如何使用Jackson对一系列对象进行反序列化

Question

在杰克逊的数据绑定的文档表明,杰克逊支持deserialising"所有支持类型的数组",但我想不通这个确切的语法.

对于单个对象,我会这样做:

//json input
{
    "id" : "junk",
    "stuff" : "things"
}

//Java
MyClass instance = objectMapper.readValue(json, MyClass.class);

现在对于一个数组我想这样做:

//json input
[{
    "id" : "junk",
    "stuff" : "things"
},
{
    "id" : "spam",
    "stuff" : "eggs"
}]

//Java
List<MyClass> entries = ?

有人知道是否有一个神奇的缺失命令？如果没有那么解决方案是什么？

Answer 1

首先创建一个映射器:

import com.fasterxml.jackson.databind.ObjectMapper;// in play 2.3
ObjectMapper mapper = new ObjectMapper();

作为数组:

MyClass[] myObjects = mapper.readValue(json, MyClass[].class);

如清单:

List<MyClass> myObjects = mapper.readValue(jsonInput, new TypeReference<List<MyClass>>(){});

另一种指定List类型的方法:

List<MyClass> myObjects = mapper.readValue(jsonInput, mapper.getTypeFactory().constructCollectionType(List.class, MyClass.class));

Answer 2

来自Eugene Tskhovrebov

List<MyClass> myObjects = Arrays.asList(mapper.readValue(json, MyClass[].class))

这个解决方案对我来说似乎是最好的

Answer 3

尝试这个

List<MyClass> list = mapper.readerForListOf(MyClass.class).readValue(json)

Answer 4

对于通用实现:

public static <T> List<T> parseJsonArray(String json,
                                         Class<T> classOnWhichArrayIsDefined) 
                                         throws IOException, ClassNotFoundException {
   ObjectMapper mapper = new ObjectMapper();
   Class<T[]> arrayClass = (Class<T[]>) Class.forName("[L" + classOnWhichArrayIsDefined.getName() + ";");
   T[] objects = mapper.readValue(json, arrayClass);
   return Arrays.asList(objects);
}

Answer 5

首先创建一个线程安全的ObjectReader实例.

ObjectMapper objectMapper = new ObjectMapper();
ObjectReader objectReader = objectMapper.reader().forType(new TypeReference<List<MyClass>>(){});

然后使用它:

List<MyClass> result = objectReader.readValue(inputStream);

Answer 6

我无法使用这个答案，因为我的 linter 不允许未经检查的强制转换。

这是您可以使用的替代方案。我觉得这实际上是一个更清洁的解决方案。

public <T> List<T> parseJsonArray(String json, Class<T> clazz) throws JsonProcessingException {
  var tree = objectMapper.readTree(json);
  var list = new ArrayList<T>();
  for (JsonNode jsonNode : tree) {
    list.add(objectMapper.treeToValue(jsonNode, clazz));
  }
  return list;
}

Answer 7

try {
    ObjectMapper mapper = new ObjectMapper();
    JsonFactory f = new JsonFactory();
    List<User> lstUser = null;
    JsonParser jp = f.createJsonParser(new File("C:\\maven\\user.json"));
    TypeReference<List<User>> tRef = new TypeReference<List<User>>() {};
    lstUser = mapper.readValue(jp, tRef);
    for (User user : lstUser) {
        System.out.println(user.toString());
    }

} catch (JsonGenerationException e) {
    e.printStackTrace();
} catch (JsonMappingException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
}

Answer 8

使用 ArrayList，这些不同的语法都对我有用：

ArrayList<MyClass> arrayList = ...

使用objectMapper.getTypeFactory().constructCollectionType：

objectMapper.readValue(json, objectMapper.getTypeFactory().constructCollectionType(ArrayList.class, MyClass.class));

使用new TypeReference：

objectMapper.readValue(json, new TypeReference<ArrayList<MyClass>>(){});

使用new ArrayList<>(Arrays.asList)：

new ArrayList<>(Arrays.asList(objectMapper.readValue(json, MyClass[].class)));

我不能说哪一个最好用，但我最终使用了最后一个：

ArrayList<MyClass> arrayList = new ArrayList<>(Arrays.asList(objectMapper.readValue(json, MyClass[].class)));

Answer 9

这是一个实用程序，可以转换 json2object 或 Object2json，无论您的 pojo（实体 T）如何

import java.io.IOException;
import java.io.StringWriter;
import java.util.List;

import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;

/**
 * 
 * @author TIAGO.MEDICI
 * 
 */
public class JsonUtils {

    public static boolean isJSONValid(String jsonInString) {
        try {
            final ObjectMapper mapper = new ObjectMapper();
            mapper.readTree(jsonInString);
            return true;
        } catch (IOException e) {
            return false;
        }
    }

    public static String serializeAsJsonString(Object object) throws JsonGenerationException, JsonMappingException, IOException {
        ObjectMapper objMapper = new ObjectMapper();
        objMapper.enable(SerializationFeature.INDENT_OUTPUT);
        objMapper.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
        StringWriter sw = new StringWriter();
        objMapper.writeValue(sw, object);
        return sw.toString();
    }

    public static String serializeAsJsonString(Object object, boolean indent) throws JsonGenerationException, JsonMappingException, IOException {
        ObjectMapper objMapper = new ObjectMapper();
        if (indent == true) {
            objMapper.enable(SerializationFeature.INDENT_OUTPUT);
            objMapper.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
        }

        StringWriter stringWriter = new StringWriter();
        objMapper.writeValue(stringWriter, object);
        return stringWriter.toString();
    }

    public static <T> T jsonStringToObject(String content, Class<T> clazz) throws JsonParseException, JsonMappingException, IOException {
        T obj = null;
        ObjectMapper objMapper = new ObjectMapper();
        obj = objMapper.readValue(content, clazz);
        return obj;
    }

    @SuppressWarnings("rawtypes")
    public static <T> T jsonStringToObjectArray(String content) throws JsonParseException, JsonMappingException, IOException {
        T obj = null;
        ObjectMapper mapper = new ObjectMapper();
        obj = mapper.readValue(content, new TypeReference<List>() {
        });
        return obj;
    }

    public static <T> T jsonStringToObjectArray(String content, Class<T> clazz) throws JsonParseException, JsonMappingException, IOException {
        T obj = null;
        ObjectMapper mapper = new ObjectMapper();
        mapper = new ObjectMapper().configure(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY, true);
        obj = mapper.readValue(content, mapper.getTypeFactory().constructCollectionType(List.class, clazz));
        return obj;
    }