How to Handle Type Erasure in Generic Methods When Using Jackson with Type References?

Jackson and Type Reference in Generic Methods

When utilizing Jackson for serialization and deserialization in generic methods, it's possible to encounter type erasure issues leading to unexpected behavior.

Consider the following code where the tester method attempts to retrieve a generic request object from a JSON string using a generic type reference:

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

The issue occurs when the getMyObject() method is called within the request object:

<code class="java">@NotNull
@JsonIgnore
public T getMyObject() {
    return myobjects.get(0);
}</code>

Jackson interprets the generic type T as Object, resulting in the return value being a LinkedHashMap when the getMyObject() method is invoked.

To resolve this ambiguity, it's necessary to explicitly specify the actual class type for T. This can be achieved by providing the Class object as an argument to the TypeReference:

<code class="java">public MyRequest<T> tester() {
    TypeReference<MyWrapper<T>> typeRef = new TypeReference<MyWrapper<T>>() {};
    MyWrapper<T> requestWrapper = (MyWrapper<T>) JsonConverter.fromJson(jsonRequest, typeRef);
    Type type = typeRef.getType(); // Retrieve the actual type from the TypeReference
    return requestWrapper.getRequest(type);
}</code>

By introducing the type variable, it's possible to instruct Jackson to deserialize the JSON Object based on the specified type information, providing more precise and predictable behavior.

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