Hello Thorsten,
Please see my replies below, marked DGS.
-----Original Message-----
From: Thorsten Heit <heit_tom...@icloud.com.INVALID>
Sent: Friday, August 1, 2025 5:32 AM
To: users@tomcat.apache.org
Subject: Re: How to access a REST service
Hi,
> Daniel,
> You might take a look at java’s “ try with” construct
DGS: Someone else recommended this. I will look into it. I see that an issue
with my current code is that the database connection won't be closed if an
error is thrown.
This is an important step, indeed:
>> Here is a fragment of the code from the file HolidaysRESTJSONResource.jar.
>> ---------------------------------------------------------------------
>> ----- public class HolidaysRESTJSONResource {
>>
>> DataSource dataSource;
>>
>> public HolidaysRESTJSONResource() {
>> dataSource = DataSourceSingleton.getInstance().dataSource;
>> }
>>
>> @GET
>> @Path("/countries")
>> @Produces(MediaType.APPLICATION_JSON)
>> public String getJsonCountries() {
>> ...
>> Connection connection;
>> try {
>> connection = dataSource.getConnection();
>> TempDataStorage.countryList =
>> GetCountryList.doIt(connection);
>> connection.close();
>> ...
>> }
A couple of questions:
1) How often is your class instantiated/created? Is it a singleton? A new one
per each incoming request?
DGS: It is created with every database query. For each user query there can be
1-3 database queries, and the same user can make an unlimited number of
queries. I know it would be better if I could somehow identify a "user
session" and only create one connection per session, but I don't know how to do
this.
2) What happens in case of an exception in your try block? Looking at
the above snipped the connection isn't closed.
DGS: Yes, that's an issue, which I have noted in the foregoing. I'll change to
the "try-with" approach.
>> The constructor for HolidaysRESTJSONResource creates a DataSource by
>> creating an instance of the following DataSourceSingleton class and
>> retrieving the value of the DataSource instance variable. I borrowed
>> this code from somewhere. It seems to work. The MySql database is
>> called "holidays".
3) Have a look at the version of the database driver you're using. Is
there perhaps a newer one? If yes, have a look at the release notes and
check the changes. Perhaps there's a leak in the driver itself...?
DGS: I'm using the latest MySQL 8 driver.
>> Back in HolidaysRESTJSONResource, in the method getJsonCountries(),
>> the DataSource object, called "dataSource", is used to create the
>> connection object, and this is used in a method call
>> GetCountryList.doIt() to retrieve a list of countries from the
>> database. Then this connection object is closed. I think that this
>> is all that is needed to prevent memory leaks. Is this correct? I do
>> this for every such method call that accesses the database.
AFAICT only in case of proper execution wrt to the code snippet
presented. In case of an exception during the #doIt(...) call this won't
happen (correctly) and can result in a memory leak (AFAIK).
DGS: Good point. The doIt() method can throw an exception. I will look into
this.
What happens in case of multiple simultaneously incoming requests? Does
your database (and driver) allow enough parallel connections so that
each request is processed correctly?
DGS: I have the Glassfish database connections maximum pool size set to 1000.
This seems to be enough. When I submitted this question, the system was
crashing occasionally, but then I noticed that the pool size had inadvertently
been reset to the default 32. This must have happened when I reinstalled
Glassfish and then neglected to update pool size. It seems to be working now.
No crashes in the last day.
DGS: Thanks for taking the time to respond. Do you know of a way to identify
the start and end of a user session?
Regards
Thorsten
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