Hello Felix, See my replies marked with DGS.
Dan From: Felix Schumacher <felix.schumac...@internetallee.de> Sent: Tuesday, August 12, 2025 2:47 PM To: users@tomcat.apache.org Subject: Re: How to access a REST service Am 08.08.25 um 18:21 schrieb Daniel Schwartz: Hello Robert, From your and other people's replies, I'm learning that tracking down memory leaks can be quite daunting and possibly beyond me. I'm aware of the try-with-resources instruction and will look into it. I wrote this code before learning about this. In any case, I will follow your advice and add a "finally" block to the "try" statement. I'm currently closing the connection in case an SQL exception is thrown, but I hadn't considered the possibility of other kinds of exceptions. I suppose that there could be such exceptions, but I'm mystified regarding what those might be. However, there has been a new development. After "fishing" around in the Glassfish administration system, I found that there is a way to monitor the JDBC connection pool and get all sorts of information. Based on this, I really don't think that there is any memory leak at all. When running a simple query on my website, I see that NumConnAcquired increases by three (which is what I would expect), and immediately thereafter NumConnReleased increases by the same number. So every connection that is being acquired is also being released. I think that, if there were a memory leak, the number of connections acquired would be increasing at a faster pace than the number being released. Also, there is a NumConnUsed statistic that has a "High Water Mark" which is described as the "maximum number of connections that were used", and this is currently 160, which is not a lot. So, if every page hit takes up three connections (as I read it above), that would imply, that you have around 50ish concurrent page hits in maximum. Is that consistent with your data? If so, you probably don't have a connection leak, but rather an unusual program design that uses (in my opinion) far too many connections for each page hit. DGS: I explained this in a previous email. Each user interaction requires either 3 or 4 database queries. That’s how my system works. There’s no way around this. But it’s really quite quick. Three of the four queries are simple SQL table look-up queries; only one requires any complex computations. If you don't see a correlation between concurrent requests to your service and your database (now, that you are able to view those metrics), I would set the reclaim option for the database pool as described at https://glassfish.org/docs/latest/administration-guide.html#statement-leak-detection-and-leaked-statement-reclamation (note I haven't run Glassfish myself, but I like the log abandoned feature from tomcats db pool). DGS: Thanks for the link. I took a quick look, but it’s quite long and would take me a lot of work to absorb it. Maybe later. Felix So it seems that the system is behaving normally. And I have noticed that since setting the maximum pool size to 1000, the system has not crashed for several days. Probably, 1000 is more than necessary, and you just need make sure you have more possible connections available than the anticipated future high-water mark. But this seems to be working. So maybe I'm just imagining problems that no longer exist. As long as the system keeps running, I'm fine with this. Thanks to everyone for their replies. Dan -----Original Message----- From: Robert Turner <rtur...@e-djuster.ca.INVALID><mailto:rtur...@e-djuster.ca.INVALID> Sent: Friday, August 8, 2025 1:32 AM To: Tomcat Users List <users@tomcat.apache.org><mailto:users@tomcat.apache.org> Subject: Re: How to access a REST service (sorry for any errors, I'm bashing this out quickly on my phone before going to bed) On Fri, Aug 8, 2025, 00:59 Daniel Schwartz <d...@danielgschwartz.com><mailto:d...@danielgschwartz.com> wrote: Hello Chuck, You are correct, I only catch SQL exceptions. I have modified my code so that the Connection object will be closed if an SQL exception is thrown, but I wasn’t aware that there are other types of exceptions that I should look for. I will look into this. I would encourage entering "avoid java resource leak" into Google and reading the "AI" generated details or any other articles that come up. The "AI" description it gave me was a reasonable overview of what we are trying to convey to you and how to resolve it. Unchecked exceptions can be thrown and the compiler will not warn about it. This may be the issue, but it it not guaranteed, but it definitely is a potential and obvious problem with the code. You need to ensure that