来自外部 API 的“无法反序列化实例”答案

【问题标题】："Cannot deserialize instance of" from outside API来自外部 API 的“无法反序列化实例”
【发布时间】：2019-08-20 04:50:29
【问题描述】：

我正在尝试制作在线歌唱家。我需要从外部 API 获取具有货币价值的表格，正是从该页面：http://api.nbp.pl/api/exchangerates/tables/A?format=json 我想在货币课上得到答案。有人可以帮我完成这项任务吗？

@Service
public class CurrentFromNBPImpl implements CurrentFromNBP {

@Override
public Currency getValueOfCurrency(String currencyCode) throws WrongCurrencyCode {

    Currency currency = null;

    try {
        URL url = new URL("http://api.nbp.pl/api/exchangerates/tables/A?format=json");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("GET");
        connection.setRequestProperty("Content-Type", "application/json");

        if (connection.getResponseCode() == 404)
            throw new WrongCurrencyCode(currencyCode);

        InputStreamReader inputStreamReader = new InputStreamReader(connection.getInputStream());
        BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
        String jsonOutput = bufferedReader.readLine();

        connection.disconnect();

        ObjectMapper objectMapper = new ObjectMapper();
        Currency list = objectMapper.readValue(jsonOutput, Currency.class);
        System.out.println(list);
    } catch (IOException e) {
        e.printStackTrace();
    }

    assert false;
    return currency;
}
}



@Data
public class Currency {

    @JsonProperty("table")
    private String table;
    @JsonProperty("no")
    private String no;
    @JsonProperty("effectiveDate")
    private String effectiveDate;
    @JsonProperty("rates")
    private List<Rate> rates = null;

}
@Data
public class Rate {

    @JsonProperty("currency")
    private String currency;
    @JsonProperty("code")
    private String code;
    @JsonProperty("mid")
    private Double mid;

    }

日志： com.fasterxml.jackson.databind.exc.MismatchedInputException：无法从 START_ARRAY 令牌中反序列化 com.kolej.bartosz.challenge.domain.Currency 的实例在 [Source: (String)"[{"table":"A","no":"062/A/NBP/2019","effectiveDate":"2019-03-28","rates":[{ "currency":"bat (Tajlandia)","code":"THB","mid":0.1202},{"currency":"dolar amerykański","code":"USD","mid":3.8202} ,{"currency":"dolar australijski","code":"AUD","mid":2.7098},{"currency":"dolar Hongkongu","code":"HKD","mid":0.4867} ,{"currency":"dolar kanadyjski","code":"CAD","mid":2.8461},{"currency":"dolar nowozelandzki","code":"NZD","mid":2.6006} ,{"currency":"dolar singapurski","code":"SGD","mid":2.8179},{"currency":"eu"[截断 1616 个字符]；行：1，列：1]

【问题讨论】：

这里有什么不清楚的地方？你说杰克逊你的 JSON 将是一个单一的 Currency 对象，但它不是。

标签： java json

【解决方案1】：

您要反序列化的 json 对象是一个 jsonArray。您需要反序列化为Currency 的列表，而不是Currency。

【讨论】：

List list = (List) objectMapper.readValue(jsonOutput, Currency.class);我试过这样做，但还是同样的问题，你能给我举个例子吗？

【解决方案2】：

你可以试试这个

ArrayList<Currency> list = new ArrayList<>();
list = objectMapper.readValue(data, new TypeReference<ArrayList<Currency>>() {});

【讨论】：

完美运行！谢谢。