TestWatcher for reporting in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this TestWatcher override?

Consider this JUnit 5 TestWatcher extension code. What will be printed when the test fails?

JUnit

import org.junit.jupiter.api.extension.TestWatcher;
import org.junit.jupiter.api.extension.ExtensionContext;

public class MyWatcher implements TestWatcher {
    @Override
    public void testFailed(ExtensionContext context, Throwable cause) {
        System.out.println("Test failed: " + context.getDisplayName());
    }
}

// Assume this watcher is registered for a test named "exampleTest" which fails.

A Test failed: exampleTest

B Test failed: null

C No output printed

D Compilation error due to missing method

💡 Hint

The testFailed method prints the test name on failure.

The testFailed method is called on test failure and prints the display name of the test, which is 'exampleTest'.

❓ assertion

intermediate

Which assertion correctly verifies a test was skipped using TestWatcher?

You want to assert inside a TestWatcher that a test was skipped. Which assertion is correct?

JUnit

import org.junit.jupiter.api.extension.TestWatcher;
import org.junit.jupiter.api.extension.ExtensionContext;
import java.util.Optional;

public class SkipWatcher implements TestWatcher {
    @Override
    public void testSkipped(ExtensionContext context, Optional<String> reason) {
        // Which assertion below is correct?
    }
}

A assertFalse(reason.isPresent());

B assertEquals("Skipped", reason.get());

C assertTrue(reason.isPresent());

D assertNull(reason);

💡 Hint

The reason parameter is Optional and should be present if skipped.

The reason parameter is an Optional containing the skip reason. It should be present when a test is skipped.

🔧 Debug

advanced

Why does this TestWatcher not print on test success?

This TestWatcher override does not print anything when a test passes. Why? public class SilentWatcher implements TestWatcher { @Override public void testSuccessful(ExtensionContext context) { // Intentionally left blank } }

A Because testSuccessful is never called on success

B Because ExtensionContext is null on success

C Because TestWatcher requires calling super.testSuccessful() to print

D Because testSuccessful method is empty, no output is printed

💡 Hint

Check what the method implementation does.

The testSuccessful method is called on success but since it is empty, no output occurs.

🧠 Conceptual

advanced

What is the main purpose of TestWatcher in JUnit 5?

Choose the best description of what TestWatcher is used for in JUnit 5 testing.

A To replace assertions inside test methods

B To monitor test lifecycle events and perform actions on test success, failure, or skip

C To configure test execution order

D To mock dependencies during tests

💡 Hint

Think about what events TestWatcher listens to.

TestWatcher listens to test lifecycle events like success, failure, and skip to allow custom reporting or actions.

❓ framework

expert

How to register a TestWatcher extension for all tests in a class?

You want to apply a TestWatcher implementation to all tests in a JUnit 5 test class. Which is the correct way?

A @ExtendWith(MyWatcher.class) on the test class

B Implement TestWatcher interface in the test class directly

C Call MyWatcher.testFailed() manually in each test

D Register MyWatcher in a static block inside the test class

💡 Hint

JUnit 5 uses annotations to register extensions.

The @ExtendWith annotation registers the TestWatcher extension for all tests in the class.

Practice

1. What is the primary purpose of using TestWatcher in JUnit tests?

easy

A. To react to test lifecycle events like start, success, failure, and finish

B. To write assertions inside test methods

C. To create mock objects for testing

D. To configure database connections before tests

Solution

1. Step 1: Understand TestWatcher role

   TestWatcher is a JUnit class designed to listen to test lifecycle events such as when a test starts, succeeds, fails, or finishes.

2. Step 2: Differentiate from other test tasks

   Writing assertions, mocking, or configuring databases are separate concerns and not the main purpose of TestWatcher.

3. Final Answer:

   To react to test lifecycle events like start, success, failure, and finish -> Option A

4. Quick Check:

   TestWatcher = react to test events [OK]

Hint: TestWatcher listens to test events, not assertions or mocks [OK]

Common Mistakes:

• Confusing TestWatcher with assertion writing
• Thinking TestWatcher creates mocks
• Assuming TestWatcher manages test setup like DB connections

2. Which method signature correctly overrides the TestWatcher method that runs when a test fails?

easy

A. public void failed(ExtensionContext context, Throwable cause)

B. protected void failed(Throwable e, Description description)

C. void failed(Description description, Throwable e)

D. public void testFailed(ExtensionContext context, Throwable cause)

Solution

1. Step 1: Recall TestWatcher method signature

   The failed method in TestWatcher is public void failed(ExtensionContext context, Throwable cause).

