@Override
	public int hashCode() {
		return (getExecutable().hashCode() * 31 + this.parameterIndex);
	}

HandlerMethodArgumentResolverComposite uses MethodParameter as a cache key for HandlerMethodArgumentResolver.

Consider:

@FunctionalInterface
public interface GenericHandler<P> {

	Object handle(P payload, MessageHeaders headers);

}

		public static class MapHandler implements GenericHandler<Map<String, String>> {

			@Override
			public String handle(Map<String, String> mapPayload, MessageHeaders messageHeaders) {
				return "Hello " + mapPayload.get("key");
			}

		}

		public static class StringHandler implements GenericHandler<String> {

			@Override
			public String handle(String stringPayload, MessageHeaders messageHeaders) {
				return stringPayload + " World!";
			}

		}

MethodParameter.executable in both cases is the interface Method:

public abstract java.lang.Object org.springframework.integration.handler.GenericHandler.handle(java.lang.Object,org.springframework.messaging.MessageHeaders)

When invoking the second method, we get a cache hit and fail with a class cast exception because the wrong argument resolver is returned.

Spring Integration recently changed to use a shared DefaultMessageHandlerMethodFactory @Bean, and hence a shared HandlerMethodArgumentResolverComposite.

We can work around this by changing the bean scope to prototype, but I think the hashCode() and equals() should include at least the containingClass and/or parameterType field.

See spring-projects/spring-integration#3004