例外:无法在 START_OBJECT 标记之外反序列化 java.util.ArrayList 的实例

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时间:2020-08-11 00:59:21  来源:igfitidea点击:

Exception: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token at

javajson

提问by FooBayo

I am trying to display data from a web service and getting this error : Exception in thread "main" com.fasterxml.Hymanson.databind.JsonMappingException: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token at...

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

Find below my code :

在我的代码下面找到:

Model object :

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 + "]";
    }
}

Main class:

主要类:

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();        
    }
}

Here is the data getting from the url :

这是从 url 获取的数据:

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

Help me please to check what i did wrong.

请帮助我检查我做错了什么。

Thank you

谢谢

采纳答案by kami

You're trying to deserialize a list of Commune, but in HTTP response you're getting an object, containing such a list, but not a list itself. So, you need another wrapper object for deserialisation:

您正在尝试反序列化 Commune 列表,但在 HTTP 响应中,您得到一个对象,其中包含这样一个列表,但不是列表本身。因此,您需要另一个用于反序列化的包装器对象:

Wrapper

包装器

public class Towns implements Serializable {
    private List<Commune> towns;

    public Towns() {}

    public List<Commune> getTowns() {
        return towns;
    }

    public void setTowns(List<Commune> towns) {
        this.towns = towns;
    }

    @Override
    public String toString() {
        return "Towns: " + towns.toString();
    }
}

main app

主应用程序

    TypeReference<Towns> mapType = new TypeReference<Towns>() {};
    Towns towns = mapper.readValue(output, mapType);
    System.out.println(towns);

回答by nullptr

You are getting this error because you are trying to deserialize something that is not actually a JSON Arrayinto a Collection

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

If you are able to change your JSON to send just a JSON Arrayformat, it will look like this one below:

如果您能够将 JSON 更改为仅发送一种JSON Array格式,它将如下所示:

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

Otherwise, to parse it correctly you also can create a new class which will represent the townof your JSON and parse using it:

否则,要正确解析它,您还可以创建一个新类,该类将代表town您的 JSON 并使用它进行解析:

public class TownRecords implements Serializable {

    private List<Commune> communes;

    ... getters and setters
}