Resolving MapStruct Error: No Property Named 'contact.holders.emails' in Java Mapping

Understanding the MapStruct Mapping Issue Between Modules

MapStruct is a powerful tool for simplifying object mapping in Java, especially when working with large systems that consist of multiple modules. In a multi-module project, it allows developers to map objects between different versions of domain models efficiently. However, even in a robust setup, mapping discrepancies can arise, leading to errors during compilation.

One such error is the false warning: "The type of parameter 'account' has no property named 'contact.holders.emails'." This issue occurs when trying to map between two domain versions where similar fields have slightly different naming conventions. Handling such cases requires a deeper understanding of how MapStruct interprets properties.

In the scenario at hand, the challenge is mapping the field 'emails' from version 6 of the domain model to the 'email' field in version 5. Despite the correct configuration of the mapping method, an unexpected error arises, indicating a possible issue with the mapping of inherited properties.

This article will explore why MapStruct struggles to identify fields inherited from a superclass and how to resolve such issues. We'll investigate whether this behavior is a bug or a limitation and offer practical solutions for your mapping needs.

Handling Complex Mapping Issues with MapStruct in Java

The scripts provided above are designed to resolve mapping issues between two versions of a domain model using MapStruct in Java. The primary goal is to handle field mismatches where a field like 'emails' in version 6 of the domain differs from 'email' in version 5. This issue typically arises in large-scale systems with multiple modules, and MapStruct’s powerful annotation-based mapping approach helps convert objects between these modules. The first script solves the problem by explicitly mapping the fields between the source and target using the @Mapping annotation.

The key command used in the first example is the @Mapping annotation, which specifies how fields in the source object are mapped to the target. The challenge in this case is dealing with a field from the superclass of the domain model, which MapStruct struggles to map automatically. To bypass this, the expression parameter within @Mapping is used, allowing developers to write custom Java logic inside the mapping process. This technique ensures flexibility when automated mapping cannot resolve complex inheritance scenarios.

In the second approach, a more manual handling of mapping is implemented using a service class in Spring. This allows for greater control over the mapping process, particularly when custom business logic is required. The use of the @Service annotation here marks the class as a Spring-managed bean, which performs the logic of manually mapping fields, including the transformation of emails. The helper function processes a list of account holders, flattening their email lists and concatenating them, ensuring the field mismatch between 'emails' and 'email' is resolved.

Finally, to ensure that the mapping logic works as expected, the third example introduces unit tests. These tests validate that the mapping process handles all edge cases, such as empty fields or null values. The assertEquals method checks whether the result of the mapping matches the expected output. This approach is crucial for maintaining the integrity of the data as it moves between versions of the domain model. By thoroughly testing each aspect of the mapping, developers can confidently deploy these mappings in a production environment without risking incorrect data transformations.

// AccountMapper.java: Handling mapping between Account and DepositAccount models
@Mapper(componentModel = MappingConstants.ComponentModel.SPRING)
public interface AccountMapper {
    // Map the account source to depositAccount target with field corrections
    @Mapping(source = "account.contact.holders.emails", target = "depositAccount.contact.holders.email")
    com.model5.AccountWithDetailsOneOf map(com.model6.DepositAccount account);
}

// Alternative solution with custom mapping logic using expression in MapStruct
@Mapper(componentModel = MappingConstants.ComponentModel.SPRING)
public interface AccountMapper {
    @Mapping(source = "account", target = "depositAccount")
    @Mapping(target = "depositAccount.contact.holders.email", expression = "java(mapEmails(account.getContact().getHolders()))")
    com.model5.AccountWithDetailsOneOf map(com.model6.DepositAccount account);
}

// Utility method to handle the emails mapping manually
default List<String> mapEmails(List<AccountHolder> holders) {
    return holders.stream()
                  .map(AccountHolder::getEmails)
                  .flatMap(Collection::stream)
                  .collect(Collectors.toList());
}

// AccountService.java: Use a service to handle mapping logic more explicitly
@Service
public class AccountService {
    public AccountWithDetailsOneOf mapDepositAccount(DepositAccount account) {
        AccountWithDetailsOneOf target = new AccountWithDetailsOneOf();
        target.setEmail(mapEmails(account.getContact().getHolders()));
        // other mappings here
        return target;
    }

    private String mapEmails(List<AccountHolder> holders) {
        return holders.stream()
                     .flatMap(holder -> holder.getEmails().stream())
                     .collect(Collectors.joining(","));
    }
}

// AccountMapperTest.java: Unit tests for the mapper
@SpringBootTest
public class AccountMapperTest {
    @Autowired
    private AccountMapper accountMapper;

    @Test
    public void testEmailMapping() {
        DepositAccount source = new DepositAccount();
        // Set up source data with emails
        AccountWithDetailsOneOf result = accountMapper.map(source);
        assertEquals("expected email", result.getEmail());
    }

    @Test
    public void testEmptyEmailMapping() {
        DepositAccount source = new DepositAccount();
        source.setContact(new Contact());
        AccountWithDetailsOneOf result = accountMapper.map(source);
        assertNull(result.getEmail());
    }
}

Handling Superclass Fields in MapStruct: Inheritance and Mapping Challenges

One important aspect of the MapStruct issue discussed is the handling of inherited fields from a superclass. In Java, fields and methods can be inherited from a parent class, but this inheritance can cause issues when using MapStruct to automatically map fields across objects. When a field like 'emails' is declared in a superclass, MapStruct may not be able to locate it directly within the subclass, causing the infamous error: "No property named 'contact.holders.emails'". This issue often arises when multiple domain models and versions are involved, where some models are based on older, more generalized classes.

To handle this kind of problem, developers need to leverage custom mapping methods. One option is to manually extract values from the superclass using methods such as getEmails(). By specifying explicit mapping logic via the @Mapping annotation and custom Java expressions, developers can ensure that fields from the parent class are correctly referenced during the mapping process. These custom expressions can flatten collections of email lists or adapt them to meet the specific requirements of the target domain model.

It is also important to note that Lombok-generated getters and setters, which are commonly used for field access, may not always be recognized by MapStruct when they belong to a superclass. To resolve this, developers can check Lombok annotations such as @Getter and @Setter to ensure they cover inherited fields. In some cases, it may be necessary to override or extend Lombok's functionality to improve MapStruct compatibility with the inheritance structure.