[Spring Boot] JSON을 활용한 API 통신 예제 + JSON 형변환 (Gson, ObjectMapper, JSONParser)

• API 통신과 JSON에 대해 정리
• JSON으로 형변환 하는 방법 정리

JSON 이란?

• JSON(JavaScript Object Notation)은 데이터를 저장/전송 할 때 많이 사용되는 형식
• 서버와 클라이언트 간의 통신에서 주로 많이 사용됨
• 아래와 같이 Key, Value 구조를 가짐

{
    "teamName" : "Tottenham",
    "league" : "Premier League",
    "ranking" : 5,
    "players" : [
        {
        "name" : "Son Heungmin",
        "backNumber" : 7,
        "age" : 30
        },
        {
        "name" : "Harry Kane",
        "backNumber" : 10,
        "age" : 29
        },
        {
        "name" : "Hugo Lloris",
        "backNumber" : 1,
        "age" : 36
        }
    ]
}

단일 객체 송신 예제

• Player라는 단일 객체를 만들고 전송하는 예제

Player

@Data
@AllArgsConstructor
static class Player {
    private String name;
    private int backNumber;
    private int age;
}

RestController

@GetMapping("/player")
public Player getOnePlayer() {
    Player player1 = new Player("Son Heungmin", 7, 30);
    return player1;
}

결과

다중 객체 송신 예제

• 여러 객체를 묶어서 보낼때는 HashMap, ArrayList 등을 사용할 수 있음
• 이 예제에서는 ArrayList를 사용해 다중 객체를 묶어 전송해 봄

RestController

@GetMapping("/players")
public ArrayList<Player> getAllPlayer() {
    Player player1 = new Player("Son Heungmin", 7, 30);
    Player player2 = new Player("Harry Kane", 10, 29);
    Player player3 = new Player("Hugo Lloris", 1, 36);

    ArrayList<Player> players = new ArrayList<>();
    players.add(player1);
    players.add(player2);
    players.add(player3);

    return players;
}

결과

ArrayList가 포함된 객체 송신 예제

• 위에서 만든 players를 FootballTeam이라는 새로운 객체에 포함시키고, FootballTeam 고유 정보 추가

FootballTeam.class

@Data
@AllArgsConstructor
static class FootballTeam {
    private String teamName;
    private String league;
    private Integer ranking;
    private ArrayList<Player> players;
}

RestController

@GetMapping("/team")
public FootballTeam getOneTeam() {
    Player player1 = new Player("Son Heungmin", 7, 30);
    Player player2 = new Player("Harry Kane", 10, 29);
    Player player3 = new Player("Hugo Lloris", 1, 36);

    ArrayList<Player> players = new ArrayList<>();
    players.add(player1);
    players.add(player2);
    players.add(player3);

    FootballTeam footballTeam = new FootballTeam("Tottenham", "Premier League", 5, players);
    return footballTeam;
}

결과

단일 객체 수신 예제

RestController

@PostMapping("/player")
public String postOnePlayer(@RequestBody Player player) {
    String response = String.format("<선수 정보>\n이름 : %s\n등번호 : %d\n나이 : %d",
            player.getName(), player.getBackNumber(), player.getAge());
    return response;
}

결과

다중 객체 수신 예제

RestController

@PostMapping("/players")
public String postPlayers(@RequestBody List<Player> players) {
    String response = "<선수단 정보>\n";

    int i = 1;
    for (Player player : players) {
        response += String.format("선수%d: 이름 : %s\t등번호 : %d\t나이 : %d\n",
                i, player.getName(), player.getBackNumber(), player.getAge());
        i ++;
    }

    return response;
}

결과

ArrayList가 포함된 객체 수신 예제

RestController

@PostMapping("/team")
public String postOneTeam(@RequestBody FootballTeam footballTeam) {
    String response = String.format("<팀 정보>\n팀명 : %s\n리그 : %s\n리그 랭킹 : %d\n",
            footballTeam.getTeamName(), footballTeam.getLeague(), footballTeam.getRanking());

    int i = 1;
    for (Player player : footballTeam.getPlayers()) {
        response += String.format("선수%d: 이름 : %s\t등번호 : %d\t나이 : %d\n",
                i, player.getName(), player.getBackNumber(), player.getAge());
        i ++;
    }

