在地图上未经检查的投射<String, Object> （JSON 转换为 Map with Jackson）

Question

In Java 8, I want to convert a JSON string to a map, and apply "complex" transformations to the keys. As an example , that "complex" transformation will simply be a lower case transformation. Values in the input JSON could be strings or nested JSON objects. My code is actually working, but I struggle to fix an unchecked cast warning.

Example

Example JSON input ( String ) :

{
    "Key1": "value1",
    "Key2": {
        "Key2.1": "value2.1"
    }
}

Desired output ( Map ) :

"key1" -> "value1"
"key2" ->
    "key2.1" -> "value2.1"

1. JSON String -> Map

For that part, I used Jackson (2.9.8) and defined the following function :

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

Map<String, Object> convertJsonStringToMap(String json) throws IOException {
    ObjectMapper mapper = new ObjectMapper();
    TypeReference type = new TypeReference<Map<String, Object>>(){};
    return mapper.readValue(json, type);
}

Since values can be strings or JSON objects, the return type of that function is Map<String, Object> . Note also that readValue (in ObjectMapper class) uses generics, its signature is :

<T> T readValue(String content, TypeReference valueTypeRef)

2. Map -> Map with transformed keys (example : lower case)

I defined the following function :

Map<String, Object> transformKeys(Map<String, Object> map) {
    Map<String, Object> result = new HashMap<>(map.size());

    for (Map.Entry<String, Object> entry : map.entrySet()) {
        Object value = entry.getValue();
        if (value instanceof Map) {
            value = transformKeys((Map<String, Object>) value);
        }
        // toLowerCase() is the transformation here (example), but I could have used something else
        result.put(entry.getKey().toLowerCase(), value);
    }

    return result;
}

To handle nested maps, this function is recursive. But since it takes a Map<String, Object> as parameter, I must cast value to Map<String, Object> to call the method recursively.

3. Putting all together

String json = "{\"Key1\": \"value1\", \"Key2\": { \"Key2.1\": \"value2.1\" }}";
Map<String, Object> initialMap = convertJsonStringToMap(json);
Map transformedMap = transformKeys(initialMap);
System.out.println(transformedMap);

This code works, and prints, as expected :

{key1=value1, key2={key2.1=value2.1}}

But this line in transformKeys function :

value = transformKeys((Map<String, Object>) value);

produces a warning :

[WARNING] App.java:[29,74] unchecked cast
    required: java.util.Map<java.lang.String,java.lang.Object>
    found:    java.lang.Object

The warning is clear and I understand it (the compiler cannot know if value is really an instance of Map<String, Object> ), but is there a way to get rid of it?

(No @SuppressWarnings please, nor -Xlint:none ) :D

I feel like returning a Map<String, Object> from convertJsonStringToMap is not the cleanest way to convert JSON String to a Map, but I can't find an other way to do it with Jackson.

Answer 1

You need to use Object as Map value because it could be another Map , List or primitive ( String , Integer , etc.). Jackson allows also to manipulate JSON using JsonNode types. We need to traverse JSON object but also JSON array (you forgot about it). In that case we need to:

• Deserialise JSON to JsonNode .
• Traverse it using JsonNode API .
• Convert to the Map

Simple implementation:

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.node.ArrayNode;
import com.fasterxml.jackson.databind.node.ObjectNode;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class JsonApp {

    public static void main(String[] args) throws Exception {
        String json = "{\"Key1\": \"value1\", \"Key2\": { \"Key2.1\": \"value2.1\" }, \"Key3\":[{\"pRiMe\":11}]}";
        Map<String, Object> map = convertJsonStringToMap(json);

        System.out.println(map);
    }

    private static Map<String, Object> convertJsonStringToMap(String json) throws IOException {
        ObjectMapper mapper = new ObjectMapper();
        JsonNode node = mapper.readTree(json);
        transformKeys(node);

        TypeReference mapType = new TypeReference<Map<String, Object>>() {
        };

        return mapper.convertValue(node, mapType);
    }

    private static void transformKeys(JsonNode parent) {
        if (parent.isObject()) {
            ObjectNode node = (ObjectNode) parent;

            List<String> names = new ArrayList<>();
            node.fieldNames().forEachRemaining(names::add);

            names.forEach(name -> {
                JsonNode item = node.remove(name);
                transformKeys(item);
                node.replace(name.toLowerCase(), item);
            });
        } else if (parent.isArray()) {
            ArrayNode array = (ArrayNode) parent;
            array.elements().forEachRemaining(JsonApp::transformKeys);
        }
    }
}

Above code prints:

{key1=value1, key2={key2.1=value2.1}, key3=[{prime=11}]}

We got rid of unsafe casting and our implementation is more concise.