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servlet writes file that is sometimes corrupt

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posted 15 years ago

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Howdy Ranchers. I have a simple servlet that produces a jpg based on posted data. If I simply return the image it always works. However, if I create the file on the server filesystem, and forward to a view that links to it, sometimes the image file is corrupt. If I reload a few times, I'll get a good version, and reloading again might produce a corrupt one. I'm wondering if one of you might notice something obvious in my code that might explain the inconsistent result, or have any other idea about why this is happening.

Thanks!

Mike

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.ServletInputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 *
 * @author mike
 */
public class ImageServlet extends HttpServlet {

/** 
     * Handles the HTTP <code>POST</code> method.
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

//This version simply returns the image.  Always works.
        /*
        ServletInputStream is = request.getInputStream();

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();
        os.close();
        */

//This version creates the file and forwards the view.  Image sometimes corrupt.
        
        String fname = Long.toString(System.currentTimeMillis()) + ".jpg";

File f = new File(getInitParameter("dataDir") + fname);

ServletInputStream is = request.getInputStream();

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

FileOutputStream fos = new FileOutputStream(f);
        while ((read = is.read(bytes)) != -1) {
            fos.write(bytes);
        }

fos.flush();
        fos.close();

request.setAttribute("filename", f.getName());

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

}

The result jsp:

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Bear Bibeault

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posted 15 years ago

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Where is the image data coming from? If it's a file upload from the client, then you are most definitely doing it wrong.

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Mike Mitchell

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posted 15 years ago

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Hi Bear. Thanks much for the reply. The image data comes from a flash module, the source of which I don't have :-(. I did just no notice I was processing the output stream differently. In the case that always worked:

And in the case that wasn't:

When I added the other two arguments to my write, it started working every time.

From the api:

I'm happy it works, but not quite satisfied I understand why. Or why it would be unpredictable -- I could repost the same data multiple times, with an inconsistent result.... weird.

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Paul Clapham

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posted 15 years ago

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Read the API documentation for the read(byte[]) method of InputStream carefully. Then ask yourself what would happen if that method put 389 bytes into that array. Would writing out 389 bytes be the right thing to do, or would writing out 1024 bytes be the right thing to do?

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Mike Mitchell

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posted 15 years ago

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After the read I'd have 389 meaningful bytes in my 1024 array. With the first form of write, I was writing all 1024 (because bytes.length is 1024 of course), and with the second, I was writing only the number of bytes read into the buffer.

Thanks Paul!

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