This might be simpler than it looks ... where the iterator is created,
clone the src list, then make the iterator from the cloned list; this
only works when you are certain the second iterator will not modify the
list in such a way which will cause some unwanted actions / invalid
state.
For example, if both the child and parent threads were traversing a
graph structure eliminating nodes ... the child may eliminate a node
which has yet to come in the parent's iterator (aka modifying a
coverage).
David
_____
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of
Sunburned Surveyor
Sent: January 17, 2007 11:58 AM
To: List for discussion of JPP development and use.
Subject: [JPP-Devel] Help with bug 1487099...
I'm had some time on my lunch break today to work on one of the bugs I
selected for squishing before the release of OpenJUMP 01.00.02. Pedro
submitted this bug to the SourceForge tracker last May. Jon Aquino made
some comments, but the bug was never fixed. I believe a fix could be
applied to both JUMP and OpenJUMP.
Pedro received a ConcurrentModificationException from the
fireCategoryChanged() method of the LayerManager class. He said this
happened when "when you load a project with lots of layers and
categories and its behaviour is more or less random as to its
occurrence."
Jon thought this could be fixed by wrapping the collection of
LayerListeners in an ArrayList.
I took a look at the LayerManager class and saw that the LayerListeners
were already stored in an instance of ArrayList, so I started looking
for the problem elsewhere. I noted that the fireLayerEvent() method
which is called by the fireCategoryChanged() method starts a new thread.
I think this is important because the Javadoc for the
ConcurrentModificationException states that: "For example, it is not
generally permssible for one thread to modify a Collection while another
thread is iterating over it. In general, the results of the iteration
are undefined under these circumstances. Some Iterator implementations
(including those of all the collection implementations provided by the
JRE) may choose to throw this exception if this behavior is detected.
Iterators that do this are known as fail-fast iterators, as they fail
quickly and cleanly, rather that risking arbitrary, non-deterministic
behavior at an undetermined time in the future."
I also noted that in Pedro's stack trace the LayerManager's
addCategory() method is called not once, but twice before the exception
is thrown. When I look at this and Pedro's description of what he was
doing when he encountered the exception I concluded that the following
can happen in OpenJUMP/JUMP.
The LayerManager.addCategory() method is called once. This method calls
the LayerManager.fireCategoryChanged() method, which in turn calls the
LayerManager.fireLayerEvent() method. The LayerManager.fireLayerEvent()
method creates a new thread and begins to iterate over the list calling
methods of the LayerListeners contained in the ArrayList.
While the thread that was created as a part of the first call to
addCategory is still executing, the second call to the
LayerManager.addCategory() occurs. This creates a second thread that
attempts to manipulate the same ArrayList that contains the
LayerListeners.
I'm no expert at threads in Java, so I need some help determining the
solution. (I've got a book on Java threads that I tried to read over the
holiday break, but it started to give me a headache. I now I need to
learn more about threads, so I will return to the book soon.) I think I
need to create an class that extends ArrayList and makes the method to
access LayerListeners synchronized. Is this the way to fix the problem,
or is there a simpler solution?
The Sunburned Surveyor
(I knew bug squishing would be difficult, but why did my first one have
to involve threads.) :]
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