Argument aggregation in JUnit - Test Execution Trace

Test Overview

This test demonstrates how to use argument aggregation in JUnit 5 to combine multiple arguments into a single object for parameterized tests. It verifies that the aggregated object correctly holds the combined values.

Test Code - JUnit 5

JUnit

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.aggregator.AggregateWith;
import org.junit.jupiter.params.aggregator.ArgumentsAccessor;
import org.junit.jupiter.params.provider.CsvSource;

class User {
    String name;
    int age;

    User(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

class UserAggregator implements org.junit.jupiter.params.aggregator.ArgumentsAggregator {
    @Override
    public User aggregateArguments(ArgumentsAccessor accessor, java.lang.reflect.Parameter parameter) {
        return new User(accessor.getString(0), accessor.getInteger(1));
    }
}

public class ArgumentAggregationTest {

    @ParameterizedTest
    @CsvSource({"Alice,30", "Bob,25"})
    void testUserAggregation(@AggregateWith(UserAggregator.class) User user) {
        assertEquals("Alice", user.name, "First user's name should be Alice");
        assertEquals(30, user.age, "First user's age should be 30");
    }

    @ParameterizedTest
    @CsvSource({"Alice,30", "Bob,25"})
    void testUserAggregationDynamic(@AggregateWith(UserAggregator.class) User user) {
        if (user.name.equals("Alice")) {
            assertEquals(30, user.age);
        } else if (user.name.equals("Bob")) {
            assertEquals(25, user.age);
        } else {
            throw new AssertionError("Unexpected user name: " + user.name);
        }
    }
}

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test runner starts and loads ArgumentAggregationTest class	JUnit test environment initialized	-	PASS
2	JUnit invokes testUserAggregation with first CSV input: "Alice,30"	Test method called with aggregated User object (name=Alice, age=30)	assertEquals("Alice", user.name) and assertEquals(30, user.age)	PASS
3	JUnit invokes testUserAggregation with second CSV input: "Bob,25"	Test method called with aggregated User object (name=Bob, age=25)	assertEquals("Alice", user.name) fails because user.name is "Bob"	FAIL
4	JUnit invokes testUserAggregationDynamic with first CSV input: "Alice,30"	Test method called with aggregated User object (name=Alice, age=30)	assertEquals(30, user.age) for user.name == "Alice"	PASS
5	JUnit invokes testUserAggregationDynamic with second CSV input: "Bob,25"	Test method called with aggregated User object (name=Bob, age=25)	assertEquals(25, user.age) for user.name == "Bob"	PASS

Failure Scenario

Failing Condition: Using fixed assertions for specific input in testUserAggregation causes failure on second input

Execution Trace Quiz - 3 Questions

What does the @AggregateWith annotation do in this test?

A It runs the test multiple times with different CSV inputs

B It asserts that the User object is not null

C It combines multiple CSV values into a single User object

D It skips the test if the User object is invalid

Key Result

When using argument aggregation in parameterized tests, write assertions that adapt to each input value instead of fixed expected values to avoid false failures.