ObjectStringMessageConverter.toMessage() can't be used with arbitrary payload

[mrpiggi]

I am not quite sure, what the original intention of ObjectStringMessageConverter was, but currently using

ObjectStringMessageConverter converter = new ObjectStringMessageConverter();
Integer payload = 1234;
Message<?> converted = converter.toMessage(payload, null);
assertThat(converted).isNotNull();

fails, as Message<?> converted is always null when payload is anything else than String.class as AbstractMessageConverter.canConvertTo()

https://github.com/spring-projects/spring-framework/blob/80646591367b93d220b9dc0b0a126b203edc140c/spring-messaging/src/main/java/org/springframework/messaging/converter/AbstractMessageConverter.java#L231-L235

always returns null due to StringMessageConverter.supports()
https://github.com/spring-projects/spring-framework/blob/80646591367b93d220b9dc0b0a126b203edc140c/spring-messaging/src/main/java/org/springframework/messaging/converter/StringMessageConverter.java#L50-L55
returns false in this case.

The question arises, whether this is intentional. If ObjectStringMessageConverter.toMessage() should not be used with anything else than String payload explicitly throwing an IllegalArgumentException would probably be a better option. For the opposite case I will provide a PR.

[mrpiggi]

Once I am on it:
package org.springframework.messaging.converter is marked with both @NonNullApi and @NonNullFields. Should this hold for package org.springframework.integration.support.converter as well? If confirmed, I would prepare a PR.