ParameterResolver extension in JUnit - Test Execution Trace

Test Overview

This test uses JUnit's ParameterResolver extension to inject a custom object into the test method. It verifies that the injected object is not null and has the expected property value.

Test Code - JUnit 5

JUnit

import org.junit.jupiter.api.extension.ExtensionContext;
import org.junit.jupiter.api.extension.ParameterContext;
import org.junit.jupiter.api.extension.ParameterResolver;
import org.junit.jupiter.api.extension.ExtendWith;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class MyService {
    private final String name;
    MyService(String name) {
        this.name = name;
    }
    String getName() {
        return name;
    }
}

class MyServiceParameterResolver implements ParameterResolver {
    @Override
    public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
        return parameterContext.getParameter().getType() == MyService.class;
    }

    @Override
    public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
        return new MyService("InjectedService");
    }
}

@ExtendWith(MyServiceParameterResolver.class)
public class ParameterResolverTest {

    @Test
    void testInjectedService(MyService service) {
        assertNotNull(service, "Injected service should not be null");
        assertEquals("InjectedService", service.getName(), "Service name should match injected value");
    }
}

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	JUnit test runner starts the test execution	Test environment initialized, extensions registered	—	PASS
2	JUnit detects test method 'testInjectedService' requires a MyService parameter	ParameterResolver extension is available	—	PASS
3	JUnit calls MyServiceParameterResolver.supportsParameter() to check if it can provide MyService	Parameter type is MyService	supportsParameter returns true	PASS
4	JUnit calls MyServiceParameterResolver.resolveParameter() to get MyService instance	MyService instance created with name 'InjectedService'	Returned object is instance of MyService	PASS
5	JUnit invokes testInjectedService with injected MyService instance	Test method running with parameter service = MyService('InjectedService')	assertNotNull(service) passes	PASS
6	JUnit verifies service.getName() equals 'InjectedService'	service.getName() returns 'InjectedService'	assertEquals('InjectedService', service.getName()) passes	PASS
7	Test method completes successfully	Test passed	—	PASS

Failure Scenario

Failing Condition: ParameterResolver does not support the parameter type or returns null

Execution Trace Quiz - 3 Questions

What does the ParameterResolver do in this test?

A It creates and injects a MyService instance into the test method

B It runs the test method automatically

C It asserts the test results

D It configures the test environment

Key Result

Using ParameterResolver allows clean injection of dependencies into test methods, improving test readability and reusability.

Practice

1. What is the main purpose of a ParameterResolver in JUnit 5?

easy

A. To define the order of test execution

B. To generate test reports after execution

C. To create mock objects for tests

D. To automatically provide values for test method parameters

Solution

1. Step 1: Understand ParameterResolver role

   A ParameterResolver is designed to supply values to parameters of test methods automatically, so tests don't need manual setup for those parameters.

2. Step 2: Compare with other options

   Options A, C, and D relate to other testing features but not to parameter injection.

3. Final Answer:

   To automatically provide values for test method parameters -> Option D

4. Quick Check:

   ParameterResolver = automatic parameter injection [OK]

Hint: ParameterResolver injects parameters automatically [OK]

Common Mistakes:

• Confusing ParameterResolver with test ordering
• Thinking it creates mocks automatically
• Assuming it generates reports

2. Which two methods must a class implement to be a valid ParameterResolver in JUnit 5?

easy

A. supportsParameter and resolveParameter

B. beforeEach and afterEach

C. initialize and cleanup

D. createParameter and validateParameter

Solution

1. Step 1: Recall ParameterResolver interface methods

   The ParameterResolver interface requires implementing supportsParameter to check if a parameter is supported, and resolveParameter to provide the value.

2. Step 2: Eliminate incorrect method pairs

   Options A, B, and C list methods unrelated to ParameterResolver interface.

