from classes whose names end with "Tests". Which command correctly runs this selection?

hard

A. junit-platform-console --exclude-tag slow --exclude-classname .*Tests

B. junit-platform-console --include-tag fast --include-classname .*Tests

C. junit-platform-console --include-tag slow --include-classname FastTests

D. junit-platform-console --exclude-tag slow --include-classname .*Tests

Solution

1. Step 1: Understand tag and class name filters

   To exclude slow tests, use --exclude-tag slow. To include only classes ending with "Tests", use --include-classname .*Tests regex.

2. Step 2: Analyze each option

   junit-platform-console --exclude-tag slow --include-classname .*Tests excludes slow tags and includes classes matching .*Tests, which matches both FastTests and SlowTests classes but excludes slow-tagged tests.

3. Step 3: Confirm correct filtering

   junit-platform-console --include-tag fast --include-classname .*Tests includes only fast tags, which works but excludes any untagged tests. junit-platform-console --exclude-tag slow --exclude-classname .*Tests excludes classes ending with Tests, which is opposite of requirement. junit-platform-console --include-tag slow --include-classname FastTests includes slow tags, which is opposite of requirement.

4. Final Answer:

   junit-platform-console --exclude-tag slow --include-classname .*Tests -> Option D

5. Quick Check:

   Exclude slow tag + include class regex = junit-platform-console --exclude-tag slow --include-classname .*Tests [OK]

Hint: Exclude slow tag and include class regex for correct filtering [OK]

Common Mistakes:

• Using include-tag slow instead of exclude-tag
• Excluding classes instead of including
• Confusing include-tag and exclude-tag usage