Question

SUMMARY

1. I need to store both uploaded and server-generated images, with portable and
   predictable paths so my server code is aware of where these images exist.
2. I need to generate URLs to these images that can be sent to the client. These URLs will be used in HTML image elements.

PROBLEM

My web application allows the user to upload an image, using gwtupload(Apache Commons). This image is stored on the server and a URL returned to the client so the image is shown. gwtupload provides a clientside method to get this URL for the uploaded image. This works in deployment; the following aspects do not:

• In certain cases, the uploaded image must be cropped. This results in
  a new image being generated by a servlet method. I want to store this
  cropped image and return(to the client) an access URL.
• Then, this image is used by another method to generate another
  image. This method must know the location on the file system of
  the previously uploaded(and/or cropped) image. A URL for the new
  image must then be returned to client.

I have implementation working perfectly in GWT development mode. However, as I expected, after deployment to my own Tomcat server, the remote services fail due to my confusion regarding the file system. I do not know the correct way of storing these images in a predictable place on the server filesystem, nor do I know how to generate access URLs(for files residing outwith the WAR, as these images will.)

All these images are only needed for the current session, so the locations can be temporary directories. I have spent two days experimenting and trawling the web for a solution to no avail.

I will post abridged code below. This is my attempt to simply use the working directory and relative pathnames. Using the Eclipse debugger attached to my servlet container, I could see the results of String dataDir = context.getRealPath(“foo”) indicating a temp folder within the servlet: but when I navigated there using explorer, NONE of the files had been written to the disk. I am very confused.

public String[] generatePreview(String xml) {

    PreviewManager manager = new PreviewManager();
    String url;

    try{       
        preview = manager.generatePreview(xml);
    }
    catch (Exception e){e.printStackTrace();}

    //Create the preview directory
    File folder = new File("previews");
    if (!folder.exists()) folder.mkdir();

    //The file to be written to
    File output = new File(folder, "front.jpg");

    ServletContext context = getServletContext();
    String dataDir = context.getRealPath("previews");

    try {
         ImageIO.write(image, "jpg", output);
    } catch (IOException e) {
         e.printStackTrace();
    }

    url = "previews/" + output.getName();

    return url;
}

@Override
public String cropBackground(int[] coord_pair, String relativePath) {

    File backgroundsFolder = new File("backgrounds");
    if (!backgroundsFolder.exists()) backgroundsFolder.mkdir();

    ServletContext context = getServletContext();
    String dataDir = context.getRealPath("backgrounds");

    File current = new File(relativePath);
    String croppedName = "cropped_" + relativePath.replace("./backgrounds/", "");

    int x = coord_pair[0];
    int y = coord_pair[1];
    int width = coord_pair[2];
    int height = coord_pair[3];
    String croppedPath = null;

    try {
        croppedPath = imageCropper.createCroppedImage(current, "backgrounds",      croppedName, x, y, width, height);
    }
    catch (IOException e){
        e.printStackTrace();
    }

    current.delete();
    return "backgrounds/" + croppedPath;

I am aware that my current ‘return’ statements would never work in deployment: I need to generate the URLs properly and return as strings. I’m sorry about the question length but I wanted to make my problem clear.

Answer 1

Choose a directory where your images will be stored, outside of Tomcat. Assign some unique ID to each uploaded or generated image, and store the images in this directory (with the ID as file name, for example).

Generate URLs to an image servlet that will read the image by ID in this directory, and send the bytes of the image to the output stream of the servlet response : image.action?id=theImageId.