How to Deserialize Nested Generic Objects in Jackson with Type Erasure?

Jackson and Generic Type Reference

When attempting to use Jackson's JSON library with generic methods, developers may encounter an issue where a nested custom object is returned as a LinkedHashMap instead of its actual class. This problem arises due to Java's type erasure, which removes type information during compilation.

In the provided code, the tester method aims to parse a JSON request into a MyRequest object with a generic type T. However, without specifying the actual class for T, Jackson defaults to treating it as T extends Object and binds JSON objects to maps.

To resolve this, the tester method should have access to the actual class when deserializing the JSON request. This can be achieved by specifying it as a Class argument. The following code demonstrates how to do this:

<code class="java">public MyRequest<T> tester(Class<T> clazz) {
    TypeReference<MyWrapper<T>> typeRef = new TypeReference<MyWrapper<T>>() {};
    MyWrapper<T> requestWrapper = (MyWrapper<T>) JsonConverter.fromJson(jsonRequest, typeRef);
    return requestWrapper.getRequest();
}</code>

Additionally, to ensure that the actual class is used during deserialization, construct a JavaType using the TypeFactory as follows:

<code class="java">JavaType type = mapper.getTypeFactory().constructCollectionType(List.class, clazz);</code>

With this modification, Jackson will deserialize the JSON request into a List containing instances of the specified class, allowing the getMyObject() method to return the object with the appropriate type.

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