How to Deserialize Nested Generic Objects with Jackson: A Guide to Using TypeReference and JavaType

Jackson and Generic Type Reference: Handling Nested Generic Objects

The Jackson JSON library allows for powerful handling of complex object structures. However, a common challenge arises when working with generic types in custom objects.

Problem Statement:

Consider the following code:

<code class="java">public class MyWrapper<T> {

    private MyRequest<T> request;

    // ... getters and setters
}

public class MyRequest<T> {

    private List<T> myobjects;

    // ... getters and setters
}</code>

A generic method attempts to deserialize a JSON object into a MyWrapper instance:

<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 calling getMyObject() on the nested MyRequest object. Jackson returns a LinkedHashMap instead of an instance of the desired type T. This happens because Java type erasure removes the type information during compilation.

Solution:

To specify the exact type T that should be returned, you need to provide Jackson with additional information about the type. This can be achieved by using the JavaType class from the Jackson mapper.

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

Replace the line in the tester() method with:

<code class="java">TypeReference<MyWrapper<T>> typeRef = new TypeReference<MyWrapper<T>>();
JavaType requestType = mapper.getTypeFactory().
  constructCollectionType(MyRequest.class, type.getType());
MyWrapper<T> requestWrapper = 
  (MyWrapper<T>) JsonConverter.fromJson(jsonRequest, requestType);</code>

This now instructs Jackson to expect a List of the given type Foo. When deserializing the JSON object, it will translate it to the correct type.

By providing Jackson with this additional type information, you can handle nested generic objects accurately, ensuring that the objects are of the correct type when retrieved.

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