[FIXED] Spring MVC: Issues Binding Request Parameters to a Generic List

Issue

I'm unable to successfully bind request parameters to a command object having a generic list property items:

public class Holder<T> {
  private List<T> items = new ArrayList<>();

  public List<T> getItems() {
    return items;
  }

  public void setItems(List<T> items) {
    this.items = items;
  }
}

Setting items via request parameters appears to work but the values remain Strings just waiting to trigger ClassCastExceptions.

For example, the following code throws a ClassCastException given the invocation

/test?items=1

and Controller handler:

@GetMapping("/test")
public String test(Holder<Integer> holder) {
  List<Integer> items = holder.getItems();

  System.out.println(items.size()); // 1
  System.out.println(items); // [1] 
  if (!items.isEmpty()) {
    System.out.println(items.get(0).getClass()); // java.lang.ClassCastException: java.lang.String incompatible with java.lang.Integer
  }
  return "test";
}

How can I configure Spring MVC to convert the item parameter values to the correct (generic) type?

I'm using Spring Boot 2.3.2.

Solution

You can implement your own HandlerMethodArgumentResolver like this:

@Component
public class HolderMethodArgumentResolver implements HandlerMethodArgumentResolver {

    @Override
    public boolean supportsParameter(MethodParameter methodParameter) {
        return Holder.class.isAssignableFrom(methodParameter.getParameterType());
    }

    @Override
    public Holder resolveArgument(MethodParameter methodParameter, ModelAndViewContainer modelAndViewContainer,
            NativeWebRequest nativeWebRequest, WebDataBinderFactory webDataBinderFactory) {
        Class<?> genericClass = (Class<?>) ((ParameterizedType) methodParameter.getGenericParameterType()).getActualTypeArguments()[0];
        if (Integer.class.isAssignableFrom(genericClass)) {
            String[] values = nativeWebRequest.getParameterValues("items");
            List<Integer> convertedValues = Stream.of(values).map(Integer::valueOf).collect(Collectors.toList());
            return new Holder<>(convertedValues);
        }
        if (String.class.isAssignableFrom(genericClass)) {
            String[] values = nativeWebRequest.getParameterValues("items");
            return new Holder<>(Arrays.asList(values));
        }
        throw new IllegalArgumentException("Generic type " + genericClass + " doesn't support");
    }

}

And after that you can define in contoller different methods:

    @GetMapping("/test")
    public String test(Holder<Integer> holder) {
        List<Integer> items = holder.getItems();

        System.out.println(items.size());
        System.out.println(items);
        if (!items.isEmpty()) {
            System.out.println(items.get(0).getClass());
        }
        return "test";
    }

    @GetMapping("/test2")
    public String test2(Holder<String> holder) {
        List<String> items = holder.getItems();

        System.out.println(items.size());
        System.out.println(items);
        if (!items.isEmpty()) {
            System.out.println(items.get(0).getClass());
        }
        return "test2";
    }

Answered By - saver