HAL not set as default in 1.2.0

[lazee]

In Spring Boot 1.1.10 the default hateoas format is HAL as long as spring-hateoas is included in the classpath (According to the doc and real life experience). This is no longer the case and I had to add both @EnableWebMvc and @EnableHypermediaSupport(type= {EnableHypermediaSupport.HypermediaType.HAL}) to my application in order to get HAL.

Is this intended behaviour or a bug? Looking at spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/hateoas/HypermediaAutoConfiguration.java it seems a bug has been fixed for another issue (#1729). Would this fix also fix this HAL problem?

[philwebb]

I'm seeing some interesting differences between 1.1.10 and 1.2.0 but it does appear to me that HAL support is enabled. I've added a sample application and a test to this branch. The difference between 1.1.x and master seems to be that the accept header must be set to application/hal+json in order to get the correct response. With 1.1.x a header of application/json also seemed to return _links in the json.

Do you have a sample application that demonstrates the issue? Have you also tried setting the accepts header in the request?

[philwebb]

/cc @olivergierke and @wilkinsona who are much more familiar with that code.

[wilkinsona]

I believe this problem was caused by the fix for #1729. Here's what's happening:

In 1.2.0 it's only the TypeConstrainedMappingJackson2HttpMessageConverter that has the HATEOAS mixins registered with its ObjectMapper. This converter only handles application/hal+json. In 1.1.10 both the TypeConstrainedMappingJackson2HttpMessageConverter and a MappingJackson2HttpMessageConverter have the mixins registered with their ObjectMapper. It's the MappingJackson2HttpMessageConverter converter that handles the application/json request.

In 1.1.10 the TypeConstrainedMappingJackson2HttpMessageConverter and the first MappingJackson2HttpMessageConverter share an ObjectMapper and, therefore, share the same mixin configuration.

In the absence of an ObjectMapper bean, JacksonAutoConfiguration creates a @Primary ObjectMapper. In 1.2.0 it's configured to be auto-configured before HypermediaAutoConfiguration so its ObjectMapper is created and used by MappingJackson2HttpMessageConverter. When
TypeConstrainedMappingJackson2HttpMessageConverter is created it's configured to use the separate HAL ObjectMapper hence the different mixin configuration. In 1.1.10, HypermediaAutoConfiguration goes first and registers the HAL ObjectMapper bean. This prevents JacksonAutoConfiguration from creating its ObjectMapper which causes the MappingJackson2HttpMessageConverter and TypeConstrainedMappingJackson2HttpMessageConverter to share ObjectMapper instances and to, therefore, also share the mixin configuration.