HTTP Error 503 when accessing servlet

<servlet>
    <servlet-name>Server_side_code</servlet-name>
    <servlet-class>com.hmkcode.servlet.Server_side_codeServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>Server_side_code</servlet-name>
    <url-pattern>/server_side_code</url-pattern>
</servlet-mapping>
<welcome-file-list>
    <welcome-file>index.html</welcome-file>
</welcome-file-list>

Servlet code

public class Server_side_codeServlet extends HttpServlet{ private static final long serialVersionUID = 1L;

    // This will store all received articles
    List<Person> persons = new LinkedList<Person>();

    /***************************************************
     * URL: /
     * doPost(): receives JSON data, parse it, map it and send back as JSON
     ****************************************************/
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException{

        // 1. get received JSON data from request
        BufferedReader br = new BufferedReader(new InputStreamReader(request.getInputStream()));
        String json = "";
        if(br != null){
            json = br.readLine();
        }

        // 2. initiate jackson mapper
        ObjectMapper mapper = new ObjectMapper();

        // 3. Convert received JSON to Article
        Person person = mapper.readValue(json, Person.class);

        // 4. Set response type to JSON
        response.setContentType("application/json");            

        // 5. Add article to List<Article>
        if(persons.size() > 20)
            persons.remove(0);

        persons.add(person);

        // 6. Send List<Article> as JSON to client
        mapper.writeValue(response.getOutputStream(), persons);
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {

        //2. initiate jackson mapper
        ObjectMapper mapper = new ObjectMapper();


        // 4. Set response type to JSON
        resp.setContentType("application/json");            


        // 6. Send List<Article> as JSON to client
        mapper.writeValue(resp.getOutputStream(), persons);
    }
}

I made a simple app in android from which I am sending three parameters to server.From android side it works fine and no error is shown.But when I try access it on local host on local:8888 it displays error HTTP ERROR: 503 Problem accessing /. Reason: Service Unavailable Powered by Jetty://