2. Step 2: Check method modifiers and parameter order

   It must be public, return void, and parameters are ExtensionContext first, then Throwable.

3. Final Answer:

   public void failed(ExtensionContext context, Throwable cause) -> Option A

4. Quick Check:

   failed method = public void failed(ExtensionContext, Throwable) [OK]

Hint: Check parameter order and access modifier for overrides [OK]

Common Mistakes:

• Swapping parameter order
• Using protected instead of public
• Missing return type or wrong method name

3. Given the following TestWatcher subclass, what will be printed if the test exampleTest fails?

class MyWatcher extends TestWatcher {
  @Override
  public void failed(ExtensionContext context, Throwable cause) {
    context.getTestMethod().ifPresent(method -> System.out.println(method.getName() + " failed"));
  }
}

@Test
@ExtendWith(MyWatcher.class)
void exampleTest() {
  assertEquals(1, 2);
}

medium

A. No output printed

B. failed

C. exampleTest

D. exampleTest failed

Solution

1. Step 1: Understand what failed() prints

   The overridden failed method prints the test method name plus " failed".

2. Step 2: Identify test method name on failure

   The test method is named exampleTest, so the output will be "exampleTest failed".

3. Final Answer:

   exampleTest failed -> Option D

4. Quick Check:

   Print = methodName + " failed" = exampleTest failed [OK]

Hint: Print uses description.getMethodName() on failure [OK]

Common Mistakes:

• Assuming no output on failure
• Printing only 'failed' without method name
• Confusing method name with test class name

4. Identify the error in this TestWatcher subclass that aims to log test success:

class SuccessWatcher extends TestWatcher {
  @Override
  public void succeeded(Description description) {
    System.out.println(description.getDisplayName() + " succeeded");
  }
}

medium

A. Method signature is incorrect; it should include Throwable parameter

B. Method signature is incorrect; it should be public void succeeded(ExtensionContext context)

C. Method should be protected, not public

D. Method signature is incorrect; it should be public void succeeded(Description description, Throwable e)

Solution

1. Step 1: Check JUnit version and method signature

   In JUnit 5, TestWatcher methods use ExtensionContext parameter, not Description.

2. Step 2: Identify correct succeeded() signature

   The correct override is public void succeeded(ExtensionContext context) in JUnit 5.

3. Final Answer:

   Method signature is incorrect; it should be public void succeeded(ExtensionContext context) -> Option B

4. Quick Check:

   JUnit 5 TestWatcher uses ExtensionContext parameter [OK]

Hint: JUnit 5 TestWatcher methods use ExtensionContext, not Description [OK]

Common Mistakes:

• Using JUnit 4 signatures in JUnit 5
• Wrong parameter types or order
• Confusing access modifiers

5. You want to create a custom TestWatcher that logs test start and failure events, but only for tests whose names start with "critical". Which approach correctly implements this behavior in JUnit 5?

hard

A. Override starting(Description description) and failed(Throwable e, Description description), check description.getMethodName() starts with "critical" before logging

B. Override beforeTestExecution(ExtensionContext context) and afterTestExecution(ExtensionContext context), check context.getTestMethod().getName() starts with "critical" before logging

C. Override starting(ExtensionContext context) and failed(ExtensionContext context, Throwable cause), check context.getDisplayName() starts with "critical" before logging

D. Override testStarted(Description description) and testFailure(Failure failure), check failure.getDescription().getMethodName() starts with "critical" before logging

Solution

1. Step 1: Identify correct JUnit 5 TestWatcher methods

   In JUnit 5, TestWatcher methods to override are starting(ExtensionContext) and failed(ExtensionContext, Throwable).

2. Step 2: Filter test names using context.getDisplayName()

   Use context.getDisplayName() to get the test name and check if it starts with "critical" before logging.

3. Final Answer:

   Override starting(ExtensionContext context) and failed(ExtensionContext context, Throwable cause), check context.getDisplayName() starts with "critical" before logging -> Option C

4. Quick Check:

   JUnit 5 TestWatcher + ExtensionContext + name check = Override starting(ExtensionContext context) and failed(ExtensionContext context, Throwable cause), check context.getDisplayName() starts with "critical" before logging [OK]

Hint: Use ExtensionContext methods and check displayName for filtering [OK]

Common Mistakes:

• Using JUnit 4 method signatures in JUnit 5
• Checking wrong parameter types
• Using lifecycle methods not related to TestWatcher