使用Jackson映射器将JSON数据映射到Java POJO类
我正在开发一个安卓应用程序,并尝试使用Jackson的流功能实现一些Java数据的处理。为了做到这一点,我有几个代码示例要运行,并将解释每个示例在运行时的作用
以下是我正在使用的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,当数据遍历my data中的每个“用户”数组列表项时,将在其中存储该数据:
@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;
}
...
}
下面的代码我希望查看JSON数据,然后获取对象的“users”数组列表,我现在只想将每个用户的名称打印到LogCat,以检查它是否工作,正如前面所解释的,我希望使用“”方法,因为数据非常大,我希望避免内存问题
HttpRequest request = HttpRequest.get("http://www.site.com/安卓/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无法工作,我从Jackson那里得到以下错误:
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 安卓.os.AsyncTask$2.call(AsyncTask.java:287)
09-06 11:11:25.326: W/System.err(15029): at java.util.concurrent.FutureTask.run(FutureTask.java:234)
09-06 11:11:25.326: W/System.err(15029): at 安卓.os.AsyncTask$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)
我真的很感激任何能在这方面给予我的帮助,因为我被困住了,无法前进
提前谢谢
******编辑
感谢您的回复,我已尝试实现该代码,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/安卓/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JSON有关?我得到的错误如下所示:
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,再次感谢你的帮助
# 1 楼答案
您不需要
@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中大写的ID需要对解析器进行提示,因此getter上的
@JsonProperty(value="ID")
注释