@ControllerAdvice
public class GlobalInitBinder {

    @InitBinder
    public void initBinder(WebDataBinder binder, HttpServletRequest request) {
        //Spring 6 fails to retrieve Params parameter objects, with binder.getTarget() = null; 
        // please help troubleshoot the issue.
        Object target = binder.getTarget();
        if (target == null) {
            return;
        }
        Map<String, String[]> parameterMap = request.getParameterMap();
        MutablePropertyValues mpv = new MutablePropertyValues(parameterMap);
        Field[] fields = target.getClass().getDeclaredFields();
}

@RestController
@RequestMapping("v1")
public class TestController {

    @PostMapping("/test")
    public void getLoadWeightRuleList(Params params) {}
}

import lombok.Data;
@Data
public class Params implements Serializable {
    @FormFieldAlias(alternateNames = {"PageNum", "PageNumber", "pageNum"})
    private Integer pageNum;
}

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

public class Params implements Serializable {
    @BindParam("PageNumber")
    private Integer pageNum;
    public Params(Integer pageNum) {
        this.pageNum = pageNum;
    }
}