How to clean ThreadLocal's data when the Thread's task is done

Question

I have this three questions:

1.Is there a way to clean ThreadLocal's data when the Thread's task is done.
2.How about when the thread is in ThreadPool.
3.How can I know a Thread is reused and given back to the ThreadPool.

What kind of ThreadPool do you mean? These? http://docs.oracle.com/javase/tutorial/essential/concurrency/pools.html — isnot2bad, Nov 23 '13 at 13:17
@isnot2bad I don't know what ThreadPool it is.It's just a pool hold threads. — lichengwu, Nov 23 '13 at 13:20
I mean, do you use the thread pools from JDK, or any other third party library? — isnot2bad, Nov 23 '13 at 13:21
@isnot2bad it's not a particular pool. Maybe from JDK or other third part libary. — lichengwu, Nov 23 '13 at 13:24

score 4 · Accepted Answer · edited Nov 24 '13 at 11:56

4

Ad 1) To 'clean' the data of a ThreadLocal, simply call ThreadLocal#remove().

Ad 2) ThreadLocals don't behave differently if accessed by a Thread from a thread pool.

Ad 3) You can't. It is highly dependent on the thread pool you use. Some thread pools offer a way to hook into, others don't.

Sidenote: Usually it shouldn't be of any interest, if a Thread of a thread pool is newly created or reused or given back, except when you want to extend an existing thread pool.

edited Nov 24 '13 at 11:56

TwoThe

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answered Nov 23 '13 at 13:30

isnot2bad

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I hava a method cache base on thred. I want to clean cache when current thread's task is done. – lichengwu Nov 23 '13 at 13:57
Then simply remove it at the end of your task! – isnot2bad Nov 23 '13 at 14:02
I do not want to write code at each task, it's there a way do this automatically? Like AOP? – lichengwu Nov 23 '13 at 14:07
Sure. AOP! ;) You could also use a wrapper-task that does preparation before, and cleanup after the real task has been executed. – isnot2bad Nov 23 '13 at 14:11
You can override the `afterTask()` or `beforeTask()` method. – Peter Lawrey Nov 23 '13 at 14:50
@PeterLawrey where is the `afterTask()` and `beforeTask()` method? – lichengwu Nov 23 '13 at 14:55
@lichengwu On ThreadPoolExecutor which implemented ExecutorService – Peter Lawrey Nov 23 '13 at 15:13
@lichengwu In that case you need to modify the thread pool you are using. – Peter Lawrey Nov 23 '13 at 17:51

score 2 · Answer 2 · edited May 23 '17 at 12:33

Removing thread local variables from thread pools is something widely applied in application containers. I answered a question on this topic here: Why do some webservers complain about memory leaks they create?

The easiest way to get rid of thread local variables is getting rid of the tread holding them. The only other way of cleaning out these variables would be reflectively accessing the thread's thread local map which basically represents a map where thread ids point to values which is how thread local pointers are implemented. There is no JVM magic involved when using thread locals. In the end its just a map which can be cleared. (In the linked article, I explain why this is often a bad idea when sharing threads.) The reflective access will however be much more expensive then renewing the thread.

However, you might just reconsider your design. Guava offers a cache which can be built with soft references. This way, you could build a proper cache where your keys could be made up by a thread specific UUID. I do not know your application well enough to give you further advice here. Using thread locals is almost always wrong, though.

score 0 · Answer 3 · answered Nov 23 '13 at 13:28

Is there a way to clean ThreadLocal's data when the Thread's task is done.

You can explicitly set null in the task code.

How about when the thread is in ThreadPool.

Same as 1.

How can I know a Thread is reused and given back to the ThreadPool.

You can use ThreadLocal for that as well. But you shouldn't need to distinguish new and reused threads.

How to clean ThreadLocal's data when the Thread's task is done

3 Answers3