Cannot use Map as a JSON @RequestParam in Spring REST controller

Question

This controller

@GetMapping("temp")
public String temp(@RequestParam(value = "foo") int foo,
                   @RequestParam(value = "bar") Map<String, String> bar) {
    return "Hello";
}

Produces the following error:

{
    "exception": "org.springframework.web.method.annotation.MethodArgumentConversionNotSupportedException",
    "message": "Failed to convert value of type 'java.lang.String' to required type 'java.util.Map'; nested exception is java.lang.IllegalStateException: Cannot convert value of type 'java.lang.String' to required type 'java.util.Map': no matching editors or conversion strategy found"
}

What I want is to pass some JSON with bar parameter: http://localhost:8089/temp?foo=7&bar=%7B%22a%22%3A%22b%22%7D, where foo is 7 and bar is {"a":"b"} Why is Spring not able to do this simple conversion? Note that it works if the map is used as a @RequestBody of a POST request.

Answer 1

Here is the solution that worked: Just define a custom converter from String to Map as a @Component. Then it will be registered automatically:

@Component
public class StringToMapConverter implements Converter<String, Map<String, String>> {

    @Override
    public Map<String, Object> convert(String source) {
        try {
            return new ObjectMapper().readValue(source, new TypeReference<Map<String, String>>() {});
        } catch (IOException e) {
            throw new RuntimeException(e.getMessage());
        }
    }
}