使用 Jackson Mapper 将 JSON 数据映射到 Java POJO 类

Question

我正在开发一个Android应用程序，我正在尝试使用Jackson的Stream功能实现一些Java数据的处理。为此，我有几个代码示例要运行，并将解释每个代码示例的作用。

这是我正在使用的 JSON：

{
"users": [
    {
        "ID": "26",
        "data1": "Momonth",
        "data2": "2011-10-30 04:34:53"
    },
    {
        "ID": "45",
        "data1": "Porto",
        "data2": "2011-10-18 05:34:00"
    },
    {
        "ID": "21",
        "data1": "Tomson",
        "data2": "2011-10-12 02:44:13"
    }
],
"success": 1,
}

下面的代码是 POJO，当它遍历数据中的每个"用户"数组列表项时，这些数据将存储在其中：

@JsonRootName(value = "users")
public class User {
    String ID;
    String data1;
    String data2;
    ...
    public String getID() {
        return ID;
    }
    public void setID(String iD) {
        ID = iD;
    }
    public String getData1() {
        return data1;
    }
    public void setData1(String data1) {
        this.data1 = data1;
    }
    public String getData2() {
        return data2;
    }
    public void setData2(String data2) {
        this.data2 = data2;
    }
    ...
}

现在下面的代码我希望查看 JSON 数据，然后获取对象的"用户"数组列表，然后我现在只想简单地将每个用户的名称打印到 LogCat 以查看它是否有效，如前所述，我希望使用 " 方法，因为数据非常大，我想避免内存问题。

HttpRequest request = HttpRequest.get("http://www.site.com/android/app/getAllUsers.php");   
final Reader reader = request.reader();
final ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
final JsonFactory factory = mapper.getFactory();
try {
    JsonParser jp = factory.createParser(reader);
    while(jp.nextToken() == JsonToken.START_OBJECT) {
        String fieldName = jp.getCurrentName();
        jp.nextToken();
        final User a = mapper.readValue(jp.getText(), User.class);
    }
} catch (JsonParseException e1) {
    // TODO Auto-generated catch block
    e1.printStackTrace();
} catch (IOException e1) {
    // TODO Auto-generated catch block
    e1.printStackTrace();
}

但是上面的代码在当前状态下不起作用，我从杰克逊那里收到以下错误：

09-06 11:11:25.326: W/System.err(15029): com.fasterxml.jackson.databind.JsonMappingException: Current token not START_OBJECT (needed to unwrap root name 'users'), but FIELD_NAME
09-06 11:11:25.326: W/System.err(15029):  at [Source: java.io.InputStreamReader@432b1580; line: 1, column: 2]
09-06 11:11:25.326: W/System.err(15029):    at com.fasterxml.jackson.databind.ObjectMapper._unwrapAndDeserialize(ObjectMapper.java:2948)
09-06 11:11:25.326: W/System.err(15029):    at com.fasterxml.jackson.databind.ObjectMapper._readValue(ObjectMapper.java:2858)
09-06 11:11:25.326: W/System.err(15029):    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1569)
09-06 11:11:25.326: W/System.err(15029):    at com.app.tool.UsersListActivity$LoadAllUsers.doInBackground(UsersListActivity.java:381)
09-06 11:11:25.326: W/System.err(15029):    at com.app.tool.UsersListActivity$LoadAllUsers.doInBackground(UsersListActivity.java:1)
09-06 11:11:25.326: W/System.err(15029):    at android.os.AsyncTask$2.call(AsyncTask.java:287)
09-06 11:11:25.326: W/System.err(15029):    at java.util.concurrent.FutureTask.run(FutureTask.java:234)
09-06 11:11:25.326: W/System.err(15029):    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
09-06 11:11:25.326: W/System.err(15029):    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)
09-06 11:11:25.326: W/System.err(15029):    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)
09-06 11:11:25.326: W/System.err(15029):    at java.lang.Thread.run(Thread.java:856)

我真的很感激能在这方面提供的任何帮助，因为我被困住了，无法前进。

提前致谢

---

*******编辑*******

---

感谢您的回复，我已尝试实现代码，但它似乎仍然不起作用，我现在拥有的代码如下：

protected String doInBackground(String... args) {
    HttpRequest request = HttpRequest.get("http://www.site.com/android/app/getAllUsers.php");
    final Reader reader = request.reader();
    final ObjectMapper mapper = new ObjectMapper();
    final JsonFactory factory = mapper.getFactory();
    try {
            JsonParser jp = factory.createParser(reader);
            JsonNode root = mapper.readTree(jp);
            ArrayNode users = (ArrayNode) root.path("users");
            Iterator<JsonNode> iterator = users.elements();
            while (iterator.hasNext()) {
                User user = mapper.readValue(iterator.next(), User.class);
                System.out.println("User Name = " + user.data1);
            }
        } catch (JsonParseException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        } catch (IOException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        }
        return null;
}

您认为这与我在异步任务中运行的进程有关，还是与我通过 Web 服务访问我的 JSON 有关？我得到的错误以及它显示的行在下面：

错误行：

User user = mapper.readValue(iterator.next(), User.class);

错误代码：

The method readValue(JsonParser, Class<T>) in the type ObjectMapper is not applicable for the arguments (JsonNode, Class<User>)

希望你能帮我解决最后一个问题，再次感谢您的帮助。

Answer 1

你不需要@JsonRootName注释。您只需读取树并获取用户数组。然后遍历并在每个节点上使用ObjectMapper 的readValue(JsonNode node, Class<T> class)。

请注意，json 中的大写 ID 需要向解析器提供提示，因此 getter 上的@JsonProperty(value="ID")注释。

public class JacksonJSON {
    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        JsonNode root = mapper.readTree(JacksonJSON.class.getResourceAsStream("/users.json"));
        ArrayNode users = (ArrayNode) root.path("users");
        Iterator<JsonNode> iterator = users.getElements();
        while (iterator.hasNext()) {
            User user = mapper.readValue(iterator.next(), User.class);
            System.out.println("User id=" + user.id + " data1=" + user.data1 + " data2=" + user.data2);
        }

    }
}

class User {
    String id;
    String data1;
    String data2;
    @JsonProperty(value="ID")
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getData1() {
        return data1;
    }
    public void setData1(String data1) {
        this.data1 = data1;
    }
    public String getData2() {
        return data2;
    }
    public void setData2(String data2) {
        this.data2 = data2;
    }
}