Spring boot application throws http error code 500 whenever we tries to send any object with any dictionary(Map) with null key, exception is swallowed

[dhananjay-bhatt]

I am running spring boot application returning following sample response object:

@Getter
@Setter
@ToString
@JsonInclude(JsonInclude.Include.NON_NULL)
@JsonIgnoreProperties(ignoreUnknown = true)
@NoArgsConstructor
@AllArgsConstructor
@Builder
class Response {
private String name;
private Map<String, Object> attr;
}

Now since my app is java based application,
this means that atleast one null key is allowed in Map.
So my sample response look like:

Response responseObj = someServiceMethodCall()

and response looks like:

{
"name" : "some-name",
"attr" : {
          null : "some-value",
          "some-key" : "other-value"
       }
}

Now when I try to send this via my basic controller as response:

return ResponseEntity.ok(responseObj);

Now for this case in my postman/client I get 500 response.
Even in logs I cant see any errors from spring.

Now if this thing happen in production we cant identify simply, also enabling debug logs at spring level needs restart of system and can actually create problem.

It would be great if we have error logging around that.

I am using 2.X.X version of spring boot,
but I tested with 3.X.X also.

I debugged it, and found while doing jackson parsing to json it fails as in json NULL keys are not allowed.
But the exception is swallowed and not logged even

[bclozel]

This is rather surprising, as I thought that Jackson did not allow null keys being serialized in Maps. Can you provide a minimal sample application?

[spring-projects-issues]

If you would like us to look at this issue, please provide the requested information. If the information is not provided within the next 7 days this issue will be closed.

[bclozel]

I have created a sample application to try and reproduce this:

@RestController
public class NullKeyController {

	@GetMapping("/")
	public Response index() {
		HashMap<String, Object> map = new HashMap<>();
		map.put("message", "Hello World!");
		map.put(null, "other");

		return new Response("Spring", map);
	}

	record Response(String message, Map<String, Object> data) {}
}

I do see this log entry in the console, which explains quite well the issue in my opinion.

2025-04-02T10:23:53.851+02:00  WARN 99114 --- [nullkey] [nio-8080-exec-2] .w.s.m.s.DefaultHandlerExceptionResolver : Resolved [org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Null key for a Map not allowed in JSON (use a converting NullKeySerializer?)]

I can't reproduce the behavior described here. We can reopen this issue if you provide us with a sample application that reproduces the problem.