ParameterResolver extension in JUnit

Introduction

ParameterResolver helps tests get needed values automatically. It makes tests cleaner and easier to read.

When you want to provide custom objects or values to test methods without manual setup.

When multiple tests need the same setup data or resource injected.

When you want to avoid repeating code to create test parameters.

When you want to control how parameters are created for tests.

When you want to inject mocks or special objects into test methods.

Syntax

JUnit

public class MyParameterResolver implements ParameterResolver {
    @Override
    public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
        // Return true if this resolver supports the parameter
        return false; // placeholder implementation
    }

    @Override
    public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
        // Return the object to inject
        return null; // placeholder implementation
    }
}

The supportsParameter method checks if the resolver can provide a value for a parameter.

The resolveParameter method returns the actual value to inject into the test.

Examples

This resolver injects the string "Injected String" whenever a test method asks for a String parameter.

JUnit

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

    @Override
    public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
        return "Injected String";
    }
}

The test method receives the injected string automatically from the resolver.

JUnit

@ExtendWith(StringParameterResolver.class)
public class MyTest {
    @Test
    void testWithInjectedString(String injected) {
        assertEquals("Injected String", injected);
    }
}

Sample Program

This example shows a ParameterResolver that injects the number 42 into test methods that ask for an Integer. The test checks the value and prints it.

JUnit

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

public class ParameterResolverExample {

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

        @Override
        public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
            return 42;
        }
    }

    @ExtendWith(NumberParameterResolver.class)
    public static class MyTest {
        @Test
        void testInjectedNumber(Integer number) {
            assertEquals(42, number);
            System.out.println("Injected number is: " + number);
        }
    }
}

Important Notes

ParameterResolver helps avoid manual setup in tests.

Always check parameter type carefully in supportsParameter.

Use @ExtendWith to apply your resolver to test classes.

Summary

ParameterResolver injects values into test method parameters automatically.

It requires implementing two methods: supportsParameter and resolveParameter.

Use it to make tests cleaner and reduce repeated setup code.

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