HalConfiguration changed order of (default) mediatypes

[MelleD]

It took me a while to reconstruct the entire problem chain.

The problem is that with the update to Spring Boot 3.5.0 and hateoas 2.5.0, the default media type in the web test client changed. Before, it was application/json, and now it's application/hal+json, which is changed if you don't specify an explicit content type in the web test client.

The root of the problem is that MediaTypes.VND_HAL_JSON was added to the HalConfiguration.
See here: https://github.com/spring-projects/spring-hateoas/blob/main/src/main/java/org/springframework/hateoas/mediatype/hal/HalConfiguration.java#L70

With the second type the order is now different and the AutoConfiguration here no longer works:
https://github.com/spring-projects/spring-boot/blob/main/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/hateoas/HypermediaAutoConfiguration.java

Because the withMediaType
https://github.com/spring-projects/spring-hateoas/blob/main/src/main/java/org/springframework/hateoas/mediatype/hal/HalConfiguration.java#L237
is setting MediaTypes.APPLICATION+JSON to second position in the new list instead of the first position.

In the end this configurer is now setting application/hal+json as default and not application/json

https://github.com/spring-projects/spring-hateoas/blob/main/src/main/java/org/springframework/hateoas/config/HypermediaWebTestClientConfigurer.java#L62

Is there a reason why HalConfiguration is not adding the new media type to position 0
e.g
newMediaTypes.add(0, mediaType);

[odrotbohm]

Thanks for bringing this to our attention. We are definitely going to fix this. The reason we use - 1 is that we want to make repeated calls to withMediaType(…) preserve the order. In other words, if you call it with A first and B second, it should still be A listed first, B second and the assumed only original media type last. This is, as you identified, broken as we register two default media types now.

[odrotbohm]

This should be fixed in the latest snapshots. For usage with Spring Boot override spring-hateoas.version to 2.5.1-SNAPSHOT.