Java org.json JDBC only last table row from database

Question

I have a RESTful application that connects to MySQL database (raw paste here: https://pastebin.com/raw/3fBp3j0B) and prints out table data in JSON format.

If I System.out.println() some data from ResultSet, everything shows up correctly, but with the JSON API only the last row in the table is printed out, twice.

import java.sql.*;
import javax.json.*;

public class Tietokanta {

    protected Connection yhteys = null;
    protected Statement kysely = null;
    protected ResultSet tulosjoukko = null;

    public boolean avaaYhteys() {
        boolean ok = true;
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            yhteys = DriverManager.getConnection("jdbc:mysql://localhost/savukelaskuri?serverTimezone=UTC", "root", "");
        } catch (Exception e) {
            e.printStackTrace();
            ok = false;
        }
        return ok;
    }

    public boolean suljeYhteys() {
        boolean ok = true;
        try {
            this.yhteys.close();
        } catch (Exception e) {
            ok = false;
        }
        return ok;
    }
}

...

import java.math.BigDecimal;
import javax.json.Json;
import javax.json.JsonArray;
import javax.ws.rs.core.Context;
import javax.ws.rs.core.UriInfo;
import javax.ws.rs.Produces;
import javax.ws.rs.Consumes;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PUT;
import static javax.ws.rs.client.Entity.json;
import javax.ws.rs.core.MediaType;
import org.json.JSONArray;
import org.json.JSONObject;

@Path("savukkeet")
public class ApiResource extends Tietokanta {
    JSONObject jsonolio = new JSONObject();
    JSONArray jsontaulu = new JSONArray();

@Context
private UriInfo context;

public ApiResource() {
    this.avaaYhteys();
}

@GET
@Produces(MediaType.APPLICATION_JSON)
public String getJson() {
    try {
        kysely = yhteys.createStatement();
        String sql = "SELECT * FROM kulutus";
        tulosjoukko = kysely.executeQuery(sql);

        while (tulosjoukko.next()) {
            System.out.println(tulosjoukko.getString("pvm"));

            jsonolio.put("id", tulosjoukko.getInt("id"));
            jsonolio.put("pvm", tulosjoukko.getString("pvm"));
            jsonolio.put("kulutus", tulosjoukko.getInt("kulutus"));
            jsontaulu.put(jsonolio);

        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return jsontaulu.toString(4);
    } 
}

I expect the result to be

[
        {
        "kulutus": 9,
        "pvm": "2019-01-14 16:46:00",
        "id": 1
    },
    {
        "kulutus": 8,
        "pvm": "2019-01-15 21:18:00",
        "id": 2
    }
]

but instead I get this

[
    {
        "kulutus": 8,
        "pvm": "2019-01-15 21:18:00",
        "id": 2
    },
    {
        "kulutus": 8,
        "pvm": "2019-01-15 21:18:00",
        "id": 2
    }
]

Answer 1

It should work if you create a new JSONObject for each iteration in the while loop:

while (tulosjoukko.next()) {
    System.out.println(tulosjoukko.getString("pvm"));

    jsonolio = new JSONObject();
    jsonolio.put("id", tulosjoukko.getInt("id"));
    jsonolio.put("pvm", tulosjoukko.getString("pvm"));
    jsonolio.put("kulutus", tulosjoukko.getInt("kulutus"));
    jsontaulu.put(jsonolio);

}

Java is pass-by-reference, so when you put a property to jsonolio it overrides the previous value even inside the JSONArray because it is still the same object