Java Reference
In-Depth Information
Chapter 9. HTTP Content Negotiation
Within any meaningfully sized organization or on the Internet, SOA (service-oriented archi-
tecture) applications need to be flexible enough to handle and integrate with a variety of cli-
ents and platforms. RESTful services have an advantage in this area because most program-
ming languages can communicate with the HTTP protocol. This is not enough, though. Dif-
ferent clients need different formats in order to run efficiently. Java clients might like their
data within an XML format. Ajax clients work a lot better with JSON. Ruby clients prefer
YAML. Clients may also want internationalized data so that they can provide translated in-
formation to their English, Chinese, Japanese, Spanish, or French users. Finally, as our
RESTful applications evolve, older clients need a clean way to interact with newer versions
of our web services.
HTTP does have facilities to help with these types of integration problems. One of its most
powerful features is a client's capability to specify to a server how it would like its responses
formatted. The client can negotiate the content type of the message body, how it is encoded,
and even which human language it wants the data translated into. This protocol is called
HTTP Content Negotiation, or conneg for short. In this chapter, I'll explain how conneg
works, how JAX-RS supports it, and most importantly how you can leverage this feature of
HTTP within your RESTful web services.
Conneg Explained
The first part of HTTP Content Negotiation is that clients can request a specific media type
they would like returned when querying a server for information. Clients set an Accept re-
quest header that is a comma-delimited list of preferred formats. For example:
Accept: application/xml, application/json
In this example request, the client is asking the server for /stuff formatted in either XML or
JSON. If the server is unable to provide the desired format, it will respond with a status code
of 406, “Not Acceptable.” Otherwise, the server chooses one of the media types and sends a
response in that format back to the client.
Wildcards and media type properties can also be used within the Accept header listing. For
Accept: text/*, text/html;level=1
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