PermGen exhaustions in
combination with
ThreadLocal are
often caused by classloader leaks.
An example:
Imagine an application server which has a pool of worker
threads.
They will be kept alive until application server termination.
A deployed web application uses a static
ThreadLocal in
one of its classes in order to store some thread-local data, an instance of another class (lets call it SomeClass)
of the web application. This is done within the worker thread (e.g. this action originates from a HTTP request).Important:
By
definition, a reference to a
ThreadLocal value is
kept until the "owning" thread dies or if the ThreadLocal itself is no longer reachable.If the web application fails
to clear the reference to the
ThreadLocal on
shutdown, bad things will happen:Because the worker thread will usually never die and the reference to the
ThreadLocal is
static, the ThreadLocal value still
references the instance of SomeClass,
a web application's class - even if the web application has been stopped!As a consequence, the web application's classloader
cannot be garbage collected, which means that all
classes (and all static data) of the web application remain
loaded (this affects the PermGen memory pool as well as the heap).
Every redeployment iteration of the web application will increase permgen (and heap) usage.
=> This is the permgen leak