Servlet - java.lang.IllegalStateException: getWriter() has already been called for this response

Question

I'm using GlassFish as Server and Netbeans IDE 8.0 Here is my project structure.

How my program works:

1. client open localhost:8080/Beer
2. she/he selects a beer (in index.html)
3. it will POST to BeerSelect.java (BS for short)
4. BS will call BeerExpert.java and then call result.jsp for finally send Test.jar to client

Here is the important code in BS.

    /* Result.jsp */
    String c = request.getParameter("color");
    BeerExpert be = new BeerExpert();
    List result = be.getBrands(c);

    request.setAttribute("styles", result);
    RequestDispatcher view = request.getRequestDispatcher("result.jsp");
    view.forward(request, response);

    /* Test Client Download */
    response.setContentType("application/jar");

    ServletContext ctx = getServletContext();
    InputStream is = ctx.getResourceAsStream("/Test.jar");

    int read = 0;
    byte[] bytes = new byte[1024];

    OutputStream os = response.getOutputStream();
    while ((read = is.read(bytes)) != -1){
        os.write(bytes, 0, read);
    }
    os.flush();

The Error:

Answer 1

It is illegal to use both ServletRequest.getOutputStream() and ServletRequest.getWriter(). This has been answered here in detail here.

java.lang.IllegalStateException: Already using output stream