File Upload with Java (with progress bar)

Question

I'm extremely new to Java, and have mostly just been teaching myself as I go, so I've started building an applet. I'd like to make one that can select a file from the local disk and upload it as a multipart/form-data POST request but with a progress bar. Obviously the user has to grant permission to the Java applet to access the hard drive. Now I've already got the first part working: the user can select a file using a JFileChooser object, which conveniently returns a File object. But I'm wondering what comes next. I know that File.length() will give me the total size in bytes of the file, but how do I send the selected File to the web, and how do I monitor how many bytes have been sent? Thanks in advance.

Answer 1

Keep in mind that the progress bar might be misleading when an intermediate component in the network (e.g., an ISP's HTTP proxy, or a reverse HTTP proxy in front of the server) consumes your upload faster than the server does.

Answer 2

As noted by the article Vincent posted, you can use Apache commons to do this.

Little snipped

DiskFileUpload upload = new DiskFileUpload();
upload.setHeaderEncoding(ConsoleConstants.UTF8_ENCODING);

upload.setSizeMax(1000000);
upload.setSizeThreshold(1000000);

Iterator it = upload.parseRequest((HttpServletRequest) request).iterator();
FileItem item;
while(it.hasNext()){
    item = (FileItem) it.next();
    if (item.getFieldName("UPLOAD FIELD"){
       String fileName = item.getString(ConsoleConstants.UTF8_ENCODING);
       byte[] fileBytes = item.get();
    }
}

Answer 3

From the other answers you can just override the AbstractHttpEntity class children or implementations public void writeTo(OutputStream outstream) method you are using if do not want to create a class.

An example using a FileEntity instance:

FileEntity fileEntity = new FileEntity(new File("img.jpg")){
    @Override
    public void writeTo(OutputStream outstream) throws IOException {
        super.writeTo(new BufferedOutputStream(outstream){
            int writedBytes = 0;

            @Override
            public synchronized void write(byte[] b, int off, int len) throws IOException {
                super.write(b, off, len);

                writedBytes+=len;
                System.out.println("wrote: "+writedBytes+"/"+getContentLength()); //Or anything you want [using other threads]
            }
        });
    }

};

Answer 4

Look into HTTP Client for uploadign the file to the web. It should be able to to do that. I am unsure how to get the progress bar, but it would involve querying that API somehow.

Answer 5

The amount of bytes returned by the listener is different from the original file size. So, instead of having transferred++, I modified it so that transferred=len; that is the length of the actual amount of bytes being written to the output stream. And when I compute the addition of the total bytes transferred it is equal to the actual ContentLength returned by CountingMultiPartEntity.this.getContentLength();

public void write(byte[] b, int off, int len) throws IOException {
    wrappedOutputStream_.write(b,off,len);
    transferred=len;
    listener_.transferred(transferred);
}