Given an assignment to return specific data using Springboot reactive but the JSON is really complicated

Question

I am new to Springboot reactive

I was asked to call the following endpoint and return todays weather data only: https://api.weather.gov/gridpoints/MLB/33,70/forecast

I believe I need to use something like this...

WebClient.create().get()
                .uri("https://api.weather.gov/gridpoints/MLB/33,70/forecast")
                .retrieve()
                .bodyToMono(WeatherClass.class)
                .block();

Do I need to map out an entire java object to match the JSON at the endpoint? is there an easy way to perhaps just grab the a certain piece of the JSON?

How would I handle something like the @context annotation in the JSON.

Answer 1

The WebClient in spring boot automatically uses Jackson's ObjectMapper to unmarshall json to a java object when the content type of the response is application/json. So there is no need to pull in any additional libraries or have to write any specific unmarshalling code, unless you want to use an alternate json-to-java library.

When using Jackson, you don't need to map every field in the json to your java object. You can annotate your java class with @JsonIgnoreProperties to inform jackson to ignore any properties that may appear in the json but do not have a matching field in your java object.

An example WeatherClass in which you want only the @context and forecastGenerator unmarshalled would look something like this

    @JsonIgnoreProperties
    public class WeatherClass {
        private final List<Object> context;
        private final WeatherProperties weatherProperties;

        public WeatherClass(@JsonProperty("@context") List<Object> context,
                            @JsonProperty("properties") WeatherProperties weatherProperties) {
            this.context = context;
            this.weatherProperties = weatherProperties;
        }

        private class WeatherProperties {
            private final String forecastGenerator;

            private WeatherProperties(@JsonProperty("forecastGenerator") String forecastGenerator) {
                this.forecastGenerator = forecastGenerator;
            }
        }
    }

Note @context seems to be an array that can contain multiple types (both objects and strings in your example). I've used Object to work around this but obviously isn't the most graceful solution but should be adequate to demonstrate how Jackson works

Alternatively, you can unmarshall the response to a JsonNode, which you can then use to traverse the structure of the json without converting it to a java object. For example

        String forecastGenerator = WebClient.create().get()
                .uri("https://api.weather.gov/gridpoints/MLB/33,70/forecast")
                .retrieve()
                .bodyToMono(JsonNode.class)
                .block().get("properties").get("forecastGenerator").toString()

There are many other annotations provided by Jackson that can used to define how the unmarshaller functions. Too many to cover here. See Jackson Deserialisation Annotations