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As explained above, message body providers can play a major part in affecting the way target resource methods are matched. If a method returns a custom type Foo and a MessageBodyWriter<Foo> is available then it will be used only if one of the media types specified in a given request's HTTP Accept header matches or intersects with one of the media types specified by @Produces annotation in a MessageBodyWriter<Foo> implementation. The same applies if a method accepts a custom type Foo, but this time the value of @Consumes in MessageBodyReader<Foo> will be matched against a request's ContentType value.

Sometimes users would like to experiment with media types different to those statically supported by a given message body reader/writer. For example, application/xml might seem a bit too general in some cases and people may want to try some custom/xml type and still use a default JAXB provider.

In such cases it's possible to override the default @Produces and/or @Consumes types supported by a given provider. It only currently works for JAXBElementProvider and JSONProvider, but any custom provider can avail of this support by simply having setter/getter pairs for either produce and/or consume types and the JAXRS runtime will use them instead.
See this example on how to provide custom media types from Spring.

Support for data bindings

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