JSON Tutorial

06 August 2014

JSON data format is widely used nowadays. In order to use it with Java, several libraries are available. In this blog post, we will take a look at handling JSON data using the org.json library.

json.simple
Gson
Jackson - Most feature rich and best performance
org.json - Lightweight

This tutorial covers handling JSON using the org.json library.

We will see how you can use classes from org.json package to build JSON objects and also to parse JSON data. org.json package is widely used in Android and avoids dependency on other heavyweight packages like Jackson and such. Lets dive straight in to some code.

1. Creating JSON Objects

The main class to use here is JSONObject.

JSONObject json = new JSONObject();
json.put("from", "alice@example.com");
json.put("to", "bob@example.com");
json.put("type", "call");
System.out.println("Output = " + json)

Output

{"to":"bob@example.com","from":"alice@example.com","type":"call"}

Another code sample

JSONObject json = new JSONObject();
json.put("from", "alice@example.com");
json.put("to", "bob@example.com");
json.put("type", "call");
json.put("numParams", 2);
json.put("params", new JSONArray().put("v1").put("v2"));

json.append("params", "v3");
json.increment("numParams");

System.out.println("Output = " + json);

Output

{"to":"bob@example.com","numParams":3,"params":["v1","v2","v3"],"from":"alice@example.com","type":"call"}

2. Parsing from JSON in string form

If we pass the last output given above to the function below, here is how we can extract the values by first converting the string to JSONObject and then invoking the various methods on it. We use the class JSONTokener which takes a source string and extracts characters and tokens from it.

private static void readJSONString(String data) {
  JSONObject json = new JSONObject(new JSONTokener(data));
  String from = json.getString("from");
  String to = json.getString("to");
  int numParams = json.getInt("numParams");

  System.out.println("From: " + from + " To: " + to + " NumParams: " + numParams);
  JSONArray arr = json.getJSONArray("params");
  System.out.print("Params: ");
  for (int i = 0; i < arr.length(); i++) {
    System.out.print(arr.getString(i) + " ");
  }
}

Output

From: alice@example.com To: bob@example.com NumParams: 3
Params: v1 v2 v3

3. Rapidly producing JSON

Both JSONWriter and JSONStringer allow you to construct a JSON object with JSONWriter additionally taking a Writer object which outputs the contents to a stream.

private static void constructJSON() throws IOException {
  Writer myWriter = new BufferedWriter(new FileWriter(new File("json.out")));
  new JSONWriter(myWriter)
    .object()
      .key("from").value("alice@example.com")
      .key("to").value("bob@test.com")
      .key("params")
        .array()
          .value("v1")
          .value("v2")
          .value("v3")
        .endArray()
    .endObject();

  myWriter.flush();
  myWriter.close();
  System.out.println("Contents written");
}

Output: Contents of the file json.out

{"from":"alice@example.com","to":"bob@test.com","params":["v1","v2","v3"]}

4. Maven dependency

org.json is available at Maven central repository, just declares following dependency in your pom.xml file.

<dependency>
  <groupId>org.json</groupId>
  <artifactId>json</artifactId>
  <version>20140107</version>
</dependency>

5. Normalization

Suppose you had a JSON string that you want to normalize such that you can save all the values in a Map.

Input:

{
  "store": {
    "book": [
      {
        "author": "NoOne",
        "title": "SOWhat",
        "category": "reference",
        "price": 8.95
      },
      {
        "author": "SomeOne",
        "title": "HiThere",
        "category": "fiction",
        "price": 8.99,
        "isbn": "0-5667666-3"
      }
    ],
    "bicycle": {
      "price": 19.95,
      "color": "red"
    }
  },
  "expensive": 10
}

Output

store.book.author:NoOne
store.book.title:SOWhat
store.book.category:reference
store.book.price:8.95
store.book.author:SomeOne
store.book.title:HiThere
store.book.category:fiction
store.book.price:8.99
store.book.isbn:0-5667666-3
store.bicycle.price:19.95
store.bicycle.color:red
expensive:10

Here is the code that can do that.

import java.util.Iterator;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;


public class JSONNormalize {

  private static void listJson(JSONObject json) throws JSONException {
    listJSONObject("", json);
  }

  private static void listObject(String parent, Object data) throws JSONException {
    if (data instanceof JSONObject) {
      listJSONObject(parent, (JSONObject)data);
    } else if (data instanceof JSONArray) {
      listJSONArray(parent, (JSONArray) data);
    } else {
      listPrimitive(parent, data);
    }    
  }

  private static void listJSONObject(String parent, JSONObject json) throws JSONException {
    Iterator it = json.keys();
    while (it.hasNext()) {
      String key = (String)it.next();
      Object child = json.get(key);
      String childKey = parent.isEmpty() ? key : parent + "." + key;
      listObject(childKey, child);
    }
  }

  private static void listJSONArray(String parent, JSONArray json) throws JSONException {
    for (int i = 0; i < json.length(); i++) {
      Object data = json.get(i);
      listObject(parent, data);
    }
  }

  private static void listPrimitive(String parent, Object obj) {
    System.out.println(parent + ":"  + obj);
  }

  public static void main(String[] args) throws JSONException {
    String data = "{\"store\":{\"book\":[{\"category\":\"reference\",\"author\":\"NoOne\",\"title\":\"SOWhat\",\"price\":8.95},{\"category\":\"fiction\",\"author\":\"SomeOne\",\"title\":\"HiThere\",\"isbn\":\"0-5667666-3\",\"price\":8.99},],\"bicycle\":{\"color\":\"red\",\"price\":19.95}},\"expensive\":10}";
    JSONObject json = new JSONObject(data);    
    System.out.println(json.toString(2));
    listJson(json);
  }

}

5. Performance

Comparisons between the various libraries can be found here

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