How Do I Access JSON POST Data in an HttpServletRequest?

Accessing JSON POST Data in HttpServletRequest

When HTTP POSTing data in JSON format to a servlet, it's necessary to understand the different data encodings involved. By default, servlets can retrieve parameters using request.getParameter(). However, for JSON data, the standard encoding scheme of "application/x-www-form-urlencoded" is not used.

To retrieve JSON POST data, you need to use a custom decoder that processes the raw datastream from the request.getReader(). Here's an example using the org.json package:

public void doPost(HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException {

  StringBuffer jb = new StringBuffer();
  String line = null;
  try {
    BufferedReader reader = request.getReader();
    while ((line = reader.readLine()) != null)
      jb.append(line);
  } catch (Exception e) { /*report an error*/ }

  try {
    JSONObject jsonObject =  HTTP.toJSONObject(jb.toString());
  } catch (JSONException e) {
    // crash and burn
    throw new IOException("Error parsing JSON request string");
  }

  // Work with the data using methods like...
  // int someInt = jsonObject.getInt("intParamName");
  // String someString = jsonObject.getString("stringParamName");
  // JSONObject nestedObj = jsonObject.getJSONObject("nestedObjName");
  // JSONArray arr = jsonObject.getJSONArray("arrayParamName");
  // etc...
}

This code reads the JSON datastream from the request, parses it using the org.json package, and stores the result in a JSONObject. You can then access the JSON parameters using methods like jsonObject.getInt(), jsonObject.getString(), and so on.

Note that this approach is not limited to JSON data. You can use the same technique to process any type of custom POST data that is not encoded as key-value pairs.

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