3. Final Answer:

   supportsParameter and resolveParameter -> Option A

4. Quick Check:

   ParameterResolver methods = supportsParameter + resolveParameter [OK]

Hint: Remember: supportsParameter checks, resolveParameter provides [OK]

Common Mistakes:

• Confusing lifecycle methods with resolver methods
• Using incorrect method names
• Assuming initialization methods are required

3. Given this ParameterResolver implementation, what will be printed when the test runs?

class MyResolver implements ParameterResolver {
  public boolean supportsParameter(ParameterContext pc, ExtensionContext ec) {
    return pc.getParameter().getType() == String.class;
  }
  public Object resolveParameter(ParameterContext pc, ExtensionContext ec) {
    return "InjectedValue";
  }
}

@ExtendWith(MyResolver.class)
class TestClass {
  @Test
  void testMethod(String param) {
    System.out.println(param);
  }
}

medium

A. null

B. InjectedValue

C. Compilation error

D. Test fails with exception

Solution

1. Step 1: Analyze supportsParameter method

   It returns true only if the parameter type is String, so the test method's String param is supported.

2. Step 2: Analyze resolveParameter method

   It returns the string "InjectedValue", so this value will be injected into the test method parameter.

3. Final Answer:

   InjectedValue -> Option B

4. Quick Check:

   Parameter injection prints InjectedValue [OK]

Hint: Check supportsParameter return true for parameter type [OK]

Common Mistakes:

• Assuming parameter is null if not supported
• Expecting compilation error due to interface methods
• Thinking test fails without explicit parameter value

4. Identify the error in this ParameterResolver implementation:

class FaultyResolver implements ParameterResolver {
  public boolean supportsParameter(ParameterContext pc, ExtensionContext ec) {
    return pc.getParameter().getType() == Integer.class;
  }
  public Object resolveParameter(ParameterContext pc, ExtensionContext ec) {
    return 42;
  }
}

@ExtendWith(FaultyResolver.class)
class TestClass {
  @Test
  void testMethod(int param) {
    System.out.println(param);
  }
}

medium

A. Missing @Override annotations cause runtime error

B. resolveParameter returns wrong type

C. supportsParameter checks for Integer.class but parameter is primitive int

D. Test method parameter must be Integer, not int

Solution

1. Step 1: Check parameter type in supportsParameter

   The test method parameter is primitive int, but supportsParameter checks for Integer.class, which is the wrapper class. This causes supportsParameter to return false.

2. Step 2: Understand consequence of mismatch

   Because supportsParameter returns false, resolveParameter is never called, so the parameter is not injected, causing test failure or default value usage.

3. Final Answer:

   supportsParameter checks for Integer.class but parameter is primitive int -> Option C

4. Quick Check:

   Primitive vs wrapper mismatch breaks supportsParameter [OK]

Hint: Match primitive and wrapper types carefully in supportsParameter [OK]

Common Mistakes:

• Ignoring difference between int and Integer
• Assuming @Override annotations are mandatory
• Thinking resolveParameter type must differ

5. You want to create a ParameterResolver that injects a unique UUID string into any test method parameter of type String. Which implementation correctly achieves this and ensures a new UUID is generated for each test invocation?

hard

A. In resolveParameter, return UUID.randomUUID().toString() without caching

B. In supportsParameter, cache a UUID and return it for all calls

C. Return a fixed UUID string in resolveParameter for all tests

D. Generate UUID once in constructor and return it in resolveParameter

Solution

1. Step 1: Understand requirement for unique UUID per test

   Each test invocation should get a new UUID string, so the UUID must be generated fresh every time resolveParameter is called.

2. Step 2: Evaluate options for UUID generation

   In resolveParameter, return UUID.randomUUID().toString() without caching generates a new UUID string on each call, fulfilling the requirement. Options B, C, and D cache or fix the UUID, so they do not produce unique values per test.

3. Final Answer:

   In resolveParameter, return UUID.randomUUID().toString() without caching -> Option A

4. Quick Check:

   Generate UUID fresh in resolveParameter for uniqueness [OK]

Hint: Generate new UUID inside resolveParameter method [OK]

Common Mistakes:

• Caching UUID and returning same value every time
• Generating UUID once in constructor
• Returning fixed string instead of UUID