Why Streaming?

The traditional API, which is described in the User Guide, assumes, that file items must be stored somewhere, before they are actually accessable by the user. This approach is convenient, because it allows easy access to an items contents. On the other hand, it is memory and time consuming.

The streaming API allows you to trade a little bit of convenience for optimal performance and and a low memory profile. Additionally, the API is more lightweight, thus easier to understand.

How it works

Again, the FileUpload class is used for accessing the form fields and fields in the order, in which they have been sent by the client. However, the FileItemFactory is completely ignored.

Parsing the request

First of all, do not forget to ensure, that a request actually is a a file upload request. This is typically done using the same static method, which you already know from the traditional API. 

// Check that we have a file upload request
boolean isMultipart = ServletFileUpload.isMultipartContent(request);

Now we are ready to parse the request into its constituent items. Here's how we do it: 

// Create a new file upload handler
ServletFileUpload upload = new ServletFileUpload();

// Parse the request
FileItemIterator iter = upload.getItemIterator(request);
while (iter.hasNext()) {
    FileItemStream item = iter.next();
    String name = item.getFieldName();
    InputStream stream = item.openStream();
    if (item.isFormField()) {
        System.out.println("Form field " + name + " with value "
            + Streams.asString(stream) + " detected.");
    } else {
        System.out.println("File field " + name + " with file name "
            + item.getName() + " detected.");
        // Process the input stream
        ...
    }
}

That's all that's needed. Really!