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I am working on a system that pools JDBC MySQL connections.
It fell over the other day with an out of memory error.
Looking at the .hprof, it says that one MySQL JDBC connection has a retained size of 27Mb. According to the hprof that was all in the com.mysql.jdbc.Connection.openConnections field.
Why would this field be accumulating values, and is there any way to clear it?
John Farrel wrote:However, it's possible to turn this functionality off by adding dontTrackOpenResources=true to the MySQL JDBC URL.
Instead of such hacks, I'd suggest to stick to best practices and explicitly close the Statements and ResultSets after use. Every (reasonable) JDBC tutorial says so.
You should call close() on connections obtained from connection pool too, of course. This is the correct way to return the connection to the pool (mentioning it because it isn't entirely clear from your post whether you're doing so). Some connection pools might ensure that resources created on that connection which weren't properly closed are freed, but this depends on the particular connection pool you're using.