How do I detect if an embedded JSON element is a JSONObject or JSONArray in Java

Question

I am receiving json and am trying to operate on it using the JSONObject and JSONArray classes. I have nested objects and need to determine if those nested objects are arrays or objects. The problem is that if I assume it is an object and it is actually an array, when I get call getJSONObject, an exception is raised. I'm wondering if the only way to do this is just to add try/catch blocks or if there is a better way to do this. For example, say I have:

{"key1": "val1",
 "key2": {"level2": {"level3": "val3"}}
}

I may also have the level2 element with multiple values and coming in as an array:

{"key1": "val1",
 "key2": {"level2": [{"level3": "val3"}]}
}

How would I extract the level2 object (as an object) and determine if it is an object or an array? Is the only way to really do this using string manipulation?

If I do something like:

jsonObj.getJSONObject("key2").getJSONObject("level2");

It will result in an error: "org.json.JSONException: JSONObject["level2"] is not a JSONObject and conversly, if I call getJSONArray("level2") and level2 is not an array I will get an error indicating it is not an array.

Answer 1

You can use optJSONObject() & optJSONArray() methods to check if the underlying is an Object or Array. Below is an example for that.

JSONObject json = new JSONObject("{\"key1\": \"val1\",\n" + 
        " \"key2\": {\"level2\": {\"level3\": \"val3\"}}\n" + 
        "}");

JSONObject json1 = new JSONObject("{\"key1\": \"val1\",\n" + 
        " \"key2\": {\"level2\": [{\"level3\": \"val3\"}]}\n" + 
        "}");

if(json.getJSONObject("key2").optJSONObject("level2") != null) {
    System.out.println("json : it is a jsonobject");
}
else if(json.getJSONObject("key2").optJSONArray("level2") != null) {
    System.out.println("json : it is a jsonarray");
}


if(json1.getJSONObject("key2").optJSONObject("level2") != null) {
    System.out.println("json1 : it is a jsonobject");
}
else if(json1.getJSONObject("key2").optJSONArray("level2") != null) {
    System.out.println("json1 : it is a jsonarray");
}

Answer 2

使用'jsonObj.get（“ level2）'并检查类型是否为JsonArray，然后尝试强制转换。

Answer 3

If you're open to using Jackson 's Databind module, it's pretty straightforward:

import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.node.ArrayNode;

// ...

// parse your JSON data into a JsonNode
JsonNode jsonObj = new ObjectMapper().readTree(yourJsonString);

// should really do some error checking here unless you're 100% sure
// you're always going to be getting "key2" and "level2" in your JSON
// data structure; look into Jackson's "hasNonNull()" function
JsonNode level2 = jsonObj.get("key2").get("level2");

if (level2.isArray()) {
    ArrayNode level2arr = (ArrayNode)level2;
    // ... do something with level2arr as an array
    // ...
} else if (level2.isObject()) {
    // ... use it as an object node
    // ...
}

Answer 4

I would stringify the JSON object and check the index of '{' or '['.

Given a Json, I would stringify it. And if the index of '{' is 0 in the stringified object then its a Json Object and if the index of '[' is 0 in the stringified object then its a Json Array.

I use Vertx framework and I can verify them as follows:

JsonObject jsonObject = new JsonObject().put("name", "avishek");
JsonArray jsonArray = new JsonArray().add("Vizury").add("MediaIQ");

System.out.println(jsonObject.toString().indexOf("{") == 0); //Json Object
System.out.println(jsonArray.toString().indexOf("[") == 0); //Json Array