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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-1439) Investigate removing the antiGC thread in embedded Derby
Date Tue, 12 Oct 2010 13:10:33 GMT

     [ https://issues.apache.org/jira/browse/DERBY-1439?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Knut Anders Hatlen updated DERBY-1439:
--------------------------------------

    Attachment: patch1.diff

I tried the naive approach to removing AntiGC (see the attached patch1.diff), but that led
to many IllegalThreadStateExceptions in suites.All. This can also be seen by shutting down
the only booted database and rebooting it in the same JVM:

ij> connect 'jdbc:derby:db1;create=true';
ij> connect 'jdbc:derby:db1;shutdown=true';
ERROR 08006: Database 'db1' shutdown.
ij> connect 'jdbc:derby:db1';
ERROR XJ040: Failed to start database 'db1' with class loader sun.misc.Launcher$AppClassLoader@fabe9,
see the next exception for details.
ERROR XJ001: Java exception: ': java.lang.IllegalThreadStateException'.

The problem with removing AntiGC is that BaseMonitor has a daemon ThreadGroup in which all
service threads are started. Since the thread group is a daemon group, it will be destroyed
automatically when its thread count becomes zero. AntiGC would prevent the thread group from
being destroyed as long as the driver was loaded, since it would stay alive until the driver
was unloaded and keep the thread count greater than zero. When there is no AntiGC thread to
keep up the thread count, it may go down to zero on database shutdown, as in the example above,
even if the driver is still loaded. The next attempt to start a service thread will then fail,
since one cannot add threads to a destroyed thread group.

I suppose we could make the thread group a non-daemon group and destroy it manually when the
driver is unloaded, but then we'd first have to wait for all service threads to complete,
which may introduce possibilities for hangs on system shutdown (for example because of DERBY-594).
Another option may be to detect that this has happened and create a new daemon group when
needed.

> Investigate removing the antiGC thread in embedded Derby
> --------------------------------------------------------
>
>                 Key: DERBY-1439
>                 URL: https://issues.apache.org/jira/browse/DERBY-1439
>             Project: Derby
>          Issue Type: Improvement
>          Components: Services
>            Reporter: Daniel John Debrunner
>            Priority: Minor
>         Attachments: patch1.diff
>
>
> The antiGC thread was originally created to avoid the DriverManager class being garbage
collected when no refrences existed to it and it had loaded the embedded JDBC driver (and
hence shutting down the engine). This was an issue with JDK 1.1. Since Derby does not support
jdk1.1 and garbage collection of classes is clearly defined, it is possible the thread serves
no useful purpose.

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