everything related to the connection (connection, ResultSet, Statement, PreparedStatement) are also released (closed) or they will retain references to the connection and the connection may not be returned to the pool. As previously mentioned by others, using try-with-resources for all of these objects is strongly recommended to avoid leaks. If you are uaing older Java (consider upgrading), you will need to use finally blocks instead (as previously mentioned by others). Unfortunately, you have only posted a portion of the code, and it may be the case that it the problems stems from elsewhere (like where you make the queries, or anywhere else that obtains a connection). It should be easy to get a tool to analyze the code (say your IDE (if you use one), PMD, Spotbugs, Sonarqube, etc.) and they would likely point out all the potential resource leaks. Many IDEs, such as Intellij IDEA, also provide such analysis built in. They basically look for any object derived from AutoClosable and ensure you are closing the object in a finally clause (or in all potential code paths). If you aren't, you have a leak. However, I don’t think that this is causing a memory link, since my code has always run perfectly without throwing any exceptions at all, as far as I know. You may not know. The container (Glassfish or Tomcat) often catches them and unless you are checking their logs, you may not see any evidence of them. But I’m beginning to wonder if my “close” operation is actually being recognized by Glassfish. It seems possible that this is not being communicated to Glassfish, and the Connection object continues to be marked as being in use. This would be a leak. But I don’t know if this is happening. It's certainly possible that some behaviour of the container and it's pooling system is exasperating your issue, but I strongly doubt the container is at fault. It is almost guaranteed to be your code. There are other possibly issues with your code, but these are less "critical" than your leaks, but could also be a potential source of issues. Namely you have static Global's that are not protected against concurrency, and the containers typically run each request in separate threads. You could be creating more than one instance of your "singleton" as a result. Dan From: Chuck Caldarale <n82...@gmail.com><mailto:n82...@gmail.com> Sent: Thursday, August 7, 2025 11:52 PM To: Tomcat Users List <users@tomcat.apache.org><mailto:users@tomcat.apache.org> Subject: Re: How to access a REST service On 2025 Aug 7, at 21:43, Daniel Schwartz <d...@danielgschwartz.com<mailto:d...@danielgschwartz.com> <mailto:d...@danielgschwartz.com><mailto:d...@danielgschwartz.com>> wrote: > > I have just posted everything again, this time with a few modifications. You say, "Last time you posted it, it was prone to leaking connection." NkdkJdXPPEBannerStart Be Careful With This Message From (Chuck Caldarale <n82...@gmail.com><mailto:n82...@gmail.com>)< https://godaddy1.cloud-protect.net/email-details/?k=k1&payload=53616c7 465645f5fed27a22a24a04d2b4474850fcd4085279f5deb0a4b45aff271dc217bd5b8a c8e65b868f686679b9d5ae69c216b8828c6ab976fd096f84ebdac1ecb38a6f62a2e05b 1ce3cda3ef1fc6615c2d5d63b3db6ac3d93719e10964cf6ecb92b637712794fd9814a3 6d8e401e9510d22d52433dc8526a1fd76ffd29c927dd092a286a002fa6edad24fdc289 956069ff246bd9f54272522f4c2b34608a52f7c8a6db9c157660efc922fcaa993a27ce 11dcae209b3f911b99a3e50e92edf0e120af447100e9d80cd45918b2f85aa8673efb14 e6bddb38cdfaa2e1ba5496bad5db8df0857d443aeb0399f46406f Learn More< https://godaddy1.cloud-protect.net/email-details/?k=k1&payload=53616c7 465645f5fed27a22a24a04d2b4474850fcd4085279f5deb0a4b45aff271dc217bd5b8a c8e65b868f686679b9d5ae69c216b8828c6ab976fd096f84ebdac1ecb38a6f62a2e05b 1ce3cda3ef1fc6615c2d5d63b3db6ac3d93719e10964cf6ecb92b637712794fd9814a3 6d8e401e9510d22d52433dc8526a1fd76ffd29c927dd092a286a002fa6edad24fdc289 956069ff246bd9f54272522f4c2b34608a52f7c8a6db9c157660efc922fcaa993a27ce 11dcae209b3f911b99a3e50e92edf0e120af447100e9d80cd45918b2f85aa8673efb14 e6bddb38cdfaa2e1ba5496bad5db8df0857d443aeb0399f46406f Potential Impersonation The sender's identity could not be verified and someone may be impersonating the sender. Take caution when interacting with this message. NkdkJdXPPEBannerEnd On 2025 Aug 7, at 21:43, Daniel Schwartz <d...@danielgschwartz.com<mailto:d...@danielgschwartz.com> <mailto:d...@danielgschwartz.com><mailto:d...