On 9/4/2015 9:00 PM, Semyon Sadetsky wrote:
On 9/4/2015 6:11 PM, Alexander Scherbatiy wrote:
On 9/3/2015 10:01 PM, Semyon Sadetsky wrote:
On 8/5/2015 4:50 PM, Semyon Sadetsky wrote:
On 8/5/2015 2:39 PM, Alexander Scherbatiy wrote:
On 8/5/2015 2:00 PM, Semyon Sadetsky wrote:
On 8/5/2015 1:04 PM, Alexander Scherbatiy wrote:
On 8/4/2015 8:13 PM, Semyon Sadetsky wrote:
On 8/4/2015 6:17 PM, Alexander Scherbatiy wrote:
On 8/4/2015 3:13 PM, Semyon Sadetsky wrote:
On 8/4/2015 2:03 PM, Alexander Scherbatiy wrote:
On 8/4/2015 12:32 PM, Semyon Sadetsky wrote:
On 8/4/2015 11:47 AM, Alexander Scherbatiy wrote:
On 8/3/2015 4:19 PM, Semyon Sadetsky wrote:
On 8/3/2015 3:12 PM, Alexander Scherbatiy wrote:
On 7/31/2015 9:44 AM, Semyon Sadetsky wrote:
Good question. But I did not add a concrete class.
The problem is that UndoManager provided by JDK wants
to be serialized but undoable objects knows nothing
about that. The contract between UndoManager and
undoable is UndoableEditListener which only notifies
UndoManager to add a new edit. AbstractDocument does
not care about the specific UndoManager implementation
and it can contain plenty different
UndoableEditListener. That is the current API approach.
If our specific UndoManager wants to be serialized it
should also take into account that the undoable it
controls may require serialization. For that it needs
undoable's synchronization monitor and AbstractDocument
can provide it using writeLock()/writeUnlock() methods.
I assumed that in the first turn UndoManger should work
well with JDK undoables than to serve as a general
implementation. Also I tried to preserve the current API.
And your suggestion is to change the existing
UndoableEditListener API by introducing synchronization
methods in it. Am I correctly understand you?
What I said is that UndoManager can be used not only
by AbstractDocument but also in other classes which can
have the same synchronization problems.
There should be a way to solve these problems without
storing links of external classes inside the UndoManager.
As well as AbstractDocument can use another undo
managers. It can be addressed to both parties. They need
each others locks to serialize changes without deadlock.
AbstarctDocument is related to UndoableEditListener as
one to many that means a lock should be taken for each
undo manager before the document change.
Undo manager does not have any methods to get its lock
because it is an UndoableEditListener implementation.
AbstarctDocument has API to receive its lock.
Do you still propose to fix the issue on AbstractDocument
side?
Yes.
Could you clarify how do you see such fix?
Put an UndoableEdit/UndoableEditEvent/necessary
information to a queue instead of firing the undoable edit
event under the write lock. Do not read the queue under
the write lock. The queue allows to preserve the order of
UndoableEdit's adding to an UndoManager.
Is not it the same as the previous attempt to fix this
issue (see 8030118 )?
8030118 fix does a strange thing like firing
InsertUpdate document event out of the lock. Do not do that.
Document change event need to be fired under write lock
because the change to the document should be atomic. Queue
of changes is undo manager's responsibility not the document.
And such queue in the AbstractDocument would require
complex coordination with all its undo managers queues.
What if undo called on undo manager during the doc's queue
processing? The right undo/redo requests and edit events
order need to be preserved in this case and it would be too
complex or we would have to change the concept and make
AbstractDocument to maintain its undo/redo history
internally instead of external undo managers.
It only needs to pass undoable edits in the right order
from abstract document to the UndoManager.
Consider the scenario: UndoManager executes undo/redo before
it receives the undoable edits. As result it will undo not
the last edit but intermediate and it will crash because the
document state is changed and intermediate the undoable edit
cannot be applied to the final document state.
This is a good point. But this does not work neither with
the current behavior nor with your proposed fix.
Consider the following scenario:
-----------------------
document.insertString("AAA") // "AAA" UndoableEdit is
added to the UndoManager
document.insertString("BBB")
writeLock();
handleInsertString();
// a user press undo, the "AAA" UndoableEdit is
selected in UndoManager but not executed, because of the write
lock
fireUndoableEditUpdate("BBB") // UndoManager is
waiting for the "AAA" UndoableEdit execution
writeUnlock() // "AAA" UndoableEdit is executed
instead of "BBB"
// "BBB" UndoableEdit is
added to the UndoManager
-----------------------
It will work after the fix. When undo() method is called it
will be blocked on the document lock until the edit is done in
another thread. Then undo() will acquire the document lock and
call editToBeUndone() method which will return the actual last
edit added with addEdit() during the undoable callback execution.
Is it possible to use the same undo manager with several
abstract documents? If so, how are you going to map a document
lock with the document undoable edit without querying it?
