There is now a JsonMapper strategy with implementations and
autoconfiguration for Gson and Jackson. If Jackson is present
it is preferred (just like in Spring Boot).
Fixes gh-150
This results in a better experience for users because the consumer
that they write is only applied to a Flux that is subscribed to
by the framework once. It gives better control over the flow of
foos, e.g. if some component wants to subscribe on a thread.
User can switch off source or sink behaviour (the default is to bind
to input and output streams), and then configure the name of a
supplier (for a source) or consumer (for a sink).
With this change there are now 2 choices for web endpoints. The
stream servlet binder is useful for multi-binder use cases
(e.g. HTTP -> message broker).
A registered singleton doesn't have a BeanDefinition, but it might have
a compiled type with enough generic information to pull out the
input and output types.
If the output is a Message we grab the headers from the first one
and send those as response headers.
A function can add headers to messages. The HTTP response will
contain only headers that are in x-* _or_ were added to the message
by user (i.e. they weren't in the request).
We certainly do not want to pass through all HTTP headers (request
headers can conflict with or invalidate response headers).
Compiled functions always show up with no metadata, and you can only
guess what the types are from the bean definition. Probably we should
add more information to the bean definition if we have it when we
compile the function.
There is still a problem if user defines functions that are not of
Flux<String> (but that has always been the case).
If the request is a single value, we already know, so if we also
know that the function returned a single-valued type, then we can
render the single value, instead of an array with a single item.
See gh-36
So that single Strings can be POSTed without JSON conversion.
There's still some work to do to support single POJOs in JSON, and
to reach parity with the WebFlux reactive type handlers, but it's
now closer to what we had before we moved the String conversion
out of the function layer.
When the user has sent us a single value, we can make the signature
of the handler and the format of the HTTP response much more
natural if it is single valued too (i.e. a Mono).
An HTTP response does not have to be an infinite stream, and in fact
life is simpler if it is not. The timeout in the web wrappers can
be used to close the response and return normally to a client
that has been waiting more than (say) 1s, instead of treating
it as an error condition.
Error handling is still kind of unsolved.
If user sends "Accept: */*" we don't want to default to sending
an SSE (for instance). So the logic for detecting those preferences
has to take MediaType.ALL into account as a special case.
We don't need to cover all the possible uses of Flux (only
Flux<String> really), so this isn't comprehensive coverage of
all the features in Spring WebFlux, but it's good enough for
functions to run with Spring Boot 1.5.
User can POST to web endpoint in SSE style, i.e:
HTTP/1.1
Content-Type: text/event-stream
data:foo
data:bar
Will be converted to a Flux with values foo and bar
