Question

I have a Servlet that sends back a JSON Object and I would like to use this servlet in another Java project. I have this method that gets me the results:

public JSONArray getSQL(String aServletURL)
{
 JSONArray toReturn = null;
 String returnString = "";
 try
 {
    URL myUrl = new URL(aServletURL);
        URLConnection conn = myUrl.openConnection();
        conn.setDoOutput(true);
        BufferedReader in = new BufferedReader( new InputStreamReader( conn.getInputStream() ) );
        String s;
        while ((s = in.readLine()) != null )
         returnString += s;
        in.close();

        toReturn = new JSONArray(returnString);
 }
 catch(Exception e)
 {
    return new JSONArray();
 }
        return toReturn;
}

This works pretty will, but the problem I am facing is the following:
When I do several simultaneous requests, the results get mixed up and I sometimes get a Response that does not match the request I send.

I suspect the problem to be related to the way I get the response back: The Reader reading a String from the InputStream of the connection.

How can I make sure that I get one reques -> one corresponding reply ?
Is there a better way to retrieve my JSON object from my servlet ?

Cheers,
Tim

Answer 1

When I do several simultaneous requests, the results get mixed up and I sometimes get a Response that does not match the request I send.

Your servlet is not thread safe. I’d bet that you’ve improperly assigned request scoped data either directly or indirectly as instance or class variables of the servlet. This is a common beginner’s mistake.

Carefully read this How do servlets work? Instantiation, sessions, shared variables and multithreading and fix your servlet code accordingly. The problem is not in the URLConnection code shown so far, although it indicates that you’re doing exactly the same job in both doGet() and doPost(), which in turn is already a smell as to how the servlet is designed.