Actuator endpoints support for Jackson's Serialization Views

[zeljko-mirovic]

I am writing actuator endpoint having two @ReadOperation-s, one without @Selector, one with:

• @ReadOperation without the selector is returning a collection of DTOs. That DTO contains a potentially long List of objects. To avoid cluttering output with all elements of the list, I would like to return only last element of the list in each DTO.
• @ReadOperation with the selector is returning one DTO. In that case, I want to return all elements of the inner DTO list.

A convenient way to do this is by using Jackson's Serialization Views. Two views are defined, and @JsonView annotations are added to DTO getters and methods annotated with @ReadOperation. The output is not affected by annotations presence.

Simplified example:

public interface DtoView {

    public interface Last {
    }

    public interface All {
    }
}

public class Dto {

    private List<Element> elements;
    ...

    @JsonView(DtoView.Last.class)
    public Element getLastElement() {
        ...
    }

    @JsonView(DtoView.All.class)
    public List<Element> getElements() {
        ...
    }
}

@Endpoint(id = "example")
public class ExampleEndpoint {

    @ReadOperation
    @JsonView(DtoView.Last.class)
    public Collection<Dto> read() {
        ...
    }

    @ReadOperation
    @JsonView(DtoView.All.class)
    public Dto read(@Selector String selector) {
        ...
    }
}

After converting actuator endpoint to @RestContoler output is affected.

Simplified example:

@RestController
public class ExampleControler {

    @GetMapping(path = "/example", produces = MediaType.APPLICATION_JSON_VALUE)
    @JsonView(DtoView.Last.class)
    public Collection<Dto> read() {
        ...
    }

    @GetMapping(path = "/example/{selector}", produces = MediaType.APPLICATION_JSON_VALUE)
    @JsonView(DtoView.All.class)
    public Dto read(@PathVariable String selector) {
        ...
    }
}

Having in mind that Jackson's Serialization Views are supported in Spring Framework since version 4.2, it will be nice to have similar support for actuator endpoints.

[philwebb]

I'm personally not too keen to tie our actuator infrastructure any further Jackson. We have a long standing issue to support other serialization technologies and I think adding direct @JsonView support would make that harder.

If you're happy only supporting Spring MVC, you could look at the @ControllerEndpoint annotation which should allow you to use more MVC features.

[philwebb]

Flagging to see what the rest of the team think about adding @JsonView support.

[bclozel]

See #20291 for the actuator-specific JSON mapper instance.
Also, we probably want to deprecate/transform the existing @ControllerEndpoint if we want to achieve #20290.

[wilkinsona]

I'm personally not too keen to tie our actuator infrastructure any further Jackson

That's my feeling too. An alternative to @ControllerEndpoint that also allows you to continue to use the Actuator endpoint infrastructure, you could use an ObjectMapper in your operation methods to turn the DTO into a Map and apply any views at that time.

[zeljko-mirovic]

@wilkinsona, thank you for the idea to turn the DTO into a Map and return Map instead.

[philwebb]

Since there's a work-around, I'm going to politely decline this one. Thanks anyway for the suggestion.