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.