Improve generic warnings when implementing BodyExtractor [SPR-16909]

[spring-projects-issues]

GanMing opened SPR-16909 and commented

 I was looking forward to extracting a tool class from methods such as readToMono, readToFlux, and findReader in BodyExtractors. This would improve the problem of generic warnings.

 Or is there a better way?In short, it would be great to get rid of that annoying warning and make it more reliable to use.

BodyExtractor<Mono<Map<String, Object>>, ReactiveHttpInputMessage> bodyExtractor = (msg, context) -> {
		ResolvableType elementType = ResolvableType.forClassWithGenerics(Map.class, String.class, Object.class);
		HttpMessageReader<Map<String, Object>> reader = context.messageReaders()
				.stream().filter(messageReader -> messageReader.canRead(elementType, MediaType.APPLICATION_JSON))
				.findFirst()
				//What actually intimidates me is the following unchecked generic cast-cause warning, because it interferes with the developer's focus.
				.map(r -> (HttpMessageReader<Map<String, Object>>) r)
				.orElseThrow(() -> new IllegalStateException(
						"No HttpMessageReader for \"" + MediaType.APPLICATION_JSON_VALUE + "\" and \"" + elementType + "\""));
		return reader.readMono(elementType, msg, context.hints());
	};

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Reference URL: #21396

[spring-projects-issues]

Arjen Poutsma commented

Given that messageReaders() returns a List<HttpMessageReader<?>> (note the wildcard), i.e. a list of message readers that can handle any kind of type, there is nono other way but to cast. We do it ourselves too, see

https://github.com/poutsma/spring-framework/blob/d77797f42c2da26286d5bb9681d6f2a0018792ce/spring-webflux/src/main/java/org/springframework/web/reactive/function/BodyExtractors.java#L244