Cannot use Map as a JSON #RequestParam in Spring REST controller - spring

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());
}
}
}

Answer 2

If you want to use Map<String, String> you have to do the following:
#GetMapping("temp")
public String temp(#RequestParam Map<String, String> blah) {
System.out.println(blah.get("a"));
return "Hello";
}
And the URL for this is: http://localhost:8080/temp?a=b
With Map<String, String>you will have access to all your URL provided Request Params, so you can add ?c=d and access the value in your controller with blah.get("c");
For more information have a look at: http://www.logicbig.com/tutorials/spring-framework/spring-web-mvc/spring-mvc-request-param/ at section Using Map with #RequestParam for multiple params
Update 1: If you want to pass a JSON as String you can try the following:
If you want to map the JSON you need to define a corresponding Java Object, so for your example try it with the entity:
public class YourObject {
private String a;
// getter, setter and NoArgsConstructor
}
Then make use of Jackson's ObjectMapper to map the JSON string to a Java entity:
#GetMapping("temp")
public String temp(#RequestParam Map<String, String> blah) {
YourObject yourObject =
new ObjectMapper().readValue(blah.get("bar"),
YourObject.class);
return "Hello";
}
For further information/different approaches have a look at: JSON parameter in spring MVC controller