If you expect really large file uploads, then it would be nice to report to your users, how much is already received. Even HTML pages allow implementing a progress bar by returning a multipart/replace response, or something like that. Tracking the upload progress may be done by supplying a ProgressListener.
ProgressListener is called quite frequently, depending on the servlet engine and other environment factors it may be called for any network packet. In other words, your ProgressListener may become a performance problem! A typical solution might be to reduce the activity of ProgressListener to only emit a message if the number of megabytes has changed beyond a range. The following example shows a ProgressListener which implements this solution. We will probably want to communicate the progress to the progress bar on the client. The Real-time server push recipe in Chapter 11, Advanced Tips, can be used as a starting point for implementing pushing updates to the client.
/Create a progress listener
ProgressListener progressListener = new ProgressListener(){
private long megaBytes = -1;
// only update the percent if more than a MB has been uploaded.
public void update(long pBytesRead, long pContentLength, int pItems) {
long mBytes = pBytesRead / 1000000;
if (megaBytes == mBytes) {
return;
}
megaBytes = mBytes;
// compute percent uploaded.
if(pContentLength > 0) {
float percent = pBytesRead / pContentLength;
// percent needs to be communicated to the client
// progress bar
}
}
};