How to Fix class java.util.LinkedHashMap cannot be cast to class... in Spring Boot

Описание: Encountering "class java.util.LinkedHashMap cannot be cast to class..." in Spring Boot? Learn the causes and steps to resolve this common issue effectively.
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How to Fix class java.util.LinkedHashMap cannot be cast to class... in Spring Boot

If you're developing a Spring Boot application, you may run into the error message: class java.util.LinkedHashMap cannot be cast to class.... This is a common issue that generally arises when working with JSON data, especially when using Jackson for serialization and deserialization.

Understanding the root cause and finding an effective solution can save you a lot of debugging time. Let's delve into the causes and remedies for this error.

Why Does This Error Occur?

This error typically indicates a casting issue where a LinkedHashMap object is being cast to a specific class. This often happens during the conversion of JSON data to Java objects. Here's why:

Automatic Type Inference: Jackson, the default JSON parser in Spring Boot, automatically infers key-value pairs in JSON objects as LinkedHashMap. If you're trying to map this JSON directly to a specific class, it can cause casting issues.

Generic Type Erasure: Java's type erasure mechanism can cause issues with generic types, leading to Jackson deserializing JSON into LinkedHashMap rather than the specified type.

Incorrect Type Information: When working with collections or nested objects, insufficient type information may cause Jackson to default to LinkedHashMap.

Steps to Resolve the Issue

Use TypeReference for Generics

If you're working with generic collections like List, Set, or Map, use TypeReference to specify the type explicitly:

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Custom Deserializer

Create a custom deserializer for your specific class to ensure proper deserialization:

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Then register the deserializer with Jackson:

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Configure Jackson Properly

Ensure that your Jackson configuration is properly set up to handle conversion to the desired class:

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Use @JsonDeserialize Annotation

Annotate your class with @JsonDeserialize to use a specific deserializer:

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Conclusion

The class java.util.LinkedHashMap cannot be cast to class... error in Spring Boot is a common challenge when dealing with JSON data and Jackson. By understanding the causes and employing the appropriate solutions, you can effectively resolve this issue and ensure smooth data conversion in your application. Whether you use TypeReference, custom deserializers, or annotations, selecting the right approach depends on your specific use case and application requirements.

Happy coding!