[java] Using Spring RestTemplate in generic method with generic parameter

To use generic types with Spring RestTemplate we need to use ParameterizedTypeReference (Unable to get a generic ResponseEntity<T> where T is a generic class "SomeClass<SomeGenericType>")

Suppose I have some class

public class MyClass {
    int users[];

    public int[] getUsers() { return users; }
    public void setUsers(int[] users) {this.users = users;}
}

And some wrapper class

public class ResponseWrapper <T> {
    T response;

    public T getResponse () { return response; }
    public void setResponse(T response) {this.response = response;}
}

So if I'm trying to do something like this, all is OK.

public ResponseWrapper<MyClass> makeRequest(URI uri) {
    ResponseEntity<ResponseWrapper<MyClass>> response = template.exchange(
        uri,
        HttpMethod.POST,
        null,
        new ParameterizedTypeReference<ResponseWrapper<MyClass>>() {});
    return response;
}

But when I'm trying to create generic variant of the above method ...

public <T> ResponseWrapper<T> makeRequest(URI uri, Class<T> clazz) {
   ResponseEntity<ResponseWrapper<T>> response = template.exchange(
        uri,
        HttpMethod.POST,
        null,
        new ParameterizedTypeReference<ResponseWrapper<T>>() {});
    return response;
}

... and calling this method like so ...

makeRequest(uri, MyClass.class)

... instead of getting ResponseEntity<ResponseWrapper<MyClass>> object I'm getting ResponseEntity<ResponseWrapper<LinkedHashSet>> object.

How can I solve this problem? Is it a RestTemplate bug?

UPDATE 1 Thanks to @Sotirios I understand the concept. Unfortunately I'm newly registered here so I cant comment on his answer, so writing it here. Im not sure that I clearly understand how to implement the proposed approach to solve my problem with Map with Class key (Proposed by @Sotirios in the end of his answer). Would someone mind to give an example?

I have another way to do this... suppose you swap out your message converter to String for your RestTemplate, then you can receive raw JSON. Using the raw JSON, you can then map it into your Generic Collection using a Jackson Object Mapper. Here's how:

Swap out the message converter:

    List<HttpMessageConverter<?>> oldConverters = new ArrayList<HttpMessageConverter<?>>();
    oldConverters.addAll(template.getMessageConverters());

    List<HttpMessageConverter<?>> stringConverter = new ArrayList<HttpMessageConverter<?>>();
    stringConverter.add(new StringHttpMessageConverter());

    template.setMessageConverters(stringConverter);

Then get your JSON response like this:

    ResponseEntity<String> response = template.exchange(uri, HttpMethod.GET, null, String.class);

Process the response like this:

     String body = null;
     List<T> result = new ArrayList<T>();
     ObjectMapper mapper = new ObjectMapper();

     if (response.hasBody()) {
        body = items.getBody();
        try {
            result = mapper.readValue(body, mapper.getTypeFactory().constructCollectionType(List.class, clazz));
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            template.setMessageConverters(oldConverters);
        }
        ...

I feel like there's a much easier way to do this... Just define a class with the type parameters that you want. e.g.:

final class MyClassWrappedByResponse extends ResponseWrapper<MyClass> {
    private static final long serialVersionUID = 1L;
}

Now change your code above to this and it should work:

public ResponseWrapper<MyClass> makeRequest(URI uri) {
    ResponseEntity<MyClassWrappedByResponse> response = template.exchange(
        uri,
        HttpMethod.POST,
        null,
        MyClassWrappedByResponse.class
    return response;
}

As Sotirios explains, you can not use the ParameterizedTypeReference, but ParameterizedTypeReference is used only to provide Type to the object mapper, and as you have the class that is removed when type erasure happens, you can create your own ParameterizedType and pass that to RestTemplate, so that the object mapper can reconstruct the object you need.

