How to use Jackson to deserialise an array of objects

Question

The Jackson data binding documentation indicates that Jackson supports deserialising "Arrays of all supported types" but I can't figure out the exact syntax for this.

For a single object I would do this:

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

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

Now for an array I want to do this:

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

//Java
List<MyClass> entries = ?

Anyone know if there is a magic missing command? If not then what is the solution?

Answer 1

First create a mapper :

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

As Array:

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

As List:

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

Another way to specify the List type:

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

Answer 2

From Eugene Tskhovrebov

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

This solution seems to be the best for me.

Answer 3

try this

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

Answer 4

For Generic Implementation:

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

First create an instance of ObjectReader which is thread-safe.

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

Then use it :

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

Answer 6

Working with an ArrayList, these different syntaxes all worked for me:

ArrayList<MyClass> arrayList = ...

Using objectMapper.getTypeFactory().constructCollectionType:

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

Using new TypeReference:

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

Using new ArrayList<>(Arrays.asList):

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

I can't say which one is the best to use, but I ended up using the last one:

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

Answer 7

I was unable to use this answer because my linter won't allow unchecked casts.

Here is an alternative you can use. I feel it is actually a cleaner solution.

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 8

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 9

here is an utility which is up to transform json2object or Object2json, whatever your pojo (entity T)

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;
    }

Answer 10

This works for me.

Json:

[
   {
      "question":"How are you feeling today?",
      "key":"feeling",
      "answer":"unwell"
   },
   {
      "question":"How many hours of sleep did you get last night?",
      "key":"sleep_hours",
      "answer":"8"
   },
   {
      "question":"Have you been having nightmares lately?",
      "key":"nightmares",
      "answer":"Recently"
   }
]

java code:

private String getFeelByAnswer(String answer) {
    ObjectMapper mapper = new ObjectMapper();
    try {
        JsonNode nodes = mapper.readTree(answer);
        String feeling = "";
        for (JsonNode node : nodes) {
            if (node.get("key").asText().equals("feeling")) {
                feeling = node.get("answer").asText();
            }
        }
        return feeling;

    } catch (Exception e) {
        log.info("Feeling not found.");
    }
    return "";
}

Answer 11

you could also create a class which extends ArrayList:

public static class MyList extends ArrayList<Myclass> {}

and then use it like:

List<MyClass> list = objectMapper.readValue(json, MyList.class);