Solving ModelMapper Null Property Issues: A Guide to Correct Mapping

Discover effective solutions for fixing incorrect property mapping with null objects in ModelMapper in this comprehensive guide. Learn how to achieve the desired mapping behavior!
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Solving ModelMapper Null Property Issues: A Guide to Correct Mapping

ModelMapper is a powerful tool in the Java programming ecosystem, often used for mapping between different object types. However, a common issue arises when dealing with properties set to null in source objects. If you're facing confusing behavior with your property mappings that you didn't expect, you are not alone!

The Problem at Hand

When mapping properties of an object, especially with null values, you might have noticed that when you try to map a source object where certain properties are null, the corresponding properties in the destination object don’t handle this as desired. Instead of the destination property being set to null, it sometimes gets set to an instance of the original property type with all its fields as null.

Example Scenario

Consider this example scenario:

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The Unexpected Output

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What you expected was that if the status in the User object is null, then the status in the UserDto should also be null. Instead, you get an instance of StatusDto where all properties are set to null.

Proposed Solution

While there has been a shift in libraries for some developers, the issue remains relevant. If you're still using ModelMapper, here's a solution based on the original code sample that can help you achieve the desired behavior.

Step-By-Step Solution

Create a Custom Converter:
To handle null values correctly, create a custom converter that checks if the source property is null. If it is, it returns null instead of an instance.

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Define Type Mapping:
After creating the converter, you need to set up a type map that specifies how you want the properties to be treated during the mapping process.

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Key Points to Remember

Reusable Converter: Although the addMappings method needs to be repeated for every property that suffers from this issue, the converter can be reused across different mappings where you encounter similar situations.

Decouple Logic: This approach helps decouple the mapping logic, allowing for cleaner and more maintainable code.

Customization: Depending on your complexity, consider whether to simplify your models or to handle mapping intricacies through additional layers.

Conclusion

Handling null values can be tricky in any codebase, especially when dealing with object mapping like in ModelMapper. The solution provided effectively addresses the issue of unwanted property assignments due to null backflows. By utilizing a custom converter and type mappings, you can ensure your models behave as expected without having to resort to overly complex solutions.

Whether you stick with ModelMapper or explore alternatives, understanding how to manage these scenarios is paramount to building resilient Java applications.

Feel free to share your experiences and any additional solutions you might have encountered!

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