Spring Web - How to format `Instant` in a specific way

Question

I am developing a REST controller in Spring. One of the requirements is to be able to handle dates only in ISO-8601 format.

I'm using Instant for this purpose. When used in request DTOs, it handles incoming dates perfectly. But when used in response DTOs, the output format needs a little change.

The date in json output looks like this:
2022-02-03T15:00:00Z

But it should look like this:
2022-02-03T15:00:00.000Z

I want to specify how RestController formats Instant. To that end, I can think of several solutions.

1. Is there a way to override how RestController converts a specific class to string? Perhaps in the same way that exception handling is done (with @ExceptionHandler).
2. Is there a way to intercept the DTO after it gets converted to json? Before getting sent off to the client, that is.
3. Is there a way to simply annotate the return value of the rest controller's method to achieve the desired format?

Answer 1

The solution was to use JavaTimeModule. I defined the ObjectMapper bean configuration for Spring's DI, and then used the @JsonFormat annotation on the Instant field of the DTO.

One thing to note is that when using the pattern field of @JsonFormat, the timezone field becomes mandatory for some reason. Even setting the time-zone in application.properties doesn't work. I set timezone = "0", as I need to use UTC dates.

Answer 2

If you use Jackson to serialize and deserialize JSON data in you Spring Boot project, I would like to suggest you use LocalDate/LocalDateTime types for your needs. Then, you can edit your application.properties and add here such row:

jackson.date-format=com.fasterxml.jackson.databind.util.ISO8601DateFormat

Also, you need to have a Maven/Gradle dependency jackson-modules-java8

Probably, it will also work with Instant type, but I didn't test it.

Also, if you use your own ObjectMapper (not autoconfigured by Spring) you need to add Jackson Module to it. Here is an explanation in another question.