How to Prevent Mapper Overwriting Resolved Objects in MapStruct

Learn how to effectively manage object mapping in Spring Boot using MapStruct. Discover strategies to prevent overwriting resolved properties when mapping DTOs to domain entities.
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Understanding the Problem: Mapper Overwriting Resolved Objects

When working with Java and frameworks like Spring Boot, developers often face challenges while using MapStruct for object mapping. A common issue occurs when a mapper overwrites properties of a resolved object. In this post, we will explore this specific problem and walk through effective solutions.

The Scenario

Imagine you have a DTO (Data Transfer Object) that includes an ID and need to convert this into a domain entity via a mapper. The process calls for resolving the entity based on the ID but, unfortunately, ends up overwriting essential properties after resolution. This leads to a loss of information that you might want to retain. Here's a brief overview of how this scenario looks in the code:

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In the generated mapping implementation, properties of ticketAccount could be overwritten after it's resolved, which is undesirable:

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To solve this issue, we need to prevent the mapper from automatically filling the resolved object with properties from the DTO.

Solutions to Prevent Overwriting Resolved Properties

1. Disable Auto-Mapping

The first approach to control overwriting behavior is to disable automatic property mapping for the specific mapping method you are dealing with. You can do this by using the @ BeanMapping(ignoreByDefault = true) annotation on the mapping method. Here’s how it looks:

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With this option, MapStruct will only use explicitly defined mappings. As a result, it won't overwrite resolved properties in your domain entity.

2. Utilize BeforeMapping Option

Another effective technique is the use of the @ BeforeMapping annotation. This allows you to define a method that resolves the object and returns it before anything else happens during the mapping process. Here’s an example:

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This strategy clarifies that if a resolved object is found, it should return it immediately. If no object is found, MapStruct will then create a new instance based on the DTO properties, ensuring you have the desired information without undesired overwriting.

Putting It All Together

Combining both solutions, your complete mapping implementation may look like this:

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By implementing these techniques, you ensure that resolved properties maintain their integrity and are not unintentionally overwritten, leading to cleaner and more efficient object mapping in your Spring Boot applications.

Conclusion

Learning to manage object mapping effectively is critical for Java developers working with frameworks like Spring Boot and MapStruct. By following the outlined strategies, you can avoid common pitfalls like overwriting resolved object properties, ultimately leading to cleaner, more reliable code.

For further queries or clarifications, feel free to reach out. Happy coding!