ParameterResolver extension in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a ParameterResolver in JUnit?

A ParameterResolver is an extension in JUnit that allows you to provide parameters to test methods dynamically at runtime.

beginner

Which two methods must be implemented in a ParameterResolver?

You must implement supportsParameter() to check if the parameter can be resolved, and resolveParameter() to provide the actual parameter value.

intermediate

How does JUnit decide to use a ParameterResolver for a test method parameter?

JUnit calls supportsParameter() on registered ParameterResolvers. If it returns true, JUnit uses resolveParameter() to get the value to inject.

beginner

Why use ParameterResolver instead of hardcoding test data?

ParameterResolver helps keep tests clean and reusable by injecting dependencies or test data dynamically, avoiding hardcoded values.

intermediate

Show a simple example of a ParameterResolver that provides a String parameter.

import org.junit.jupiter.api.extension.ExtensionContext;
import org.junit.jupiter.api.extension.ParameterContext;
import org.junit.jupiter.api.extension.ParameterResolver;

public class SimpleStringResolver implements ParameterResolver {
  @Override
  public boolean supportsParameter(ParameterContext pc, ExtensionContext ec) {
    return pc.getParameter().getType() == String.class;
  }

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

What does the supportsParameter() method do in a ParameterResolver?

A Provides the actual parameter value

B Checks if the resolver can provide a value for the parameter

C Runs the test method

D Registers the extension

supportsParameter() checks if the ParameterResolver can handle the parameter type before providing a value.

Which annotation is used to register a ParameterResolver in JUnit 5?

A @Test

B @BeforeEach

C @ExtendWith

D @ParameterResolver

@ExtendWith is used to register extensions like ParameterResolver in JUnit 5.

What type of parameters can a ParameterResolver inject?

A Any type supported by the resolver

B Only Strings

C Only objects with default constructors

D Only primitive types

ParameterResolver can inject any parameter type as long as it supports it in supportsParameter().

When is resolveParameter() called during test execution?

A Before supportsParameter()

B Only if the test fails

C After the test method finishes

D After supportsParameter() returns true

resolveParameter() is called only if supportsParameter() returns true for the parameter.

What is the main benefit of using ParameterResolver in tests?

A To inject parameters dynamically and improve test flexibility

B To write tests faster by skipping assertions

C To run tests in parallel

D To automatically generate test data

ParameterResolver allows dynamic injection of parameters, making tests more flexible and maintainable.

Explain how a ParameterResolver works in JUnit and why it is useful.

Think about how JUnit decides what value to pass to a test method parameter.

Describe the steps to create and use a custom ParameterResolver in a JUnit test.

Focus on the interface methods and how to connect the resolver to the test.