Hi Geoff,
I've read through the SoftReference documentation and as far as I
understand it the references do only get garbage-collected in case of
memory-pressure.
However, the behavior to keep recently used objects is only encouraged, not
explicitly required.
Looking over the source code, you mabye can replace PageSource with a
custom implementation that uses another caching implementation.
Something like this (untested) code maybe?
package tapestry;
import java.lang.ref.SoftReference;
import java.util.Map;
import org.apache.tapestry5.commons.util.CollectionFactory;
import org.apache.tapestry5.internal.services.PageLoader;
import org.apache.tapestry5.internal.services.PageSourceImpl;
import org.apache.tapestry5.internal.structure.Page;
import
org.apache.tapestry5.services.pageload.ComponentRequestSelectorAnalyzer;
import org.apache.tapestry5.services.pageload.ComponentResourceSelector;
public class CustomPageSourceImpl extends PageSourceImpl {
private PageLoader pageLoader;
private ComponentRequestSelectorAnalyzer selectorAnalyzer;
public CustomPageSourceImpl(PageLoader pageLoader,
ComponentRequestSelectorAnalyzer selectorAnalyzer) {
super(pageLoader, selectorAnalyzer);
this.pageLoader = pageLoader;
this.selectorAnalyzer = selectorAnalyzer;
}
private static final record CachedPageKey(String pageName,
ComponentResourceSelector selector) {
}
private final Map<CachedPageKey, Object> pageCache =
CollectionFactory.newConcurrentMap();
public Page getPage(String canonicalPageName)
{
var selector = selectorAnalyzer.buildSelectorForRequest();
var key = new CachedPageKey(canonicalPageName, selector);
while (true)
{
Object cachedObject = pageCache.get(key);
Page page = null;
if (cachedObject instanceof SoftReference<?> ref) {
page = ref == null ? null : (Page) ref.get();
} else {
page = (Page) cachedObject;
}
if (page != null)
{
return page;
}
// In rare race conditions, we may see the same page loaded
multiple times across
// different threads. The last built one will "evict" the
others from the page cache,
// and the earlier ones will be GCed.
page = pageLoader.loadPage(canonicalPageName, selector);
// TODO: Decide here if how you want to store the Page
Object cacheValue = new SoftReference<Page>(page);
pageCache.put(key, cacheValue);
}
}
}
I'm not sure what the implications are if a page is kept forever, but as a
SoftReference isn't guaranteed to be garbage-collected, I don't see an
immediate downside, except needing more memory.
Alternatively you could trigger the page with a cron job to keep it
"fresh", but an overriden service is the more robust solution in my opinion.
Cheers
Ben
On Tue, Dec 27, 2022 at 3:24 PM JumpStart <
[email protected]> wrote:
> Hi,
>
> I have one page in my production app which takes a long time to load -
> minimum 18 seconds. Once loaded, it is very quick to request. But from time
> to time throughout the day, when this page is requested Tapestry decides it
> has to reload it. I presume this is because the page cache uses
> SoftReference (PageSourceImpl.pageCache) and the page has been garbage
> collected. For the unlucky user, they have to wait an unbearably long time.
> Sometimes when the system is under load the request can even time out. Is
> there a simple, reliable, safe way to prevent it being garbage collected?
> Or have I misunderstood what’s going on?
>
> Cheers,
>
> Geoff
>
>
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