My first experience with Jackson.

Hi all! Once upon a time, in a galaxy far, far away, a very long JSON was found... And I became too lazy to create a POJO for it. And I asked myself: Imagine a situation in which I receive a response in the form of JSON with, for example, exchange rates. There are a lot of fields in JSON itself, and I need 2 of them. And now I would like to know - can I create a class with the fields I need and try to parse this json into a class object? Will Jackson understand what I want from him? And, accordingly, if he understands and can do it this way, how can he do it more correctly and what will work faster? So, regarding the question I had about JSON and Jackson's perception of it: Jackson will understand everything. He is smart. He just needs a little help with this. We create a POJO - a regular Java class, which will describe the variables we need from JSON. I’ll say right away that for this it is highly desirable to study the JSON itself, for which we are writing a class (most likely it will have nested classes that also need to be created). Next, using the @JsonCreator annotation above the constructor, we show that the fields of this class need to be filled in from JSON. In the constructor parameters, we can specify which Json fields to assign to the class fields using the @JsonParam("JsonFieldName") annotation. If there are more fields in json than we need (and this was originally my question), we must warn about this and explain that we do not need the remaining fields. For this we use the @JsonIgnoreProperties(ignoreUnknown=true) annotation. Then, when encountering unknown fields, the program will not crash. And, drum roll...! We have a class object with fields filled in (yes, exactly the ones we need) and we can use it. Initially, I wrote my own separate class with static methods that took out the values ​​I needed and, if necessary, brought them into divine form. It's a chore. It's artisanal. These are crutches. Here is the class itself with methods:

public class Methods {
    public static double findRes(String body, String need){
        int begin = body.indexOf(need);
        int end = body.indexOf(",", begin);
        String res = body.substring(begin, end);
        String res2 = res.substring(res.indexOf(":")+2);
        double finalRes = Double.parseDouble(res2);
        return finalRes;
    }

    public static String findUrl(String body, String need){
        int begin = body.indexOf(need) + need.length();
        int end = body.indexOf(",", begin);
        String res = body.substring(begin, end);
        String res2 = res.substring(res.indexOf(":")+1);
        return res2;
    }
}

Using Jackson is much more convenient and readable (at least it’s really prettier). In a conversation with our elder brothers, they confirmed: using libraries, in our case Jackson, is preferable.

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonProperty;
import org.springframework.stereotype.Component;

@Component
@JsonIgnoreProperties(ignoreUnknown = true)
public class CurrencyPojo {
    private Rates rates;

    public CurrencyPojo() {
    }

    @JsonCreator
    public CurrencyPojo(@JsonProperty("rates") Rates rates) {
        this.rates = rates;
    }

    public Rates getRates() {
        return rates;
    }

    public void setRates(Rates rates) {
        this.rates = rates;
    }

    @Override
    public String toString() {
        return "CurrencyPojo{" +
                "rates=" + rates +
                '}';
    }

    @Component
    @JsonIgnoreProperties(ignoreUnknown = true)
    public static class Rates {

        private double rub;
        private double inr;
        private double eur;

        public Rates() {
        }

        @JsonCreator
        public Rates(@JsonProperty("RUB") double rub,
                     @JsonProperty("INR") double inr,
                     @JsonProperty("EUR") double eur) {
            this.rub = rub;
            this.inr = inr;
            this.eur = eur;
        }

        public double getRub() {
            return rub;
        }

        public void setRub(double rub) {
            this.rub = rub;
        }

        public double getInr() {
            return inr;
        }

        public void setInr(double inr) {
            this.inr = inr;
        }

        public double getEur() {
            return eur;
        }

        public void setEur(double eur) {
            this.eur = eur;
        }

        @Override
        public String toString() {
            return "Rates{" +
                    "rub=" + rub +
                    ", inr=" + inr +
                    ", eur=" + eur +
                    '}';
        }
    }
}

I would like to draw your attention to the use of a nested class. Before this task, I couldn’t think of a use for them =) And here is an example of incoming JSON: I don’t know how to make a “spoiler”, so let’s train our finger))

