Custom argument providers in JUnit

Introduction

Custom argument providers let you supply test data in your own way. This helps run the same test with different inputs easily.

You want to test a method with many different inputs that are not simple lists.

You need to generate test data dynamically, like reading from a file or database.

You want to reuse complex test data setup across multiple tests.

You want to keep your test code clean by separating data generation from test logic.

Syntax

JUnit

public class MyArgumentsProvider implements ArgumentsProvider {
    @Override
    public Stream<? extends Arguments> provideArguments(ExtensionContext context) {
        return Stream.of(
            Arguments.of("input1", 1),
            Arguments.of("input2", 2)
        );
    }
}

@ParameterizedTest
@ArgumentsSource(MyArgumentsProvider.class)
void testWithCustomArguments(String input, int number) {
    // test code here
}

The class must implement ArgumentsProvider interface.

Override provideArguments to return a stream of Arguments.

Examples

This provider returns two sets of arguments: a string and a number.

JUnit

public class SimpleArgumentsProvider implements ArgumentsProvider {
    @Override
    public Stream<? extends Arguments> provideArguments(ExtensionContext context) {
        return Stream.of(
            Arguments.of("apple", 5),
            Arguments.of("banana", 6)
        );
    }
}

This test uses the custom provider to check string lengths.

JUnit

@ParameterizedTest
@ArgumentsSource(SimpleArgumentsProvider.class)
void testFruitLength(String fruit, int expectedLength) {
    assertEquals(expectedLength, fruit.length());
}

This provider generates arguments dynamically from a list.

JUnit

public class DynamicArgumentsProvider implements ArgumentsProvider {
    @Override
    public Stream<? extends Arguments> provideArguments(ExtensionContext context) {
        List<String> data = List.of("dog", "cat", "bird");
        return data.stream().map(s -> Arguments.of(s, s.length()));
    }
}

Sample Program

This test uses a custom argument provider that supplies pairs of words and numbers. The test checks if the word's length matches the number.

JUnit

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.extension.ExtensionContext;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.Arguments;
import org.junit.jupiter.params.provider.ArgumentsProvider;
import org.junit.jupiter.params.provider.ArgumentsSource;

import java.util.stream.Stream;

public class CustomArgumentProviderTest {

    static class NumberWordProvider implements ArgumentsProvider {
        @Override
        public Stream<? extends Arguments> provideArguments(ExtensionContext context) {
            return Stream.of(
                Arguments.of("one", 3),
                Arguments.of("two", 3),
                Arguments.of("three", 5)
            );
        }
    }

    @ParameterizedTest
    @ArgumentsSource(NumberWordProvider.class)
    void testWordLength(String word, int number) {
        assertEquals(number, word.length());
    }
}

Important Notes

Custom argument providers help keep tests clean and flexible.

Always return a Stream of Arguments for best compatibility.

Use @ArgumentsSource annotation to link the provider to the test.

Summary

Custom argument providers supply test data in your own way.

Implement ArgumentsProvider and override provideArguments method.

Use @ArgumentsSource to connect the provider to your parameterized test.

Practice

1. What is the main purpose of a custom argument provider in JUnit?

easy

A. To generate test reports

B. To create new test methods automatically

C. To supply test data to parameterized tests in a custom way

D. To replace the @Test annotation

Solution

1. Step 1: Understand the role of argument providers

   Argument providers supply data to parameterized tests, allowing tests to run multiple times with different inputs.

2. Step 2: Identify the purpose of custom argument providers

   Custom argument providers let you define your own way to supply test data beyond built-in sources.

3. Final Answer:

   To supply test data to parameterized tests in a custom way -> Option C

4. Quick Check:

   Custom argument providers = supply test data [OK]

Hint: Custom providers supply test data uniquely for parameterized tests [OK]

Common Mistakes:

• Confusing argument providers with test method creators
• Thinking they replace @Test annotation
• Assuming they generate reports

2. Which interface must you implement to create a custom argument provider in JUnit?

easy

A. ArgumentsSource

B. ArgumentsProvider

C. TestTemplateInvocationContextProvider

D. TestInstancePostProcessor

Solution

1. Step 1: Recall JUnit interfaces for argument providers

   The interface to implement for custom argument providers is ArgumentsProvider.

