How to Ignore Fields When Converting String to Object with ObjectMapper in Java

Описание: A comprehensive guide on how to use ObjectMapper from the Jackson library to ignore specific fields when converting strings to objects in Java. Learn best practices and troubleshooting tips!
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This video is based on the question https://stackoverflow.com/q/72971601/ asked by the user 'Shivangi Gupta' ( https://stackoverflow.com/u/7697959/ ) and on the answer https://stackoverflow.com/a/72972147/ provided by the user 'Ausgefuchster' ( https://stackoverflow.com/u/11480721/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Ignore Fields When Converting String to Object with ObjectMapper in Java

In Java programming, especially when working with APIs and JSON data, you often encounter situations where you need to serialize and deserialize objects. A common scenario is using Jackson's ObjectMapper to convert Java objects to JSON strings and vice versa. However, you may have fields in your object that you want to ignore during these conversions. In this guide, we'll address a specific case where you want to exclude a field annotated with @ JsonIgnore both during serialization and deserialization. Let's dive into the problem and solution!

The Problem

We have a simple User class defined as follows:

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In this class, we want to avoid including the id field when the object is converted to a JSON string. However, an issue arises when we try to convert the string back into an object. Despite successfully excluding the id field from the serialized string, we still end up with a default value (0) for the id field once we deserialize the string back to the User object.

Here’s how we convert the User object to a string:

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This gives us the expected JSON string without the id field:

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However, upon deserializing the string:

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We end up with a User object showing the following output:

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This is not the expected behavior. We want the id field entirely excluded when we create the User object from the string.

Solution

To effectively ignore columns during both serialization and deserialization, follow these steps:

Step 1: Use Integer Instead of int

First, it's important to note that int is a primitive data type and cannot be null. If you want to omit it during deserialization, consider changing int to Integer. This way, it can hold a null value, indicating the absence of data:

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Step 2: Ignore Unknown Properties

You can add an annotation to your class to tell Jackson to ignore any unknown properties during deserialization. This is done using @ JsonIgnoreProperties as shown:

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Step 3: Override toString Method

To customize the string representation of your User class, you can override the toString method. This allows you to specify that the string representation should only include the name:

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Alternatively, if you’re using the @ Data annotation from Lombok, you can simply exclude the id field from the automatically generated toString method with the @ ToString.Exclude annotation:

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Putting It All Together

Here’s the final version of your User class:

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Conclusion

By making these changes, you can successfully ignore a field during both JSON serialization and deserialization. This approach provides a clean and necessary control over your JSON data with Jackson's ObjectMapper, enabling you to manage your data effectively while keeping your model classes clear.

If you have other custom User objects or similar classes, consider applying the same principles wherever appropriate. Happy coding!