Selecting tests by tags and classes in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of JUnit test run with tag filter

Given the following JUnit 5 test classes with tags, what will be the output when running tests with the tag @Tag("fast") only?

JUnit

import org.junit.jupiter.api.Tag;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertTrue;

public class FastTests {
    @Test
    @Tag("fast")
    void fastTest1() {
        assertTrue(true);
    }

    @Test
    @Tag("slow")
    void slowTest1() {
        assertTrue(true);
    }
}

public class MixedTests {
    @Test
    @Tag("fast")
    void fastTest2() {
        assertTrue(true);
    }

    @Test
    void noTagTest() {
        assertTrue(true);
    }
}

A Only slowTest1 runs because it has a tag; others are skipped.

B All tests run regardless of tags and all pass.

C Only tests without tags run; fastTest1, fastTest2, and slowTest1 are skipped.

D Only fastTest1 and fastTest2 run and pass; slowTest1 and noTagTest are skipped.

💡 Hint

Remember that JUnit 5 allows filtering tests by tags using the @Tag annotation and test runners can include or exclude tests by tag.

When running tests filtered by the tag "fast", only tests annotated with @Tag("fast") will run. Tests with other tags or no tags are skipped.

❓ assertion

intermediate

Correct assertion to verify only tagged tests ran

You run JUnit tests filtered by tag "integration". Which assertion correctly verifies that only tests with this tag ran?

JUnit

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

// Assume testResults is a list of test names that ran
List<String> testResults = List.of("integrationTest1", "integrationTest2");

A assertEquals(2, testResults.size());

B assertEquals(4, testResults.size());

C assertEquals(0, testResults.size());

D assertEquals(1, testResults.size());

💡 Hint

Count how many tests have the "integration" tag and expect only those to run.

If only tests tagged "integration" run, and there are exactly two such tests, the size of testResults should be 2.

🔧 Debug

advanced

Why does filtering by tag fail to exclude untagged tests?

You configured your JUnit 5 test run to include only tests with tag "ui" but untagged tests still run. What is the most likely cause?

A The test runner configuration is missing the tag filter include directive.

B Tests without tags are always included by default and cannot be excluded.

C JUnit 5 does not support filtering by tags.

D The @Tag annotation is only for documentation and does not affect test execution.

💡 Hint

Check your test runner or build tool configuration for tag filtering settings.

JUnit 5 supports tag filtering, but the runner must be explicitly configured to include or exclude tags. Without this, all tests run.

❓ framework

advanced

Selecting tests by class and tag in JUnit 5

Which JUnit 5 command line option runs only tests in class LoginTests tagged with smoke?

A junit-platform-console --select-package=LoginTests --include-tags=smoke

B junit-platform-console --select-package=LoginTests --exclude-tags=smoke

C junit-platform-console --select-class=LoginTests --include-tags=smoke

D junit-platform-console --select-class=LoginTests --exclude-tags=smoke

💡 Hint

Use --select-class to specify the test class and --include-tags to filter by tag.

The option --select-class=LoginTests selects that class, and --include-tags=smoke filters tests to only those tagged "smoke".

🧠 Conceptual

expert

Impact of multiple tags on test selection

If a test method is annotated with @Tag("fast") and @Tag("database"), and the test run includes only tests tagged "fast", what happens?

A The test is excluded because it has more than one tag.

B The test runs because it has the "fast" tag, even though it also has "database".

C The test runs only if both tags are included in the filter.

D The test is excluded because it has the "database" tag.

💡 Hint

Think about how tag filters include tests that have at least one matching tag.

JUnit 5 includes tests that have any of the included tags. Having multiple tags does not exclude a test if one tag matches the filter.

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