2. Step 2: Differentiate from other interfaces

   ArgumentsSource is an annotation, not an interface. The others serve different purposes.

3. Final Answer:

   ArgumentsProvider -> Option B

4. Quick Check:

   Implement ArgumentsProvider for custom data [OK]

Hint: Implement ArgumentsProvider interface for custom data supply [OK]

Common Mistakes:

• Confusing ArgumentsSource annotation with interface
• Choosing unrelated interfaces
• Not overriding provideArguments method

3. Given this custom argument provider code snippet, what will be the output when the parameterized test runs?

public class MyProvider implements ArgumentsProvider {
    @Override
    public Stream<Arguments> provideArguments(ExtensionContext context) {
        return Stream.of(Arguments.of("apple"), Arguments.of("banana"));
    }
}

@ParameterizedTest
@ArgumentsSource(MyProvider.class)
void testFruits(String fruit) {
    System.out.println(fruit);
}

medium

A. apple\nbanana (each on separate lines)

B. apple banana

C. Compilation error due to missing imports

D. Runtime error due to wrong method signature

Solution

1. Step 1: Analyze the provideArguments method

   The method returns a stream with two Arguments: "apple" and "banana".

2. Step 2: Understand parameterized test execution

   The test runs twice, once with "apple" and once with "banana", printing each on its own line.

3. Final Answer:

   apple\nbanana (each on separate lines) -> Option A

4. Quick Check:

   Stream of Arguments runs test twice [OK]

Hint: Each argument runs test once, prints on separate lines [OK]

Common Mistakes:

• Assuming output is on one line
• Thinking code has syntax errors without imports
• Confusing method signature correctness

4. Identify the error in this custom argument provider implementation:

public class BadProvider implements ArgumentsProvider {
    public Stream<Arguments> provideArguments() {
        return Stream.of(Arguments.of(1), Arguments.of(2));
    }
}

medium

A. Method provideArguments must have ExtensionContext parameter

B. Missing @Override annotation

C. Arguments.of() cannot take integers

D. Class must extend ArgumentsSource

Solution

1. Step 1: Check method signature requirements

   The provideArguments method must override the interface method and accept ExtensionContext as a parameter.

2. Step 2: Identify the missing parameter

   The code's provideArguments method lacks the ExtensionContext parameter, causing a signature mismatch.

3. Final Answer:

   Method provideArguments must have ExtensionContext parameter -> Option A

4. Quick Check:

   Correct method signature requires ExtensionContext [OK]

Hint: provideArguments must accept ExtensionContext parameter [OK]

Common Mistakes:

• Ignoring method signature parameters
• Thinking @Override is mandatory but not fixing signature
• Confusing Arguments.of() usage

5. You want to create a custom argument provider that supplies pairs of integers where the second is always double the first. Which implementation correctly achieves this?

public class DoubleProvider implements ArgumentsProvider {
    @Override
    public Stream<Arguments> provideArguments(ExtensionContext context) {
        // Fill in the blank
    }
}

hard

A. return Stream.of(Arguments.of(1, 3), Arguments.of(2, 5), Arguments.of(3, 7));

B. return Stream.of(Arguments.of(1, 1), Arguments.of(2, 2), Arguments.of(3, 3));

C. return Stream.empty();

D. return Stream.of(Arguments.of(1, 2), Arguments.of(2, 4), Arguments.of(3, 6));

Solution

1. Step 1: Understand the requirement

   Each pair must have the second integer double the first.

2. Step 2: Evaluate each option

   return Stream.of(Arguments.of(1, 2), Arguments.of(2, 4), Arguments.of(3, 6)); correctly pairs (1,2), (2,4), (3,6). Other options provide pairs like (1,3),(2,5),(3,7) or equal numbers (1,1),(2,2),(3,3), or an empty stream.

3. Final Answer:

   return Stream.of(Arguments.of(1, 2), Arguments.of(2, 4), Arguments.of(3, 6)); -> Option D

4. Quick Check:

   Second integer = 2 * first integer [OK]

Hint: Check if second number is double the first in pairs [OK]

Common Mistakes:

• Providing pairs where second is not double
• Returning empty stream by mistake
• Confusing argument order