    return response;
}

결과

JsonString <-> Object 변환 방법

• 입력이 Json 형태의 String으로 들어오는 상황
  • ex) String("{ "name": "Son Heungmin", "backNumber": 7, "age": 30 }") => Player 객체
• Object를 Json 형태의 String으로 변환시켜야 되는 상황
  • ex) Player 객체 => String ("{ "name": "Son Heungmin", "backNumber": 7, "age": 30 }")
• 이런 상황에서는 JSONParser, gson, ObjectMapper 등을 사용하여 변환 가능
  • gson 사용 방식이 가장 쉬운것 같음

JSONParser 사용 예제

라이브러리 추가

implementation 'com.googlecode.json-simple:json-simple:1.1.1'

JsonString -> Player

• String -> JSONObject -> Player

import org.json.simple.JSONObject;
import org.json.simple.parser.JSONParser;

@PostMapping("/json-parser/string-to-player")
public Player convertStringToPlayer1(@RequestBody String jsonStr) throws ParseException {
    JSONParser jsonParser = new JSONParser();
    JSONObject jsonObject = (JSONObject) jsonParser.parse(jsonStr);

    Player player = new Player(jsonObject.get("name").toString(),
            Integer.valueOf(jsonObject.get("backNumber").toString()),
            Integer.valueOf(jsonObject.get("age").toString()));
    return player;
}

결과

Player -> JsonString

• Player -> JSONObject -> String

import org.json.simple.JSONObject;
import org.json.simple.parser.JSONParser;

@PostMapping("/json-parser/player-to-string")
public String convertPlayerToString1(@RequestBody Player player) {
    JSONObject jsonPlayer = new JSONObject();
    jsonPlayer.put("name", player.getName());
    jsonPlayer.put("backNumber", player.getBackNumber());
    jsonPlayer.put("age", player.getAge());

    String jsonStr = jsonPlayer.toJSONString();
    return jsonStr;
}

결과

gson 사용 예제

라이브러리 추가

implementation 'com.google.code.gson:gson:2.8.6'

JsonString -> Player, FootballTeam

import com.google.gson.*;

@PostMapping("/gson/string-to-player")
public Player convertStringToPlayer2(@RequestBody String jsonStr) {
    Gson gson = new Gson();
    Player player = gson.fromJson(jsonStr, Player.class);
    return player;
}

@PostMapping("/gson/string-to-team")
public FootballTeam convertStringToTeam2(@RequestBody String jsonStr) {
    Gson gson = new Gson();
    FootballTeam footballTeam = gson.fromJson(jsonStr, FootballTeam.class);
    return footballTeam;
}

결과

 

FootballTeam -> JsonString

import com.google.gson.*;

@PostMapping("/gson/team-to-string")
public String convertTeamToString2(@RequestBody FootballTeam footballTeam) {
    Gson gson = new Gson();
    String jsonStr = gson.toJson(footballTeam);
    return jsonStr;
}

결과

object-mapper 사용 예제

JsonString -> FootballTeam

• String -> Map -> FootballTeam

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

@PostMapping("/object-mapper/string-to-team")
public FootballTeam convertStringToTeam3(@RequestBody String jsonStr) throws JsonProcessingException {
    ObjectMapper objectMapper = new ObjectMapper();

    Map<String, Object> footballTeamMap = objectMapper.readValue(jsonStr, new TypeReference<>(){});
    FootballTeam footballTeam = new FootballTeam(
            footballTeamMap.get("teamName").toString(),
            footballTeamMap.get("league").toString(),
            Integer.valueOf(footballTeamMap.get("ranking").toString()),
            (ArrayList<Player>) footballTeamMap.get("players"));

    return footballTeam;
}

결과

FootballTeam -> JsonString

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

@PostMapping("/object-mapper/team-to-string")
public String convertTeamToString3(@RequestBody FootballTeam footballTeam) throws JsonProcessingException {
    ObjectMapper objectMapper = new ObjectMapper();

    String jsonStr = objectMapper.writeValueAsString(footballTeam);
    return jsonStr;
}

결과