How to clean up ThreadLocals

Question

Does any one have an example how to do this? Are they handled by the garbage collector? I\'m using Tomcat 6.

Answer 1

There is no way to cleanup ThreadLocal values except from within the thread that put them in there in the first place (or when the thread is garbage collected - not the case with worker threads). This means you should take care to clean up your ThreadLocal's when a servlet request is finished (or before transferring AsyncContext to another thread in Servlet 3), because after that point you may never get a chance to enter that specific worker thread, and hence, will leak memory in situations when your web app is undeployed while the server is not restarted.

A good place to do such cleanup is ServletRequestListener.requestDestroyed().

If you use Spring, all the necessary wiring is already in place, you can simply put stuff in your request scope without worrying about cleaning them up (that happens automatically):

RequestContextHolder.getRequestAttributes().setAttribute("myAttr", myAttr, RequestAttributes.SCOPE_REQUEST);
. . .
RequestContextHolder.getRequestAttributes().getAttribute("myAttr", RequestAttributes.SCOPE_REQUEST);