Return data from Servlet to Java Client

Question

Hi i have a problem with returning data from Servlet to Java Client. This is a first time that i use a servlet. All examples that i saw on the web return data to an HTML page but i want make a Server-Client software where Server do something and return a String List.

How can i return from a GET/POST method an Array to a Client? What do i set in setContentType? I didnt understand how can i put in response the information that i want (like int , array , String) and return to the Client.

If someone could make an example where a Java Client make a POST request and a Servlet return to him a Array or ArrayList i would be very happy.

Answer 1

You are running into the problem of serialization. Serialization is where you convert some data into a format that can be transmitted. There are several ways of doing this, some are mentioned in other answers.

I would suggest using JSON as your format. You can get a nice JSON library for java from json.org. Then you can simply create a JSON array with the library and write it to the servlet's OutputStream.

public void service(ServletRequest req, ServletResponse res) {
    final JSONArray arr=new JSONArray();
    for (String s : this.myListOfStrings){
        arr.put(s);
    }
    //Here we serialize the stream to a String.
    final String output = arr.toString();
    res.setContentLength(output.length());
    //And write the string to output.
    res.getOutputStream().write(output.getBytes());
    res.getOutputStream().flush();
    res.getOutputStream().close();
}

Now from your client, you can make the request and get back your ArrayList like so:

public ArrayList<String> contactServer(){
    final URL url = new URL(serverURL);
    final URLConnection connection=url.openConnection();
    connection.setDoOutput(true);
    /*
     * ...
     * write your POST data to the connection.
     * ...
     */
    final BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream()));
    final char[] buffer=new char[Integer.parseInt(connection.getHeaderField("Content-Length"))];
    int bytesRead=0;
    while (bytesRead < buffer.length){
        bytesRead += br.read(buffer, bytesRead, buffer.length - bytesRead + 1);
    }
    final JSONArray arr = new JSONArray(new String(buffer));
    final ArrayList<String> ret = new ArrayList<String>(arr.length());
    for (int i=0; i<arr.length(); i++) {
        ret.add(arr.get(i));
    }
    return ret;
}

Answer 2

You seem to need a RESTful service over http. You should choose the way you want to serialize your objects. The typical choice is JSON - you serlialize the object to JSON and write it to the response (with Content-Type set to application/json

There are frameworks that do that - take a look at Spring MVC or Jersey/Resteasy

If you want something more low-level, you can use RMI or sockets directly, without using a servlet. Servlets are aimed to respond to HTTP requests, which can only transmit textual data.