Agent Skills: Unit Testing Mappers and Converters

Unit Testing Mappers and Converters

Overview

Provides patterns for unit testing MapStruct mappers and custom converter classes. Covers field mapping accuracy, null handling, type conversions, nested object transformations, bidirectional mapping, enum mapping, and partial updates.

When to Use

• Writing mapping tests for MapStruct mapper implementations
• Testing custom entity-to-DTO converters and bean mappings
• Validating nested object mapping and collection transformations

Instructions

1. Validate Generated Mapper Classes

Before testing, verify generated mapper classes exist:

# Maven
ls target/generated-sources/

# Gradle
ls build/generated/sources/

If generated classes are missing:

1. Run mvn compile (Maven) or ./gradlew compileJava (Gradle)
2. Check that the MapStruct annotation processor is configured
3. Verify @Mapper interfaces are in a compiled source set

2. Test Null Handling

assertThat(mapper.toDto(null)).isNull();

Configure nullValueMappingStrategy in mapper if null should return empty/default.

If null tests fail:

1. Add nullValueMappingStrategy = NullValueMappingStrategy.RETURN_NULL to @Mapper
2. Or use nullValuePropertyMappingStrategy for nested property handling

3. Test Bidirectional Mapping

User restored = mapper.toEntity(mapper.toDto(original));
assertThat(restored).usingRecursiveComparison().isEqualTo(original);

If bidirectional tests fail:

1. Check @Mapping annotations for field name mismatches
2. Verify both directions are explicitly mapped if auto-mapping fails
3. Use unmappedTargetPolicy = ReportingPolicy.ERROR to catch missing mappings

4. Test Nested Object Mapping

assertThat(dto.getNested()).usingRecursiveComparison().isEqualTo(expected);

If nested tests fail:

1. Ensure nested mapper exists or is referenced via uses = NestedMapper.class
2. Check collection element mappings with elementMappingStrategy

5. Test Custom Expressions

Custom expressions in @Mapping(target = "field", expression = "java(...)") are not compile-time validated.

If expression tests fail:

1. Verify the expression syntax and method signatures
2. Check that imported classes are accessible from the expression context

6. Test Enum Mappings

Use @ValueMapping for enum-to-enum translations. Test all enum values exhaustively.

Best Practices

• Use Mappers.getMapper() for standalone tests, Spring injection for integration tests
• Use usingRecursiveComparison() for complex nested structures
• Test all mapper methods including collection transformations
• Verify null handling for all nullable source fields
• Test bidirectional mapping catches asymmetries between entity→DTO and DTO→entity
• Keep mapper tests focused on transformation correctness, not implementation details

Constraints and Warnings

• Compile-time generation: MapStruct generates code at compile time—verify generated classes exist before running tests
• Null handling: Configure nullValueMappingStrategy and nullValuePropertyMappingStrategy appropriately
• Expression validation: Expressions in @Mapping are not validated at compile time—test them explicitly
• Circular dependencies: MapStruct cannot handle circular dependencies between mappers
• Collection immutability: Mapping immutable collections may require special configuration
• Date/Time: Verify date/time objects map correctly across timezones

Examples

Complete executable test with imports:

package com.example.mapper;

import org.junit.jupiter.api.Test;
import org.mapstruct.factory.Mappers;
import static org.assertj.core.api.Assertions.*;

class UserMapperCompleteTest {
  private final UserMapper mapper = Mappers.getMapper(UserMapper.class);

  @Test
  void shouldMapUserToDto() {
    User user = new User(1L, "Alice", "alice@example.com", 25);
    UserDto dto = mapper.toDto(user);
    assertThat(dto)
      .isNotNull()
      .extracting(UserDto::getName, UserDto::getEmail)
      .containsExactly("Alice", "alice@example.com");
  }

  @Test
  void shouldMaintainRoundTrip() {
    User original = new User(1L, "Alice", "alice@example.com", 25);
    assertThat(mapper.toEntity(mapper.toDto(original)))
      .usingRecursiveComparison()
      .isEqualTo(original);
  }

  @Test
  void shouldHandleNullInput() {
    assertThat(mapper.toDto(null)).isNull();
  }
}

Additional examples in: references/examples.md