How to parse JSON in Java

Question

In java, how can I parse json text to get value of variables like post_id ?

Here is my json text 

{ 

  "page": {
         "name": "tommas",
    },
    "posts": [
         {
              "post_id": "123456789012_123456789012",
              "actor_id": "1234567890",
              "name": "Alex",
              "message": "Nice!",
              "likes_count": "3",
              "time": "1234567890"
         }
    ]
}

Answer 1

JSON basically stands for JavaScript Object Notation. It is simply a format in which in which you can transfer data from client to server and vice versa. In the initial days of web development we used to work with HTML later with CSS as it all evolved, as it evolved we had more dynamic web pages, what it meant was we had different operating languages on both ends and complex strings which need to be communicated needed to be transferable to both ends, which is when JSON came in. JSON is a lightweight, text-based, language-independent data exchange format which is used for the purpose.

Parsing JSON in java

We assume here we want to parse some data from server side to client side. Passing JSON is much easier if let’s say we are operating in JavaScript, but if we are using it in some other language ( Java in our case) we need to take help of certain libraries in order to convert normal text to JSON format first on the client side and parse it to client side and again convert it to normal text using libraries. Library which is commonly used are gson, jackson  all these are available and can be used to convert java code into Json and vice versa. We will use org.json library here,

import org.json.*;

JSONObject obj = new JSONObject(" .... ");

String name = obj.getJSONObject("page").getString("name");

JSONArray arr = obj.getJSONArray("posts");

for (int i = 0; i < arr.length(); i++)

 {

   String id = arr.getJSONObject(i).getString("post_id");

   ......

 }

Answer 2

Let's assume you have a class called Person with just name as an attribute.

For example:

private class Person {

    public String name;

              public Person(String name) {

        this.name = name;

    }

}

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For the serialization/de-serialization of objects, you can do this:

Gson g = new Gson();

Person person = g.fromJson("{\"name\": \"John\"}", Person.class);

System.out.println(person.name); //John

System.out.println(g.toJson(person)); // {"name":"John"}

If you want to get a single attribute out you can do it easily with the Google library as well, do like this:

JsonObject jsonObject = new JsonParser().parse("{\"name\": \"John\"}").getAsJsonObject();

System.out.println(jsonObject.get("name").getAsString()); 

But, if you don't need object de-serialization but to simply get an attribute, you can try org.json 

JSONObject obj = new JSONObject("{\"name\": \"John\"}");

System.out.println(obj.getString("name")); 

ObjectMapper mapper = new ObjectMapper();

Person user = mapper.readValue("{\"name\": \"John\"}", Person.class);

System.out.println(user.name);