Clearing up Vaadin StreamResource after file download

Question

Vaadin version 14.4.8

I am using the StreamResource to download large files that are generated on the fly and never required again like so:

StreamResource streamResource = new StreamResource("export.zip", () -> generateFile()); 
streamResource.setCacheTime(10); //does this work?
Anchor hiddenDownloadLink = new Anchor(streamResource, "Workaround");
hiddenDownloadLink.setId("DownloadLinkWorkaround-" + System.currentTimeMillis());
hiddenDownloadLink.getElement().setAttribute("style", "display: none");
UI.getCurrent().getElement().appendChild(hiddenDownloadLink.getElement());
UI.getCurrent().getPage().executeJs("$0.click();", hiddenDownloadLink.getElement());

After the file has been successfully downloaded if I open dev tools on my browser and click the anchor link the file is downloaded straight away (no file generation) which makes me think the stream resource is cached somewhere. How do I clear up the stream after use and get rid of anything related to it in memory? I have tried removing the Anchor which obviously removes the link on the page but this doesn't seem to release the memory...

Edit The generateFile() method look something like:

private ByteArrayInputStream generateFile() {
   try (FastByteArrayOutputStream byteOutStream = new 
   FastByteArrayOutputStream();
             ZipOutputStream zipOut = new ZipOutputStream(byteOutStream)) {
   //do some processing that can take minutes
   return new ByteArrayInputStream(byteOutStream.toByteArray());
        }
        catch (IOException ex) {
            log.error("Failed to crete streams", ex);
        }

Answer 1

You didn't share what generateFile() looks like, but I did a simple implementation of it to be able to try out your example:

public InputStream generateFile() {
    System.out.println("Generating file");
    return new ByteArrayInputStream("Hello world".getBytes(StandardCharsets.UTF_8));
}

With this implementation, I see that it prints Generating file to the server-side console every time the link is clicked. Are you sure the file is cached in your case rather than just being generated again very quickly?

setCacheTime defines for how long the browser is allowed to cache the content but it has no impact on what the server does. With your example, the browser cache time is set to 10 milliseconds which effectively means that the browser does no caching. The default setting if you don't do setCacheTime is to instruct the browser to not cache the file at all.

Vaadin keeps a reference to the stream resource and its stream generator callback (() -> generateFile() in this case) as long as at least one Anchor component that references that stream resource is attached to the server-side component tree. The memory use for this is minimal as long as long as generateFile() doesn't internally cache any data.

The pattern that you are showing isn't particularly user friendly since they don't have any easy way of restarting the download. It's instead recommended to keep the link visible ("If your download does not start automatically, then click here") until the user navigates to some other part of the application.