Spring Custom Annotation to process request body

GreetingController.java

package hello;

import org.springframework.web.bind.annotation.*;

import java.util.Map;
import java.util.concurrent.atomic.AtomicLong;

@RestController
public class GreetingController {
    private static final String template = "Hello, %s!";
    private final AtomicLong counter = new AtomicLong();

    @RequestMapping("/greeting")
    public Greeting greeting(@UpperCase(value="name") String name) {
        return new Greeting(counter.incrementAndGet(),
                String.format(template, name));
    }

    @PostMapping("/save")
    @ResponseBody
    public Object save(@UpperCase("req") Object req) {
        return ((Map<String, String>) req).get("decoded");
    }
}

UpperCase.java

package hello;

import java.lang.annotation.*;

@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface UpperCase {
    String value();
}

UpperCaseResolver.java

package hello;

import org.apache.commons.io.IOUtils;
import org.springframework.core.MethodParameter;
import org.springframework.web.bind.support.WebDataBinderFactory;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.method.support.HandlerMethodArgumentResolver;
import org.springframework.web.method.support.ModelAndViewContainer;

import javax.servlet.http.HttpServletRequest;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public final class UpperCaseResolver implements HandlerMethodArgumentResolver {
    private static final String JSONBODYATTRIBUTE = "JSON_REQUEST_BODY";

    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        return parameter.getParameterAnnotation(UpperCase.class) != null;
    }

    @Override
    public Object resolveArgument(
            MethodParameter parameter,
            ModelAndViewContainer mavContainer,
            NativeWebRequest webRequest,
            WebDataBinderFactory binderFactory
    ) {
        String body = getRequestBody(webRequest);
        Map<String, String> resultMap = new HashMap<>();
        StringBuilder sb = new StringBuilder();
        sb.append("decoded: ");
        sb.append(body);
        resultMap.put("decoded", sb.toString());
        resultMap.put("someotherfield", "yes");
        return resultMap;
    }

    private String getRequestBody(NativeWebRequest webRequest){
        HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
        String jsonBody = (String) servletRequest.getAttribute(JSONBODYATTRIBUTE);
        if (jsonBody==null){
            try {
                String body = IOUtils.toString(servletRequest.getInputStream());
                servletRequest.setAttribute(JSONBODYATTRIBUTE, body);
                return body;
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return "";

    }
}