How to modify request body before reaching controller in spring boot

Another alternative would be adding an attribute to the HttpServletRequest object. And after that you can read that attribute in the Controller class with @RequestAttribute annotation.

In the Interceptor

@Component
    public class SimpleInterceptor extends HandlerInterceptorAdapter {

        @Override
        public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
                throws ServletException, IOException {
            String parameter = request.getParameter("parameter");
            if (parameter == "somevalue") {
                request.setAttribute("customAttribute", "value");
            }

            return true;
        }

    }

In the Controller

@RestController
@RequestMapping("")
public class SampleController {


    @RequestMapping(value = "/sample",method = RequestMethod.POST)
    public String work(@RequestBody SampleRequest sampleRequest, @RequestAttribute("customAttribute") String customAttribute) {
        System.out.println(customAttribute);
        return "This works";
    }
}

This has advantage of not modifying the request body.