java - 异常 : Can not deserialize instance of java. util.ArrayList 超出 START_OBJECT token

Question

我正在尝试显示来自网络服务的数据并收到此错误:线程“main”com.fasterxml.jackson.databind.JsonMappingException 中的异常:无法从 START_OBJECT token 中反序列化 java.util.ArrayList 的实例......

在下面找到我的代码:

模型对象:

public class Commune implements Serializable{

    private static final long serialVersionUID = 1L;

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Commune [name=" + name + "]";
    }
}

主类:

public class Test {

    public static void main(String[] args) throws IOException {

        URL url = new URL("https://bj-decoupage-territorial.herokuapp.com/api/v1/towns");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("GET");
        connection.setRequestProperty("Accept", "application/json");

        if(connection.getResponseCode() != 200){
            throw new RuntimeException("Failed : Http code : "+connection.getResponseCode());
        }

        BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));

        String output;

        while((output = reader.readLine()) != null){

            ObjectMapper mapper = new ObjectMapper();
            TypeReference<List<Commune>> mapType = new TypeReference<List<Commune>>() {};
            List<Commune> jsonToCommuneList = mapper.readValue(output, mapType);

            for(Commune c : jsonToCommuneList) {
                System.out.println(c.getName());
            }
        }
        connection.disconnect();        
    }
}

这是从 url 获取的数据:

{"towns":
   [
      {"name":"BANIKOARA"},
      {"name":"GOGOUNOU"},
      {"name":"KANDI"},
      {"name":"KARIMAMA"}
   ]
}

请帮我检查一下我做错了什么。

谢谢

Answer 1

您收到此错误是因为您正在尝试将实际上不是 JSON 数组 的内容反序列化为 Collection

如果您能够将 JSON 更改为仅发送 JSON 数组 格式，它将如下所示:

[
  {"name":"BANIKOARA"},
  {"name":"GOGOUNOU"},
  {"name":"KANDI"},
  {"name":"KARIMAMA"}
]

否则，要正确解析它，您还可以创建一个新类来表示您的 JSON 的 town 并使用它进行解析:

public class TownRecords implements Serializable {

    private List<Commune> communes;

    ... getters and setters
}