Unlocking the Power of ModelMapper: Efficiently Mapping Attributes from Root to Elements in Java

Discover how to configure `ModelMapper` to effortlessly map properties from a root object to elements in a list, ensuring maintainable and organized code.
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Unlocking the Power of ModelMapper: Efficiently Mapping Attributes from Root to Elements in Java

In software development, particularly when working with Java, mapping properties between different DTOs (Data Transfer Objects) is a common task. A popular library for this purpose is ModelMapper, which simplifies the process by automating the mapping of objects. However, developers often face challenges, especially when they need to transfer specific attributes from a root object to items within a list.

The Problem

Consider a scenario where you have a RootDTO class with a foo attribute, and you want this same attribute to be replicated for each Element when mapping to Root. The following class structure illustrates this problem:

Class Definitions

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In this case, ModelMapper does a great job of mapping the foo attribute from RootDTO to Root, but fails to map this value into Element objects, resulting in null for Element.foo. This leads to the critical question:

How do we configure ModelMapper to replicate the foo value from the root object to each element without needing to change the mapping every time new attributes are added?

Solution Overview

The solution involves setting up a custom converter that allows you to control the order of mapping. With this approach, you can ensure that after the initial implicit mapping occurs, you append a step to replicate the foo value to each Element in the list.

Step-by-Step Guide

Define a Custom Converter

We first need to create a new converter class that will handle the assignment of the foo values. This converter will run after the standard mapping is done.

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Set Up ModelMapper Mapping

Use implicit mappings for your DTO classes and then set the custom converter as a post-converter:

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Expected Behavior

With this configuration:

When you create an instance of Root using ModelMapper and map from RootDTO, the foo value from RootDTO will correctly replicate into all Element instances within Root.

This ensures that even if new fields are added later to either the Root or Element classes, the mapping remains intact and manageable.

Conclusion

By implementing the above process, developers can achieve seamless data transfer between RootDTO and Root while populating fields within Element.

This approach enhances the maintainability of your code, allowing you to focus on more critical tasks rather than manual mapping of attributes. The power of ModelMapper lies not just in simplifying object mapping, but also in making it adaptable to changes in your data model without requiring numerous code alterations.

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