An example showing Spring Feign as a HATEOAS client of Data Rest
This example contains three projects:
- data-rest-service: a Data REST service using an embedded H2 Database, JpaRepository and HATEOAS/HAL (by default).
- feign-client-boot: a Feign REST client configured to understand HATEOAS/HAL. It uses Spring Boot.
- feign-client-nonboot: even though Feign falls under the Spring Cloud umbrella, which in turn falls under Spring Boot, the use of Feign is also useful outside Boot, like RestTemplate. This example is a version of feign-client-boot without the dependencies on Spring Boot.
I was trying to get Springs RestTemplate (as a REST client) to work with a REST service. RestTemplate works fine when using basic JSON (level 2 on the Richardson REST Maturity Model), but when adding hypermedia (level 3) to the REST service (which is considered as a good practice) in combination with requests that return a list of results, the use of RestTemplate is complicated a lot: you suddenly have to configure the RestTemplate to be able to understand the HAL (Hypertext Application Language) JSON standard convention followed by the use of classes such as Resource, Resources, PagedResources, followed by a verbose/ugly method call to perform the request: exchange() method with 4 arguments, of which one is null and of which one is an empty anonymous class (ParameterizedTypeReference) that is required to give the type parameter to the RestTemplate, otherwise it won't be able to map it back to the original class and then thus creates a basic JSON hashmap instead.
Information about getting Spring Feign to work with HATEOAS/HAL (which is used by default by Spring Data REST), is pretty scarce: examples are often without HATEOAS, which makes it difficult to get it to work with Data REST. People easily run into JSON deserialization problems like http://stackoverflow.com/questions/30515483/feign-and-hal-resources
Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token
Fortunately in https://github.com/joshlong/not-a-last-minute-demo I found a working example of Feign with HATEOAS, on which my example is based. My example shows a Feign project stripped to the minimum, communicating with a Data REST project.
See in code.