How to parse a Spring 5 WebClient response in a non-blocking way?

Question

I'm using Spring WebFlux WebClient to retrieve data from an external API, like this:

public WeatherWebClient() {
    this.weatherWebClient = WebClient.create("http://api.openweathermap.org/data/2.5/weather");
}

public Mono<String> getWeatherByCityName(String cityName) {
    return weatherWebClient
            .get()
            .uri(uriBuilder -> uriBuilder
                                .queryParam("q", cityName)
                                .queryParam("units", "metric")
                                .queryParam("appid", API_KEY)
                                .build())
            .accept(MediaType.APPLICATION_JSON)
            .retrieve()
            .bodyToMono(String.class);
}

This works fine and produces a response like this:

{
    "coord":{
        "lon":-47.06,
        "lat":-22.91
    },
    "weather":[
    {
        "id":800,
        "main":"Clear",
        "description":"clear sky",
        "icon":"01d"
    }
    ],
    "base":"stations",
    "main":{
        "temp":16,
        "pressure":1020,
        "humidity":67,
        "temp_min":16,
        "temp_max":16
    },
    "visibility":10000,
    "wind":{
        "speed":1,
        "deg":90
    },
    "clouds":{
        "all":0
    },
    "dt":1527937200,
    "sys":{
        "type":1,
        "id":4521,
        "message":0.0038,
        "country":"BR",
        "sunrise":1527932532,
        "sunset":1527971422
    },
    "id":3467865,
    "name":"Campinas",
    "cod":200
}

But I'm only interested in the "temp" property (main -> temp). How could I transform the response (using Jackson's ObjectMapper, for example) to return only "temp" value in a reactive/non-blocking way?

I understand the first thing is replacing ".retrieve()" by ".exchange()" but I can't figure out how to make it work.

PS: This is my first question here. Please let me know if I'm doing something wrong or if you need more details.

Thanks!

Answer 1

You need to create a type that corresponds to the response sent by the server. A very minimal example could be like this:

@JsonIgnoreProperties(ignoreUnknown = true)
public class WeatherResponse {
    public MainWeatherData main;
}

and the MainWeatherData class could be:

@JsonIgnoreProperties(ignoreUnknown = true)
public class MainWeatherData {
    public String temp;
}

Finally, you could use WeatherResponse in bodyToMono:

...
   .retrieve()
   .bodyToMono(WeatherResponse.class);

The @JsonIgnoreProperties(ignoreUnknown = true)annotation instructs Jackson to not give any errors if it encounters any value in JSON string that is not present in you POJO.

You can access the WeatherResponseobject with a chained map operator:

getWeatherByCityName(cityName)
     .map(weatherResponse -> weatherResponse.main.temp)