WebFlux can't serialize "Object" return types as response body [SPR-15731]

[spring-projects-issues]

Brian Clozel opened SPR-15731 and commented

Given this WebFlux endpoint:

@RestController
public class TestController {

	@GetMapping("/test")
	public Object myTest() {
		return Mono.just(new User("spring"));
	}
}

Calling that endpoint will result in the following:

java.lang.IllegalArgumentException: Unrecognized Type: org.springframework.core.ResolvableType$EmptyType@6b003199
	at com.fasterxml.jackson.databind.type.TypeFactory._fromAny(TypeFactory.java:1222) ~[jackson-databind-2.9.0.pr4.jar:2.9.0.pr4]
	at com.fasterxml.jackson.databind.type.TypeFactory.constructType(TypeFactory.java:616) ~[jackson-databind-2.9.0.pr4.jar:2.9.0.pr4]
	at org.springframework.http.codec.json.Jackson2CodecSupport.getJavaType(Jackson2CodecSupport.java:89) ~[spring-web-5.0.0.BUILD-20170703.083012-345.jar:5.0.0.BUILD-SNAPSHOT]
	at org.springframework.http.codec.json.Jackson2JsonEncoder.encodeValue(Jackson2JsonEncoder.java:144) ~[spring-web-5.0.0.BUILD-20170703.083012-345.jar:5.0.0.BUILD-SNAPSHOT]
	at org.springframework.http.codec.json.Jackson2JsonEncoder.lambda$encode$0(Jackson2JsonEncoder.java:125) ~[spring-web-5.0.0.BUILD-20170703.083012-345.jar:5.0.0.BUILD-SNAPSHOT]

It seems that the ResponseBodyResultHandler looks at the method return type to get the type information and because this endpoint exposes Object, this leads to an instance of ResolvableType.EmptyType(), which Jackson doesn't know how to deal with.

In those cases, should we derive the type at runtime?
If this is intended behavior, should we forbid Object return types in handlers and perform a hard check at startup?

---

Affects: 5.0 RC2

Issue Links:

• Returning ResponseEntity from WebFlux controller method declared to return Object serialises entire response entity to response body [SPR-15785] #20340 Returning ResponseEntity from WebFlux controller method declared to return Object serialises entire response entity to response body

[spring-projects-issues]

Rossen Stoyanchev commented

It does work if the actual type is the POJO to render. I applied a small fix so that the generic type we determine from Mono doesn't end up being ResolvableTypeNone.