Using Spring RestTemplate in generic method with generic parameter

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No, it is not a bug. It is a result of how the ParameterizedTypeReference hack works.

If you look at its implementation, it uses Class#getGenericSuperclass() which states

Returns the Type representing the direct superclass of the entity
(class, interface, primitive type or void) represented by this Class.

If the superclass is a parameterized type, the Type object returned
must accurately reflect the actual type parameters used in the source
code.

So, if you use 

new ParameterizedTypeReference<ResponseWrapper<MyClass>>() {}

it will accurately return a Type for ResponseWrapper<MyClass>.

If you use 

new ParameterizedTypeReference<ResponseWrapper<T>>() {}

it will accurately return a Type for ResponseWrapper<T> because that is how it appears in the source code.

When Spring sees T, which is actually a TypeVariable object, it doesn’t know the type to use, so it uses its default.

You cannot use ParameterizedTypeReference the way you are proposing, making it generic in the sense of accepting any type. Consider writing a Map with key Class mapped to a predefined ParameterizedTypeReference for that class.

You can subclass ParameterizedTypeReference and override its getType method to return an appropriately created ParameterizedType, as suggested by IonSpin.

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