Content negotiation: how does Spring decide JSON vs XML?

Content negotiation is how Spring decides which HttpMessageConverter (JSON vs XML vs …) serializes your response, and by default it's driven by the request's Accept header. The same OrderDto can come back as JSON or XML depending on what the client asks for — your controller code doesn't change.

The default: the Accept header

A ContentNegotiationManager (configured by a ContentNegotiationStrategy) inspects the request. The default strategy, HeaderContentNegotiationStrategy, reads the Accept header:

Accept: application/json   -> Jackson JSON converter
Accept: application/xml    -> Jackson XML / JAXB converter (if on classpath)

Spring then finds a registered converter whose supported media type intersects the requested one, and whose produces (if you set it on the mapping) is compatible. With only spring-boot-starter-web, Jackson JSON is present, so JSON is what you get unless an XML converter is also on the classpath.

Other strategies (opt-in)

You can broaden how the media type is determined via WebMvcConfigurer:

@Override
public void configureContentNegotiation(ContentNegotiationConfigurer c) {
    c.favorParameter(true)          // ?format=xml
     .parameterName("format")
     .ignoreAcceptHeader(false)
     .defaultContentType(MediaType.APPLICATION_JSON)
     .mediaType("xml", MediaType.APPLICATION_XML)
     .mediaType("json", MediaType.APPLICATION_JSON);
}

• Header strategy (default) — Accept.
• Parameter strategy — a query param like ?format=json (handy for browsers that can't set Accept).
• Path extension — /orders.xml; largely deprecated for security reasons in modern Spring.
• Fixed default — fall back to defaultContentType when the client expresses no preference.

How it ties to the mapping

produces on @GetMapping(produces = …) narrows which requests a handler even matches, and feeds negotiation. If the client's Accept can't be satisfied by any converter, Spring returns 406 Not Acceptable.

@GetMapping(value = "/{id}", produces = {APPLICATION_JSON_VALUE, APPLICATION_XML_VALUE})
OrderDto get(@PathVariable Long id) { ... } // negotiated per Accept