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https://issues.apache.org/jira/browse/TOREE-374?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15855262#comment-15855262
]
David Taieb commented on TOREE-374:
-----------------------------------
[~jodersky] Simple steps to reproduce.
In cell 1, create a ReferenceQueue with the following code, run cell 1 only
once:
```
import scala.ref.WeakReference
import scala.ref.ReferenceQueue
val queue:ReferenceQueue[AnyRef] = new ReferenceQueue
```
In cell 2, create an obj and WeakReference to it
```
var obj = new Object()
val weakRef = new WeakReference(obj, queue)
```
Run cell 2 twice, the expected behaviour is that the first instance of obj
should be marked for gc and placed in the ReferenceQueue
In cell 3, poll the ReferenceQueue:
```
System.gc()
println(queue.poll)
```
Run cell 3 and observe that it output None. No object has been marked for
deletion.
Now the positive test, add obj = null in cell 3 as such (note: that's why I
used var in the cell2, which means that val can never be gc'ed since you can't
dereference them)
```
obj=null
System.gc()
println(queue.poll)
```
Output is: Some(scala.ref.WeakReferenceWithWrapper@25ac93fd) which is expected.
> Variables declared on the Notebook are not garbage collected
> ------------------------------------------------------------
>
> Key: TOREE-374
> URL: https://issues.apache.org/jira/browse/TOREE-374
> Project: TOREE
> Issue Type: Bug
> Affects Versions: 0.1.0
> Reporter: David Taieb
>
> I'm not sure if it's a bug or a limitation of the underlying scala REPL.
> As part of supporting PixieDust (https://github.com/ibm-cds-labs/pixiedust)
> auto-visualization feature within Scala gateway, I have implemented a weak
> hashmap that tracks objects declared on the Scala REPL. However, I have found
> that objects are not correctly gc'ed when the object is declared in a cell
> with a val or var keyword and then the cell is ran again. One would expect
> that the original object has no more references and should be gc'ed but it's
> not.
> However, when the object is declare with var keyword and then set to null in
> another cell, then it is correctly gc'ed.
> I'm concerned that users who run the same cell multiple times would
> unwittingly have memory leaks which can eventually lead to OOM errors.
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