Solution to “Tomcat can’t stop [Abandoned connection cleanup thread]”

I am preparing a web service for one of my larger projects, using Tomcat, Hibernate and MySQL. While developing I stumbled across several Tomcat error messages that suggest some threads could not be stopped. Most of the threads could simply be vanished by calling Hibernate’s SessionFactory.close() method. However, a few threads seem to remain, e.g.

Jul 09, 2014 10:55:40 AM org.apache.catalina.loader.WebappClassLoader clearReferencesJdbc
SEVERE: The web application [/examples] registered the JDBC driver [com.mysql.jdbc.Driver] but failed to 
unregister it when the web application was stopped. To prevent a memory leak, the JDBC Driver has been forcibly unregistered.
Jul 09, 2014 10:55:40 AM org.apache.catalina.loader.WebappClassLoader clearReferencesThreads
SEVERE: The web application [/examples] appears to have started a thread named [Abandoned connection cleanup 
thread] but has failed to stop it. This is very likely to create a memory leak.

and in fact, Tomcat stopped working after a few redeployments due to memory problems. Two main reasons:

  1. The MySQL AbandonedConnectionCleanupThread does not shutdown correctly.
  2. The MySQL J/Connector driver doesn’t unregister anymore from the DriverManager.

At least with J/Connector V5.1.31 the problem disappears by using this shutdown procedure (hooked in by a ServletContextListener):

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   try {
      com.mysql.jdbc.AbandonedConnectionCleanupThread.shutdown();
   } catch (Throwable t) {}
   // This manually deregisters JDBC driver, which prevents Tomcat 7 from complaining about memory leaks
   Enumeration<java .sql.Driver> drivers = java.sql.DriverManager.getDrivers();
   while (drivers.hasMoreElements()) {
      java.sql.Driver driver = drivers.nextElement();
      try {
         java.sql.DriverManager.deregisterDriver(driver);
      } catch (Throwable t) {}
   }
   try { Thread.sleep(2000L); } catch (Exception e) {}
</java>

I still have to observe long-term effects and will keep you informed.

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One Response to “Solution to “Tomcat can’t stop [Abandoned connection cleanup thread]””

  1. Ralph Schuster Says:

    Update July 3rd 2015: No side effects observed. 🙂

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