@danielgschwartz.com>> wrote: I have just posted everything again, this time with a few modifications. You say, "Last time you posted it, it was prone to leaking connection." Could you say more exactly where you see saw the leak? You only catch errors of the type SQLException, which means anything else will lose the connection. This is the reason you should always use try - catch - finally (or the newer with-resources syntax) around database manipulations, as recommended in Chris’ blog post. - Chuck Thanks, Dan Schwartz -----Original Message----- From: Robert Turner <rtur...@e-djuster.ca.INVALID<mailto:rtur...@e-djuster.ca.INVALID><mailto: rtur...@e-djuster.ca.INVALID<mailto:rtur...@e-djuster.ca.INVALID>>> Sent: Thursday, August 7, 2025 5:07 PM To: Tomcat Users List <users@tomcat.apache.org<mailto:users@tomcat.apache.org><mailto: users@tomcat.apache.org<mailto:users@tomcat.apache.org>>> Subject: Re: How to access a REST service Dan, On Thu, Aug 7, 2025 at 5:01 PM Daniel Schwartz <d...@danielgschwartz.com<mailto:d...@danielgschwartz.com> <mailto:d...@danielgschwartz.com><mailto:d...@danielgschwartz.com>> wrote: Hello Chris, Thank you for your reply, but I'm still unsure. You seem to be implying that I have a memory leak, i.e., many connection objects being created that are not being closed. However, I really don't think this is happening. My code closes each connection immediately after using it. Maybe post your code again. Last time you posted it, it was prone to leaking connections. If it hasn't changed, that is likely your problem. My understanding is that the only way the maximum pool size of X, whatever that is, would be a limitation is if there was an attempt to create X+1 simultaneous connections. When you do this in Glassfish, it outputs an error message saying that no more connections can be created and then crashes. You have to go back in and manually restart it. I believe that the essential problem, as explained in a previous email to Rob Sargent, is that I'm getting several hundred database requests per day from web crawlers. I just spent some time reading through my ngnix access.log and found that the vast majority of these are from GoogleBot. My guess it that, due to a time lag between opening and closing connections, many connections will be opened simultaneously. This is why a small pool size won't work. Also, I'm advised to not block the web crawlers because this assists with SEO. My understanding is that you just have to live with this. I don't think there is an issue with my code. The only answer I can come up with is to have a large maximum pool size, larger that the expected number of simultaneous accesses. There is almost definitely a problem with your code (unfortunately), or your database requests are very slow and triggered by any connection. We run servers that handle much more traffic than you are describing and make thousands of DB requests per minute, and we rarely go over 10 DB connections being used at a time. There is almost for sure something leaking in your code. This is very unlikely to be a problem with the pooling ("select isn't broken"). You are looking for unlikely causes to the problem. I originally wrote to this email list because I was thinking of shifting from Glassfish to Tomcat, and was trying to learn how to do this. I think I do know how to do this now, and might try doing this. My understanding is that the connection pooling that works with Tomcat doesn't have that same limitation as Glassfish, and one can have connections that exist outside the pool. This would resolve the issue I'm currently having with Glassfish. Best regards, Dan Schwartz ...snip.. -------------------------------------------------------------------- - To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org<mailto:users-unsubscr...@tomcat.apache.org><mailto: users-unsubscr...@tomcat.apache.org<mailto:users-unsubscr...@tomcat.apache.org>> For additional commands, e-mail: users-h...@tomcat.apache.org<mailto:users-h...@tomcat.apache.org><mailto: users-h...@tomcat.apache.org<mailto:users-h...@tomcat.apache.org>> --------------------------------------------------------------------- To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org<mailto:users-unsubscr...@tomcat.apache.org><mailto:<mailto:users-unsubscr...@tomcat.apache.org> users-unsubscr...@tomcat.apache.org><mailto:users-unsubscr...@tomcat.apache.org> For additional commands, e-mail: users-h...@tomcat.apache.org<mailto:users-h...@tomcat.apache.org><mailto:<mailto:users-h...@tomcat.apache.org> users-h...@tomcat.apache.org><mailto:users-h...@tomcat.apache.org> --------------------------------------------------------------------- To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org<mailto:users-unsubscr...@tomcat.apache.org> For additional commands, e-mail: users-h...@tomcat.apache.org<mailto:users-h...@tomcat.apache.org>