That scenario is possible. As well as several undo managers can
be assigned to the same document. I think I can improve the fix
in that direction when you agree with the general approach.
It is interesting how it is possible to do that without
querying an undoable edit. Your fix is relaying that an abstract
document lock should precede the undo manager lock but to get the
right abstract manager lock you need to take an undoable edit
under the undo manager lock first.
We always have only two possible directions forward and backward so
it will require only 2 references.
Hi Alexander,
Please, take a look on the updated version which works for any
number documents shared one undo manager.
Also I removed the reference you did not like. This has some
disadvantages but I think they are negligible.
http://cr.openjdk.java.net/~ssadetsky/8030702/webrev.01/
You code looks like:
----------------------
public void undo() throws CannotUndoException {
synchronized (this) {
lockedDoc = getLockedDocument(edit);
}
// TP: 1
while (!done) {
lockedDoc.writeLock();
// ...
lockedDoc.writeUnlock();
}
}
----------------------
Is it possible that on the line "TP: 1" a new UndoableEdit will be
added to the UndoManager so the the lockedDoc will not point to the
latest UndoableEdit which is taken on the line 438.
No. It is not possible because of
438 UndoableEdit edit = editToBeUndone();
It always return the last significant edit so we preserve the
consistency.
I see.
There is one more question about the undoOrRedo() method there the
synchronization is removed.
Lets look at the sequences of calls to an UndoManager:
addEdit(anEdit1), addEdit(anEdit2), undoOrRedo(), undoOrRedo().
The result for two undoOrRedo() calls should neutralize each other
(it is just undo and redo calls).
Is it possible that after the fix the the first undoOrRedo() from one
thread fails to do the redo and before starting to do the undo
the second undoOrRedo() call from another thread also fails to do a
redo action?
In this cases two undo actions could be called instead of undo and redo.
I still think that updating the UndoableManager for one particular
AbstarctManager class can be made only after investigation of other
possibilities.
You could start with choosing behavior which you want to achieve
or to keep, like:
- fix the deadlock
- atomic undo() method
- serialization
- immediate roll back action
- abstract document consistency after undo() action
- ...
We need to pay attention to the deadlock at first of cause.
Serialization and consistency are achieved. Any concrete doubts?
immediate roll back action -- ?what is that?
"if user starts a long edit operation and press undo after that he
expects when the long edit is finished it will be rolled back
immediately." - what ever does it mean.
I sacrificed undo/redo call atomicity because you did not like doc
references in undo manager. I think it is not important for the most
multithreaded undo/redo scenarios.
Could you give more details about it. Which doc references do you mean?
and look which of the following approaches can better solve them
(where the fist is more preferred and the last is less preferred case):
- using UndoManager as is without adding links from some specific
classes to it
- provide an API for UndoManager to work with UndoableEdit-s which
have synchronization for undo/redo methods
- adding links of external classes directly to UndoManager
What do you mean under link term? A reference or dependency?
There are two options. If UndoManager is a class designed to be
only used with the AbstractDocument and placed in the javax.swing.text
package it definitly can have special code to handle an abstract
document instance in a special way.
If UndoManager is a general purpose class, it looks strange that
it handles some special classes in different way as all others. It
usually mean that there are some design problems in this class. That is
why I just asked to look at other ways at first. Only if other solutions
are not suitable it has sense to look at the way that you are provided.
We are constrained by compatibility requirements. UndoManager is a
broadly used class we cannot change the API so drastically.
I think that you can generalize your solution just adding an
internal interface like sun.swing.UndoableEditLock.
Every UndoableEdit which implements this interface can provide a
lock for its synchronization.
If this will work it can be made public in some days so other
application can also have proper synchronization for their undo/redo
actions.
Thanks,
Alexandr.
"adding links of external classes directly to UndoManager" - Sorry,
did not catch what are you about? Could you clarify?
--Semyon
Thanks,
Alexandr.
--Semyon
There is a mistake in your scenario steps:
fireUndoableEditUpdate() is called before the freeing the lock
(see AbstractDocument.handleInsertString() method).
Yet another argument do not do this from the user
experience: if user starts a long edit operation and press
undo after that he expects when the long edit is finished
it will be rolled back immediately.
It is not true. The first process adds his undo edit to
the UndoManager. While a user trying to press undo the
second long process can be started.
That is what led to this issue because when undo is in
progress document writing should be allowed.
Sorry but I didn't see why is "It not true"? Then what is
your expectation when you press undo button while edit is not
finished yet and there is no way to abort it?
It would be good if it works as you described. But it does
not work in this way with or without your fix.
undo() action has writeLock in AbstractDocument and
because of it is always executed after insert string action.
If a user sees that undo is available, he can call it but
the second long insertString process can start earlier and
acquire the writeLock.
That is what we are going to fix. And this does work after this
fix. Undo call will be blocked by the long edit until the last
is done without any deadlocks. And when edit is done undo()
will acquire the lock and prevent any new edits until undo() is
done. Please provide a scenario when in your opinion it does
not wok.
