is there a way to return a list from servlet to html page

Question

I want to send request from html page to servlet then create a list in java servlet then i want to return this list to the same html page .

Answer 1

The problem is that you never initialize your HttpSession s variable:

@WebServlet(urlPatterns = {"/ShowPersonServlet"})
public class ShowPersonServlet extends HttpServlet {

    HttpSession s; //null by default
    //...

    protected void processRequest(...) {
        //...
        //since never initialized, s is null
        user.add((Person) s.getAttribute("person"));
    }

    @Override
    protected void doGet(...) {
        //...
        //since never initialized, s is null
        s.setAttribute("person",person);
    }
}

Make-it-work solution: Set the value of s

s = request.getSession();

Real world solution:

• Remove all the fields from the Servlet, NEVER try to handle state in your servlet unless they are resources managed by the container like EJBs.
• Change the scope of your HttpSession s to be local per method. Also, change its name from s to session or something more useful.
• Move your HTML code to a component that deals with view details like a JSP, then perform a forward to the view.

So, your code would look like this:

@WebServlet(urlPatterns = {"/ShowPersonServlet"})
public class ShowPersonServlet extends HttpServlet {
    /*
    HttpSession s ; //moved as local variable
    Person person = new Person(); //moved as local variable
    private List<Person> user = new ArrayList<Person>(); //not sure where you want to store this
    */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        HttpSession session = request.getSession();
        List<Person> personList = (List<Person>) session.getAttribute("personList");
        if (personList == null) {
            personList = new ArrayList<>();
            session.setAttribute("personList", personList);
        }
        personList.add((Person) session.getAttribute("person"));
        /*
        try (PrintWriter out = response.getWriter()) {
            //removed to shorten this answer
        }
        */
        request.getRequestDispatcher("/showPerson.jsp").forward(request, response);
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
        Person person = new Person();
        person.setKey(request.getParameter("txt_Key"));
        person.setFirstName(request.getParameter("txt_firstName"));
        person.setLastName(request.getParameter("txt_lastName"));
        processRequest(request, response);
        HttpSession session = request.getSession();
        session.setAttribute("person",person);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
        processRequest(request, response);
    }
}

More info:

• How do servlets work? Instantiation, sessions, shared variables and multithreading
• StackOverflow Servlets wiki

Answer 2

You need to get the session from the request, you cannot simply expect to get it with HttpSession s ; - There s is null.

// Something like this (in processRequest), although I 
// would prefer a local session variable.
s = request.getSession();

Answer 3

You need to provide stacktrace of your exception. But it looks like it could be in 2 places:

1. s.setAttribute("person",person); where s is never set;
2. out.println("" + p.getKey() + ""); where attribute "person" is never set;

Good luck.

Answer 4

The value of var s is not defined. It is null so using s.getAttribute("str") will throw an exception.

HttpSession s ;

s = request.getSession(false);