TestWatcher for reporting in JUnit

Introduction

TestWatcher helps you see what happened during tests. It reports when tests start, succeed, fail, or skip.

You want to log messages when tests pass or fail.

You need to perform actions after each test, like taking a screenshot.

You want to track test results for better reports.

You want to debug why a test failed by adding extra info.

You want to clean up resources after tests based on their result.

Syntax

JUnit

public class MyTestWatcher extends TestWatcher {
    @Override
    protected void succeeded(Description description) {
        // code when test passes
    }

    @Override
    protected void failed(Throwable e, Description description) {
        // code when test fails
    }

    @Override
    protected void starting(Description description) {
        // code when test starts
    }

    @Override
    protected void finished(Description description) {
        // code when test finishes
    }
}

Override only the methods you need.

Use Description to get test method name and class.

Examples

This prints a message when a test passes.

JUnit

public class MyWatcher extends TestWatcher {
    @Override
    protected void succeeded(Description description) {
        System.out.println(description.getMethodName() + " passed");
    }
}

This prints the failure reason when a test fails.

JUnit

public class MyWatcher extends TestWatcher {
    @Override
    protected void failed(Throwable e, Description description) {
        System.out.println(description.getMethodName() + " failed: " + e.getMessage());
    }
}

This shows messages when tests start and finish.

JUnit

public class MyWatcher extends TestWatcher {
    @Override
    protected void starting(Description description) {
        System.out.println("Starting test: " + description.getMethodName());
    }

    @Override
    protected void finished(Description description) {
        System.out.println("Finished test: " + description.getMethodName());
    }
}

Sample Program

This test class uses TestWatcher to print messages for each test event. One test passes, the other fails.

JUnit

import org.junit.Rule;
import static org.junit.Assert.*;
import org.junit.Test;
import org.junit.rules.TestWatcher;
import org.junit.runner.Description;

public class SampleTest {

    @Rule
    public TestWatcher watcher = new TestWatcher() {
        @Override
        protected void starting(Description description) {
            System.out.println("Starting: " + description.getMethodName());
        }

        @Override
        protected void succeeded(Description description) {
            System.out.println("Passed: " + description.getMethodName());
        }

        @Override
        protected void failed(Throwable e, Description description) {
            System.out.println("Failed: " + description.getMethodName() + " - " + e.getMessage());
        }

        @Override
        protected void finished(Description description) {
            System.out.println("Finished: " + description.getMethodName());
        }
    };

    @Test
    public void testSuccess() {
        // This test will pass
        int a = 1 + 1;
        assertEquals(2, a);
    }

    @Test
    public void testFailure() {
        // This test will fail
        int a = 1 + 1;
        assertEquals(3, a);
    }
}

Important Notes

TestWatcher works with JUnit 4 rules.

Use Description methods like getMethodName() to identify tests.

TestWatcher helps add custom logging without changing test code.

Summary

TestWatcher lets you react to test events like start, success, failure, and finish.

It helps add useful reports and logs during test runs.

Override only the methods you need for simple and clear code.

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