The first process starts for 5 minutes. When it is finished
a user sees that he can press undo. While he is pressing undo
button, the second long process starts for 10 minutes and
acquire the write lock. The user presses undo but he needs to
wait 10 more minutes until the second process is finished.
Actually, if two or more threads are waiting for a monitor it is
not determined which one will get the control after the signal.
To order that the ReentrantLock API could be used but
AbstractDocument uses wait/notify for locking. I think it is not
worth to dig so deep. It does not cause any issues
The issue that is considered is "if user starts a long edit
operation and press undo after that he expects when the long edit
is finished it will be rolled back immediately."
If you are agree that it is not always possible to do the roll
back "immediately" there is no point to discussion.
I agree. On that level it is not possible to predict the order
exactly in such scenario. But the state of the document will be
consistent. And it is possible to have it predictable using lock
fairness.
because undo() always get the last edit anyway. If it will be
important for somebody to preserve the execution order on that
level of details we will fix it.
So undo should be executed after the edit is fully
performed because the corresponding UndoableEdit which
undos this edit can be produced only after the edit is done.
I think at first we need to look on the situation
externally rather than concentrate on implementation
questions like in which class do references go.
Yes, please look on this situation from a user point of
view which wants to implement simple Java Painter.
But could you describe this scenario? Just steps when this
simple Painter fails under the proposed fix?I
Note, if this Painter's content is not an AbstarctDocument it
will work as before the fix.
Any application that uses UndoManager and wants to have the
same synchronization (have the same lock both for UndoableEdit
adding and undo() method execution) will have the same
deadlock problems.
As I have already written:
---------------
Consider someone writes Java Painter application where it
is possible to draw lines and images and uses UndoManager for
undo/redo actions.
He might want that it was possible to work with copied
images. He can get lock on ctrl+v action, process an image,
prepare UndoableEdit and notify the UndoManager.
He also can use lock/unlock in the undo action to have a
consistent state with the processed image. If someone calls
undo action during the image processing and gets a deadlock
does it mean that link from Java Painter need to be added to
the UndoManager?
---------------
Still do not understand the steps for your Painter scenario. A
link (reference?) can be added if it is required to implement
functionality. If the content is not an AbstarctDocument it may
be required to implement custom UndoManager to support the same
behavior.
What is the difference between the AbstractDocument and
other classes (in Swing or user defined)? Do you mean that the
UndoManager is intended only to be used with AbstractDocument
and it shouldn't be used in other cases where undo/redo actions
are required for non text data?
No, undo manager can be used with any classes. But since we have
it assigned to AbstarctDocument so often we need to do our best
to make undo manager working with it correctly because users do
not like deadlocks usualy. For other classes we cannot provide
synchronization by default because there is no API to get the
lock. So it remains up to user how to provide the undo manager
synchronization with the object it controls for other classes
What we should do just to understand that the same deadlock
can happen in an user applications because he wants to use the
same synchronization both for the data processing and for the undo
action. If so, there should be two investigations:
1. Is it possible to achieve the requested goals without
changing UndoManager? In other words The UndoManager should be
used in proper way as it is required by its design.
2. Is it possible to update the UndoManager API to provide
functionality that meets new requests?
With API change it is reachable. But I would preserve the current
API as less constrained. If we add some methods for locking we will
determine the way how a user should synchronize his undoable
content. And user may not need any synchronization at all. We
should keep in mind this opportunity as well.
Only after this discussion there can be a reason to look to
other ways.
Thanks,
Alexandr.
I think our undo manager implementation do not pretend to be used
as the global undo manager for big complex applications and it
cannot cover all possible undo approaches. But some basic
functionality can be provided and it should be usable. Without
edits serialization approach it is not usable for multithreaded
use. So either we do not pretend to provide a multithreaded undo
manager and remove all synchronize keywords from UndoManager
class, either we need to support serialization approach which
does not cause deadlocks.
Thanks,
Alexandr.
I don't see a contradiction here, could you point on it more
precisely?
Thanks,
Alexandr.
Thanks,
Alexandr.
Thanks,
Alexandr.
--Semyon
Thanks,
Alexandr.
--Semyon
On 7/30/2015 5:27 PM, Alexander Scherbatiy wrote:
Consider someone writes Java Painter application
where it is possible to draw lines and images and uses
UndoManager for undo/redo actions.
He might want that it was possible to work with
copied images. He can get lock on ctrl+v action,
process an image, prepare UndoableEdit and notify the
UndoManager.
He also can use lock/unlock in the undo action to
have a consistent state with the processed image. If
someone calls undo action during the image processing
and gets a deadlock does it mean that link from Java
Painter need to be added to the UndoManager?
Thanks,
Alexandr.
It looks like AbstractDocument violates
UndoManager synchronization contract when it both
use lock to work with UndoManager and in the
implemented undo() method.
Thanks,
Alexandr.
--Semyon