{
    "disclaimer": "Usage subject to terms: https://openexchangerates.org/terms",
    "license": "https://openexchangerates.org/license",
    "timestamp": 1638143999,
    "base": "USD",
    "rates": {
        "AED": 3.672934,
        "AFN": 95.889778,
        "ALL": 107.277494,
        "AMD": 483.27152,
        "ANG": 1.802446,
        "AOA": 585,
        "ARS": 100.99011,
        "AUD": 1.400119,
        "AWG": 1.80025,
        "AZN": 1.700805,
        "BAM": 1.734322,
        "BBD": 2,
        "BDT": 85.804468,
        "BGN": 1.727247,
        "BHD": 0.377164,
        "BIF": 1994.344572,
        "BMD": 1,
        "BND": 1.370223,
        "BOB": 6.905713,
        "BRL": 5.593606,
        "BSD": 1,
        "BTC": 0.000017488138,
        "BTN": 74.895631,
        "BWP": 11.828585,
        "BYN": 2.560653,
        "BZD": 2.015918,
        "CAD": 1.27258,
        "CDF": 2003.492833,
        "CHF": 0.92439,
        "CLF": 0.030154,
        "CLP": 831.315923,
        "CNH": 6.395085,
        "CNY": 6.393,
        "COP": 3975.845415,
        "CRC": 639.731775,
        "CUC": 1,
        "CUP": 25.75,
        "CVE": 97.95,
        "CZK": 22.7447,
        "DJF": 178.031664,
        "DKK": 6.585348,
        "DOP": 56.599119,
        "DZD": 139.135508,
        "EGP": 15.756894,
        "ERN": 15.000155,
        "ETB": 47.819833,
        "EUR": 0.885541,
        "FJD": 2.12473,
        "FKP": 0.749595,
        "GBP": 0.749595,
        "GEL": 3.095,
        "GGP": 0.749595,
        "GHS": 6.142755,
        "GIP": 0.749595,
        "GMD": 52.425,
        "GNF": 9472.013443,
        "GTQ": 7.738789,
        "GYD": 209.235741,
        "HKD": 7.7981,
        "HNL": 24.17051,
        "HRK": 6.661782,
        "HTG": 98.81349,
        "HUF": 327.09539,
        "IDR": 14379.716018,
        "ILS": 3.185445,
        "IMP": 0.749595,
        "INR": 75.050444,
        "IQD": 1458.680982,
        "IRR": 42275,
        "ISK": 130.231848,
        "JEP": 0.749595,
        "JMD": 155.740793,
        "JOD": 0.709,
        "JPY": 113.7185,
        "KES": 112.535405,
        "KGS": 84.774702,
        "KHR": 4069.37439,
        "KMF": 436.000041,
        "KPW": 900,
        "KRW": 1195.716418,
        "KWD": 0.30268,
        "KYD": 0.833396,
        "KZT": 436.292325,
        "LAK": 10839.499888,
        "LBP": 1520.868483,
        "LKR": 202.516227,
        "LRD": 142.25,
        "LSL": 16.236278,
        "LYD": 4.615464,
        "MAD": 9.244198,
        "MDL": 17.763696,
        "MGA": 3988.128848,
        "MKD": 54.637275,
        "MMK": 1790.896161,
        "MNT": 2854.559306,
        "MOP": 8.033255,
        "MRO": 356.999828,
        "MRU": 36.094075,
        "MUR": 43.067396,
        "MVR": 15.45,
        "MWK": 816.475065,
        "MXN": 21.738389,
        "MYR": 4.239,
        "MZN": 63.857001,
        "NAD": 16.26,
        "NGN": 410.875846,
        "NIO": 35.230131,
        "NOK": 9.0605,
        "NPR": 119.833306,
        "NZD": 1.465193,
        "OMR": 0.385109,
        "PAB": 1,
        "PEN": 4.033921,
        "PGK": 3.51889,
        "PHP": 50.480705,
        "PKR": 176.598456,
        "PLN": 4.168379,
        "PYG": 6826.299832,
        "QAR": 3.646364,
        "RON": 4.37388,
        "RSD": 103.877366,
        "RUB": 75.58127,
        "RWF": 1024.40338,
        "SAR": 3.7514,
        "SBD": 8.064563,
        "SCR": 14.654883,
        "SDG": 438,
        "SEK": 9.148279,
        "SGD": 1.370086,
        "SHP": 0.749595,
        "SLL": 11119.30017,
        "SOS": 580.721202,
        "SRD": 21.52,
        "SSP": 130.26,
        "STD": 21187.940504,
        "STN": 22.195,
        "SVC": 8.750748,
        "SYP": 2512.5,
        "SZL": 15.967534,
        "THB": 33.757117,
        "TJS": 11.286041,
        "TMT": 3.51,
        "TND": 2.882,
        "TOP": 2.277258,
        "TRY": 12.378954,
        "TTD": 6.78112,
        "TWD": 27.866934,
        "TZS": 2302.544214,
        "UAH": 27.094403,
        "UGX": 3563.214629,
        "USD": 1,
        "UYU": 44.148288,
        "UZS": 10783.399861,
        "VES": 4.57705,
        "VND": 22678.30849,
        "VUV": 111.998805,
        "WST": 2.563531,
        "XAF": 580.876668,
        "XAG": 0.04289544,
        "XAU": 0.00055691,
        "XCD": 2.70255,
        "XDR": 0.714635,
        "XOF": 580.876668,
        "XPD": 0.00055962,
        "XPF": 105.673123,
        "XPT": 0.00101782,
        "YER": 250.249937,
        "ZAR": 16.1344,
        "ZMW": 17.776133,
        "ZWL": 322
    }
}

To convert json into a class object we use a mapper:

ObjectMapper mapper = new ObjectMapper();
currencyPojo(an object нашего класса) = mapper.readValue(jsonResponse(Джсон в стринге), CurrencyPojo.class(класс, который нам нужен));

The code is working, it pulls out of this hulk exactly what I need. Here is the output of the class object after parsing: CurrencyPojo{rates=Rates{rub=73.6944, inr=75.621547, eur=0.885436}} I'm delighted. The second day I clap all over my face and smile with my hands... Or something like that. Rate values ​​may vary because JSON text and object print from different dates, but in general the result should be clear. PS I’m writing an article for the first time, so don’t judge strictly. And I’m just a beginner hacker =) I love and accept criticism;) PS2 I apologize for the “many letters”, but I tried to present what I’ve been working on for days.