Copy past json-simple.jar file in to jmeter lib folder (you can download jar file from http://www.java2s.com/Code/Jar/j/Downloadjsonsimple11jar.htm)

Example JSON

{
    "users": {
        "name": "myname",
        "age": "20"
    }
}

Shell

String response = prev.getResponseDataAsString();
JSONParser parser = new JSONParser();

try {
    Object obj = parser.parse(response);
    JSONObject jsonObject = (JSONObject) obj;
    JSONObject user= (JSONObject) jsonObject.get("user");

    log.info("Name : "+ user.name);

} catch (FileNotFoundException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
} catch (ParseException e) {
    e.printStackTrace();
}

Iterating through JSON Object

String response = prev.getResponseDataAsString();
JSONParser parser = new JSONParser();

try{
    Object obj = parser.parse(response);
    JSONObject jsonObject = (JSONObject) obj;
    JSONObject message = (JSONObject) jsonObject.get("message");

    Set keys = message.keySet();
    Iterator a = keys.iterator();
    while(a.hasNext()) {
        String key = (String)a.next();
        // loop to get the dynamic key
        String value = (String)jsonObject.get(key);

        log.info("key : "+key);
        log.info(" value :"+value);
    }

} catch (FileNotFoundException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
} catch (ParseException e) {
    e.printStackTrace();
}

Iterating through JSON Array

String response = prev.getResponseDataAsString();
JSONParser parser = new JSONParser();

try {

    Object obj = parser.parse(response);
    JSONObject jsonObject = (JSONObject) obj;
    
    JSONArray  changesArray = (JSONArray) jsonObject.get("changes");
    
    int le= changesArray .size();
    for(int i = 0 ; i < le; i++){
       log.info("\n"+ changesArray .get(i));
    }

} catch (FileNotFoundException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
} catch (ParseException e) {
    e.printStackTrace();
}