Customize Spring @RequestParam Deserialization for Maps and/or Nested Objects

Question

@RestController
class MyController {
     @RequestMapping(...)
     public void test(Container container) { ... }
}

Spring by default uses Dot-Notation to deserialize a nested @RequestParam:

class Container {
    A a;
}

class A {
    String val;
}

works with:

http://.../myController?a.val=foo

---

But for Maps it uses Square Bracket notation:

class Container {
    Map<String, String> a;
}

works with:

http://.../myController?a[val]=foo

---

When using JavaScript there's of course no difference between a HashMap and a Nested Object, so everything will get serialized either with Dots or Square-Brackets.

---

Question:

How / where can I tell Spring (or Spring Boot if that's easier) to use Dot-Notation (or Square Brackets) for both, nested objects and Maps?

Or is there any reason why Spring makes a difference between those types?

Answer 1

Spring Boot supports the use of dot-separated paths to bind maps thanks to its custom DataBinder subclass, RelaxedDataBinder. The good news is that its also a DataBinder that's used in Spring MVC to perform the request parameter binding. The bad news is that plugging in your own binder isn't straightforward and that it needs to be a WebDataBinder. You can plug one in by declaring your own RequestMappingHandlerAdapter bean named requestMappingHandlerAdapter. For example:

@Bean
public RequestMappingHandlerAdapter requestMappingHandlerAdpter() {
    return new RequestMappingHandlerAdapter() {

        @Override
        protected InitBinderDataBinderFactory createDataBinderFactory(
                List<InvocableHandlerMethod> binderMethods)
                throws Exception {
            return new ServletRequestDataBinderFactory(binderMethods, getWebBindingInitializer()) {

                @Override
                protected ServletRequestDataBinder createBinderInstance(
                        final Object target, String objectName,
                        NativeWebRequest request) {

                    return new ServletRequestDataBinder(target) {

                        private RelaxedDataBinder relaxedBinder = new RelaxedDataBinder(target);

                        @Override
                        protected void doBind(MutablePropertyValues mpvs) {
                            this.relaxedBinder.bind(mpvs);
                        }
                    };
                }
            };
        }   
    };
}

You may well want to refactor this to avoid the use of multiple nested anonymous inner classes, but it hopefully illustrates the general approach.