From: Christopher Schultz
Sent: 20 February 2022 14:22
To: users@tomcat.apache.org
Subject: Re: <Context … reloadable = "true"> is too quick to respond
John,
On 2/20/22 05:50, John Barrow wrote:
Neil,
Thanks for your useful feedback. I am still feeling my way as you can
probably see from my earlier emails trying to setup a development
environment.
I did actually think of this but didn't put it in scope for a couple of reasons.
Firstly, the Tomcat documentation for readloadable quotes
"Set to true if you want Catalina to monitor classes in
/WEB-INF/classes/ and /WEB-INF/lib for changes, and automatically
reload the web application if a change is detected. This feature is
very useful during application development, but it requires
significant runtime overhead and is not recommended for use on
deployed production applications. That's why the default setting for
this attribute is false. You can use the Manager web application,
however, to trigger reloads of deployed applications on demand."
Therefore, I took it to mean that this flag was geared at development,
not production which is what I assume when you would deploy a .war
file. So Tomcat would be listening to specific changes in .classes and
.jar files that had just been compiled and these are normally small in
size. But then I suppose that a single .jar file may be so sized that
Tomcat could react while the file was still being written to the disk.
The patch you are currently working on should fix this aspect of the
overall problem you are trying to solve.
Secondly, I sort of assumed that since the feature was already in
place and handles changes to single files that this check for
completeness has already been implemented, but then as I can't get a
development environment to run, I don't have enough skills to drill
into the sources without it being interactive to help me explore and
learn.
However, it makes sense that your recommendation is implemented,
although I was imagining setting the delay to (say) 500ms to ensure
that whatever IDE had time to complete the copying of all the files as
that is a small price to pay for automatic refresh. Also by resetting
the timer after each event it would have to be quite a large upload
for Tomcat to start reacting.
Like you, I am not sure how to formally check that a file has
completed its copy to the destination. The most common suggestion I
hear is to try and change its name and then change it back again and
capture the exception which will be raised if the file is locked. I
wonder whether attempting to set an attribute (e.g.toggle read-only)
would have the same effect (i.e. only allow if file wasn't locked) and
be a little more elegant. I would have to try it.
Don't do anything like that; it won't work on various environments. For
example, Windows obtains exclusive file-locks for even sometimes
read-only operations. But *NIX does /not/. So you may develop something
that works on Windows but doesn't work at all anywhere else.
You basically can't check to see if a file is "done uploading"" or
whatever else may be happening. What you *can* do is check to see if any
file in the list-of-files-to-be it *too recent* indicating that a
compile/copy/upload/whatever may still be in progress.
I assume that Windows has a way of querying a file lock but not sure
(a) whether that is exposed via a Java API and (b) whether that would
apply to Unix as well (as I have only ever used Windows for
development).
How does Tomcat test if a file has been updated?
It's just relative timestamps. Dive into the code Mark suggested and
you'll find it.
Again, I don't know this yet (lack of IDE again), but I assumed that
it would be similar to the method I implemented in the attached source
code, i.e. Create a listener for events being triggered on file
changes to either /WEB-INF/classes/ and /WEB-INF/lib, as they are both
hard-coded file paths.
I don't think your attachment made it to the list. Maybe you can host it
somewhere else and then post a URL to the list? Attachments tend to be
stripped. I'm actually surprised your ZIP file made it through.
As an aside, I should have mentioned, for anyone interested in this
thread and in case not obvious from the source, but to see the sample
source code in action, you need to add, modify, rename or delete files
within the specified directory in a File Explorer.
-chris
On Sun, 20 Feb 2022 at 00:04, Neil Aggarwal <n...@propfinancing.com> wrote:
John:
If anyone has a moment, can you have a quick look and see if what I am
proposing seems acceptable.
Thinking about when a large file (Such as a war file) is being uploaded
to the server, we don't want Tomcat to reload it until after the file has
completed upload and is fully formed.
How does Tomcat test if a file has been updated?
I assume it uses File.lastModified() or something similar.
Does anyone know the detailed nuts and bolts of how that works?
Does last modified keep changing as the file is uploading or is it just
changed at the start of the upload? Is the behavior the same or different
across platforms?
These questions should be taken into account when designing the solution.
Thank you,
Neil
--
Neil Aggarwal, (972) 834-1565, http://www.propfinancing.com
We offer 30 year loans on single family houses!
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