First you need to have the ParameterizedType interface implemented, you can find an implementation in Google Gson project here. Once you add the implementation to your project, you can extend the abstract ParameterizedTypeReference like this:

class FakeParameterizedTypeReference<T> extends ParameterizedTypeReference<T> {

@Override
public Type getType() {
    Type [] responseWrapperActualTypes = {MyClass.class};
    ParameterizedType responseWrapperType = new ParameterizedTypeImpl(
        ResponseWrapper.class,
        responseWrapperActualTypes,
        null
        );
    return responseWrapperType;
    }
}

And then you can pass that to your exchange function:

template.exchange(
    uri,
    HttpMethod.POST,
    null,
    new FakeParameterizedTypeReference<ResponseWrapper<T>>());

With all the type information present object mapper will properly construct your ResponseWrapper<MyClass> object

As the code below shows it, it works.

public <T> ResponseWrapper<T> makeRequest(URI uri, final Class<T> clazz) {
   ResponseEntity<ResponseWrapper<T>> response = template.exchange(
        uri,
        HttpMethod.POST,
        null,
        new ParameterizedTypeReference<ResponseWrapper<T>>() {
            public Type getType() {
                return new MyParameterizedTypeImpl((ParameterizedType) super.getType(), new Type[] {clazz});
        }
    });
    return response;
}

public class MyParameterizedTypeImpl implements ParameterizedType {
    private ParameterizedType delegate;
    private Type[] actualTypeArguments;

    MyParameterizedTypeImpl(ParameterizedType delegate, Type[] actualTypeArguments) {
        this.delegate = delegate;
        this.actualTypeArguments = actualTypeArguments;
    }

    @Override
    public Type[] getActualTypeArguments() {
        return actualTypeArguments;
    }

    @Override
    public Type getRawType() {
        return delegate.getRawType();
    }

    @Override
    public Type getOwnerType() {
        return delegate.getOwnerType();
    }

}

I am using org.springframework.core.ResolvableType for a ListResultEntity :

    ResolvableType resolvableType = ResolvableType.forClassWithGenerics(ListResultEntity.class, itemClass);
    ParameterizedTypeReference<ListResultEntity<T>> typeRef = ParameterizedTypeReference.forType(resolvableType.getType());

So in your case:

public <T> ResponseWrapper<T> makeRequest(URI uri, Class<T> clazz) {
   ResponseEntity<ResponseWrapper<T>> response = template.exchange(
        uri,
        HttpMethod.POST,
        null,
        ParameterizedTypeReference.forType(ResolvableType.forClassWithGenerics(ResponseWrapper.class, clazz)));
    return response;
}

This only makes use of spring and of course requires some knowledge about the returned types (but should even work for things like Wrapper>> as long as you provide the classes as varargs )

Note: This answer refers/adds to Sotirios Delimanolis's answer and comment.

I tried to get it to work with Map<Class, ParameterizedTypeReference<ResponseWrapper<?>>>, as indicated in Sotirios's comment, but couldn't without an example.

In the end, I dropped the wildcard and parametrisation from ParameterizedTypeReference and used raw types instead, like so

Map<Class<?>, ParameterizedTypeReference> typeReferences = new HashMap<>();
typeReferences.put(MyClass1.class, new ParameterizedTypeReference<ResponseWrapper<MyClass1>>() { });
typeReferences.put(MyClass2.class, new ParameterizedTypeReference<ResponseWrapper<MyClass2>>() { });

...

ParameterizedTypeReference typeRef = typeReferences.get(clazz);

ResponseEntity<ResponseWrapper<T>> response = restTemplate.exchange(
        uri, 
        HttpMethod.GET, 
        null, 
        typeRef);

and this finally worked.

If anyone has an example with parametrisation, I'd be very grateful to see it.

My own implementation of generic restTemplate call:

private <REQ, RES> RES queryRemoteService(String url, HttpMethod method, REQ req, Class reqClass) {
    RES result = null;
    try {
        long startMillis = System.currentTimeMillis();

        // Set the Content-Type header
        HttpHeaders requestHeaders = new HttpHeaders();
        requestHeaders.setContentType(new MediaType("application","json"));            

        // Set the request entity
        HttpEntity<REQ> requestEntity = new HttpEntity<>(req, requestHeaders);

        // Create a new RestTemplate instance
        RestTemplate restTemplate = new RestTemplate();

        // Add the Jackson and String message converters
        restTemplate.getMessageConverters().add(new MappingJackson2HttpMessageConverter());
        restTemplate.getMessageConverters().add(new StringHttpMessageConverter());

        // Make the HTTP POST request, marshaling the request to JSON, and the response to a String
        ResponseEntity<RES> responseEntity = restTemplate.exchange(url, method, requestEntity, reqClass);
        result = responseEntity.getBody();
        long stopMillis = System.currentTimeMillis() - startMillis;

