Support for MultiValueMap elements in web data binding

[MHajoha]

Affects: spring-web 5.2.11

---

Given a controller like the following:

import javax.annotation.Nullable;

import org.springframework.util.MultiValueMap;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/path")
public class MyController {

	private static class MyCriteria {

		@Nullable
		private String param1;

		@Nullable
		private MultiValueMap<String, String> otherParams;

		// getters and setter omitted
	}

	@GetMapping
	public void getByCriteria(MyCriteria criteria) {
		// body
	}
}

I would expect a GET request to /path?param1=a&otherParams[otherParam]=b to cause an instance of MyCriteria with param1 = "a" and otherParams = {"otherParam": ["b"]} to be passed in.

Instead, the following exception is thrown:

java.lang.ClassCastException: class java.lang.String cannot be cast to class java.util.List (java.lang.String and java.util.List are in module java.base of loader 'bootstrap')
	at org.springframework.util.LinkedMultiValueMap.put(LinkedMultiValueMap.java:42)
	at org.springframework.beans.AbstractNestablePropertyAccessor.processKeyedProperty(AbstractNestablePropertyAccessor.java:371)
	at org.springframework.beans.AbstractNestablePropertyAccessor.setPropertyValue(AbstractNestablePropertyAccessor.java:275)
	at org.springframework.beans.AbstractNestablePropertyAccessor.setPropertyValue(AbstractNestablePropertyAccessor.java:266)
	at org.springframework.beans.AbstractPropertyAccessor.setPropertyValues(AbstractPropertyAccessor.java:104)
	at org.springframework.validation.DataBinder.applyPropertyValues(DataBinder.java:848)
	at org.springframework.validation.DataBinder.doBind(DataBinder.java:744)
	at org.springframework.web.bind.WebDataBinder.doBind(WebDataBinder.java:197)
	at org.springframework.web.bind.ServletRequestDataBinder.bind(ServletRequestDataBinder.java:107)

This works as expected when MultiValueMap<String, String> is replaced with Map<String, List<String>>

[jhoeller]

MultiValueMap value type detection does not seem to work in the core BeanWrapper infrastructure underneath such a web data binding attempt, whereas it does work for a generically typed Map with explicit collection values. We'll revisit this for 5.3.3, with an intent for a 5.2.13 backport.