@ArgumentsSource with custom providers in JUnit - Build an Automation Script

Automate parameterized test using @ArgumentsSource with a custom provider

Preconditions (2)

Step 1: Create a test class with a parameterized test method

Step 2: Annotate the test method with @ParameterizedTest and @ArgumentsSource referencing the custom provider

Step 3: Implement the custom ArgumentsProvider to supply multiple sets of arguments

Step 4: Run the test and verify that the test method executes once per argument set

✅ Expected Result: The test method runs multiple times with different arguments supplied by the custom provider, and all assertions pass.

Automation Requirements - JUnit 5

Assertions Needed:

Verify the test method receives correct arguments from the custom provider

Verify the test method executes the expected number of times

Best Practices:

Use @ParameterizedTest with @ArgumentsSource for custom argument providers

Implement ArgumentsProvider interface correctly

Use Stream.of or Arguments.of to supply arguments

Keep test method simple and focused on assertions

Avoid hardcoding test data inside the test method

Automated Solution

JUnit

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

import java.util.stream.Stream;

public class CustomArgumentsSourceTest {

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

    @ParameterizedTest
    @ArgumentsSource(MyArgumentsProvider.class)
    void testWithCustomArgumentsSource(String fruit, int length) {
        Assertions.assertEquals(length, fruit.length());
    }
}

The code defines a test class CustomArgumentsSourceTest with a nested static class MyArgumentsProvider that implements ArgumentsProvider. This provider returns a stream of argument sets, each containing a fruit name and its expected length.

The test method testWithCustomArgumentsSource is annotated with @ParameterizedTest and @ArgumentsSource(MyArgumentsProvider.class). This tells JUnit to run the test multiple times, once for each argument set provided by MyArgumentsProvider.

Inside the test method, an assertion checks that the length of the fruit string matches the expected length. This verifies that the arguments are passed correctly.

This approach keeps test data separate from the test logic, making tests clean and maintainable.

Common Mistakes - 4 Pitfalls

Not implementing the ArgumentsProvider interface correctly

Hardcoding test data inside the test method instead of using the provider

Using incorrect annotations like @ValueSource instead of @ArgumentsSource for custom providers

Not matching the test method parameters with the arguments provided

Bonus Challenge

Now add data-driven testing with 3 different sets of fruit names and their lengths using the custom ArgumentsProvider

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