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Learn how to effectively use Jackson in Java to `deserialize multiple JSON keys` into a single list. This guide walks through step-by-step instructions on using annotations for better data handling.
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How to Deserialize Multiple JSON Keys to One Field with Jackson

When working with JSON data in Java, especially from a REST API, you might encounter situations where you need to map identical fields from different keys of a JSON object into a single property in your class. This often leads to the question: How can we effectively achieve this with Jackson? In this guide, we'll explore how to deserialize multiple JSON keys into a single field and provide a comprehensive guide on how to set this up using specific Jackson annotations.

Understanding the Problem

Consider a JSON response from a REST API that looks like this:

[[See Video to Reveal this Text or Code Snippet]]

You have a data structure that produces multiple code entries (code1, code2, etc.) but want to consolidate them into a single list of CustomCode objects instead of using a map or individual fields for each entry, such as customCodeList. Your existing model might look like this:

[[See Video to Reveal this Text or Code Snippet]]

However, this structure will not work as intended since each customCodeList will only capture one object corresponding to the keys at any given time.

The Solution

Using @ JsonAnySetter Annotation

To address this issue effectively, you can utilize the @ JsonAnySetter annotation provided by Jackson. This annotation allows you to handle unknown fields dynamically when deserializing JSON data. Below is how you can implement it:

[[See Video to Reveal this Text or Code Snippet]]

How It Works

Dynamic Map Creation: The @ JsonAnySetter annotation, when added to a method, tells Jackson to call this method for any unknown JSON properties. This allows you to handle multiple keys without predefining them in your class.

Conditional Insertion: In the set method, you check if the incoming key is part of the defined aliases (code1 to code5). If it is, the corresponding CustomCode object is added to the customCodeList.

Maintainability: Using a Set<String> ALIASES helps you manage which keys you want to capture and makes your code cleaner and more manageable.

Important Consideration

Unknown Fields Handling: While the above approach works seamlessly for known fields, be cautious: if the incoming JSON may have unknown fields (which you don't want to handle), consider changing the type for the set method's second argument from CustomCode to Object. You will need to include additional parsing logic in this case to properly extract meaningful data from unknown fields.

Conclusion

By utilizing Jackson’s @ JsonAnySetter annotation, you can efficiently deserialize multiple JSON keys into a single list, which simplifies your data model and enhances the maintainability of your code. This technique is particularly beneficial when you're dealing with a variable set of keys and don’t want to use a verbose approach with maps or individual fields.

As you work more with JSON in Java, methods like this can save you time and keep your code neat and understandable, so give it a try!