How to check if the last Asynktask is over

Question

I have an android application that communicates with the site through json that is obtained in the following way:

@Override
protected String doInBackground(String... params) {

    String url = params[0];
    String jsonString = params[1];

    Log.d("Parameters", " Url : " + url + " JSON :" + jsonString);

    try {

        if (client == null) {
            client = ((ProjectApplication) this.activity.get().getApplication()).getClient();
        }


        HttpContext context = new BasicHttpContext();


        CookieStore store = ((DefaultHttpClient) client).getCookieStore();
        context.setAttribute(ClientContext.COOKIE_STORE, store);

        HttpPost post = new HttpPost(url);
        post.setHeader("Content-type", "application/json");
        post.setHeader("Accept", "application/json");

        StringEntity stringParams = new StringEntity(jsonString);

        post.setEntity(stringParams);
        HttpResponse httpResponse;

        httpResponse = client.execute(post);


        HttpEntity httpEntity = httpResponse.getEntity();
        is = httpEntity.getContent();


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

    try {
        BufferedReader reader = new BufferedReader(new InputStreamReader(
                is, "UTF-8"), 8);
        StringBuilder sb = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) {
            sb.append(line).append("\n");
        }
        is.close();
        json = sb.toString();
        reader.close();
    } catch (Exception e) {
        Log.e("Buffer Error", "Error converting result " + e.toString());
    }


    return json;

}

Then I take the variable json, and depending on the keyword in json, I launch an Activity on onPostExecute. The problem starts when I make two of these calls, the program does not wait for the end of the previous call before executing the current one. How can I fix it? I wanted to store somewhere Status last call to see if this is over, if it is not over then the current call is dropped.

Note: I don’t want to rule out the first Asynktask, I want to rule out the second.