        Log.d(TAG, method + ":" + url + " took " + stopMillis + " ms");
    } catch (Exception e) {
         Log.e(TAG, e.getMessage());
    }
    return result;
}

To add some context, I'm consuming RESTful service with this, hence all requests and responses are wrapped into small POJO like this:

public class ValidateRequest {
  User user;
  User checkedUser;
  Vehicle vehicle;
}

and

public class UserResponse {
  User user;
  RequestResult requestResult;
}

Method which calls this is the following:

public User checkUser(User user, String checkedUserName) {
    String url = urlBuilder()
            .add(USER)
            .add(USER_CHECK)
            .build();

    ValidateRequest request = new ValidateRequest();
    request.setUser(user);
    request.setCheckedUser(new User(checkedUserName));

    UserResponse response = queryRemoteService(url, HttpMethod.POST, request, UserResponse.class);
    return response.getUser();
}

And yes, there's a List dto-s as well.

Actually, you can do this, but with additional code.

There is Guava equivalent of ParameterizedTypeReference and it's called TypeToken.

Guava's class is much more powerful then Spring's equivalent. You can compose the TypeTokens as you wish. For example:

static <K, V> TypeToken<Map<K, V>> mapToken(TypeToken<K> keyToken, TypeToken<V> valueToken) {
  return new TypeToken<Map<K, V>>() {}
    .where(new TypeParameter<K>() {}, keyToken)
    .where(new TypeParameter<V>() {}, valueToken);
}

If you call mapToken(TypeToken.of(String.class), TypeToken.of(BigInteger.class)); you will create TypeToken<Map<String, BigInteger>>!

The only disadvantage here is that many Spring APIs require ParameterizedTypeReference and not TypeToken. But we can create ParameterizedTypeReference implementation which is adapter to TypeToken itself.

import com.google.common.reflect.TypeToken;
import org.springframework.core.ParameterizedTypeReference;

import java.lang.reflect.Type;

public class ParameterizedTypeReferenceBuilder {

    public static <T> ParameterizedTypeReference<T> fromTypeToken(TypeToken<T> typeToken) {
        return new TypeTokenParameterizedTypeReference<>(typeToken);
    }

    private static class TypeTokenParameterizedTypeReference<T> extends ParameterizedTypeReference<T> {

        private final Type type;

        private TypeTokenParameterizedTypeReference(TypeToken<T> typeToken) {
            this.type = typeToken.getType();
        }

        @Override
        public Type getType() {
            return type;
        }

        @Override
        public boolean equals(Object obj) {
            return (this == obj || (obj instanceof ParameterizedTypeReference &&
                    this.type.equals(((ParameterizedTypeReference<?>) obj).getType())));
        }

        @Override
        public int hashCode() {
            return this.type.hashCode();
        }

        @Override
        public String toString() {
            return "ParameterizedTypeReference<" + this.type + ">";
        }

    }

}

Then you can use it like this:

public <T> ResponseWrapper<T> makeRequest(URI uri, Class<T> clazz) {
   ParameterizedTypeReference<ResponseWrapper<T>> responseTypeRef =
           ParameterizedTypeReferenceBuilder.fromTypeToken(
               new TypeToken<ResponseWrapper<T>>() {}
                   .where(new TypeParameter<T>() {}, clazz));
   ResponseEntity<ResponseWrapper<T>> response = template.exchange(
        uri,
        HttpMethod.POST,
        null,
        responseTypeRef);
    return response;
}

And call it like:

ResponseWrapper<MyClass> result = makeRequest(uri, MyClass.class);

And the response body will be correctly deserialized as ResponseWrapper<MyClass>!

You can even use more complex types if you rewrite your generic request method (or overload it) like this:

public <T> ResponseWrapper<T> makeRequest(URI uri, TypeToken<T> resultTypeToken) {
   ParameterizedTypeReference<ResponseWrapper<T>> responseTypeRef =
           ParameterizedTypeReferenceBuilder.fromTypeToken(
               new TypeToken<ResponseWrapper<T>>() {}
                   .where(new TypeParameter<T>() {}, resultTypeToken));
   ResponseEntity<ResponseWrapper<T>> response = template.exchange(
        uri,
        HttpMethod.POST,
        null,
        responseTypeRef);
    return response;
}

This way T can be complex type, like List<MyClass>.

And call it like:

ResponseWrapper<List<MyClass>> result = makeRequest(uri, new TypeToken<List<MyClass>>() {});