how to get the nested item of the json object using Java

DrMian
April 12, 2023
233 views
0 votes
2 Answers

I have the following json file test.json

{
  "type" : "object",
  "id" : "123",
  "title" : "Test",
  "properties" : {
    "_picnic" : {
      "type" : "boolean"
....

I am able to get the first level value of the key using the following code. E.g. for type I get object

public static void main(String[] args){

        String exampleRequest = null;
        ObjectMapper objectMapper = new ObjectMapper();
        try {
            exampleRequest = FileUtils.readFileToString(new File("src/main/resources/test.json"), StandardCharsets.UTF_8);
            JsonNode jsonNode = objectMapper.readTree(exampleRequest);

            String type = jsonNode.get("type").asText();
            System.out.println(type);

        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        // System.out.println(exampleRequest);


    }

However I do not get any response when I try to get the value for the second level. E.g. if I do

String type = jsonNode.get("_picnic").asText();
System.out.println(type);

OR the whole object e.g.

String type = jsonNode.get("properties").asText();
System.out.println(type);

I tried doing properties._picnic or properties[0]_picnic but no luck.

Answers

- dominicoder
- April 12, 2023 at 8:42 pm
- 0 votes
0
The documentation for that method states (emphasis mine):

Method that will return a valid String representation of the container value, if the node is a value node (method isValueNode() returns true), otherwise empty String.

Per the snippet of json text you shared, both "properties" and "_picnic" are objects not values so you are getting an empty string. It seems you’d need to use the get method:
```
jsonNode.get("properties").get("_picnic").get("type").asText()
```
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You can use recursion to simplify this code. Just call getText with a starting node and a variable list (array) of paths. It will determine if it need to check for object or array.

import java.io.*;
import java.nio.charset.StandardCharsets;
import org.apache.commons.io.FileUtils;
import com.fasterxml.jackson.databind.*;

public class NestedJson {
    public static void main(String[] args) {
        String exampleRequest = null;
        ObjectMapper objectMapper = new ObjectMapper();
        try {
            exampleRequest = FileUtils.readFileToString(new File("src/main/resources/test.json"),
                    StandardCharsets.UTF_8);
            JsonNode jsonNode = objectMapper.readTree(exampleRequest);
            System.out.println(getText(jsonNode, "type")); // object
            System.out.println(getText(jsonNode, "properties", "_picnic", "type")); // boolean
            System.out.println(getText(jsonNode, "stuff", "0", "foo")); // bar
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    public static String getText(JsonNode node, String... path) {
        return getText(node, 0, path);
    }

    private static String getText(JsonNode node, int cursor, String[] path) {
        if (node == null || path == null || path.length == 0)
            return null;
        if (cursor == path.length) {
            return node != null ? node.asText() : null;
        }
        if (node.isObject()) {
            return getText(node.get(path[cursor]), cursor + 1, path);
        } else if (node.isArray()) {
            return getText(node.get(Integer.parseInt(path[cursor])), cursor + 1, path);
        }
        return null;
    }
}

Modified test.json:

{
  "type": "object",
  "id": "123",
  "title": "Test",
  "properties": {
    "_picnic": {
      "type": "boolean"
    }
  },
  "stuff": [
    {
      "foo": "bar"
    }
  ]
}

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