Display list of files in webpage using servlet

Question

I'm new to Java EE, and I want to display in a webpage a list of PDF thumbnails. These PDF are stored in a folder in src/main/webapp/pdf, and I want to read this folder to get all the filenames. Here is my code :

@Override
public void doGet(HttpServletRequest req, HttpServletResponse res) {
    try {
        res.setContentType("application/json");
        res.setCharacterEncoding("UTF-8");

        PrintWriter out = res.getWriter();
        File pdfFolder = new File("/pdf");

        for (File pdf : pdfFolder.listFiles()) { // Line 27
            out.println(pdf.getName());
        }
    } catch (IOException e) {
        log.log(Level.SEVERE, e.getMessage());
    }
}

When I run this code, I get a NullPointerException :

java.lang.NullPointerException
    com.multi.services.ListFiles.doGet(ListFiles.java:27)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)

Here is my structure :

What I want to have is a web service that reads the PDF folder and returns a JSON containing the PDF filenames, and I will call this service in a JavaScript using Ajax.

Can anyone help me to make my script running well ? Or has anyone a better solution ?

Thanks :)

Answer 1

A quote from the Javadoc of File.listFiles()

returns null if this abstract pathname does not denote a directory, or if an I/O error occurs.

So your path is not correct (as the current directory of your servlet container is undefined). You have these possibilities here:

• Use the absolute path (this is appropriate, if you store the PDF outside of your webapp)

• Use getRealPath() (this should be suitable for your use case; PDFs are part of webapp):

File pdfFolder =
  new File(req.getSession().getServletContext().getRealPath("/pdf"));

Answer 2

Berilium, I've tried your suggestion and it works fine !

In order to clarify this item, I show my case:

try { 
    File pdfFolder=new File(request.getSession().getServletContext().getRealPath("img/fotos"));

    System.out.println("PATH---->"+pdfFolder);

    for (File pdf : pdfFolder.listFiles()) { 

        String s = pdf.getName();
        String t = s.substring(0, s.lastIndexOf("."));

        if (t.equals(nombre)) {
            Foto = s;
            break;
        }
    }
} 
catch (IOException e) {
     e.printStackTrace();
}