9 Replies Latest reply on Nov 27, 2004 8:27 PM by Jimmy Wilson

    Use of getChildrenNames

    Jimmy Wilson Apprentice

      I have some code that uses "getChildrenNames", and this works well except when other threads via the same VM (or a remote one upon replication) change the children a node has by removing or adding children.

      The issue is that to use a set, you need to use an iterator. When the underlying set is modified, the iterator properly throws a ConcurrentModificationException. To avoid this, I attempted to use the "toArray" method on the returned set, but this suffers from the same problem.

      In other code where I have used iterators, I have used the "clone" method to avoid this problem, but since the Set interface is returned by this method, I can not use that. I tried to see the type actually returned through "getClass().getName()", but it is an UnmodifiableSet returned from the Collections class, and it does not implement Cloneable either.

      Can anyone suggest a course of action to take? I do not care if what I get back remains up to date as changes are made to the cache node, I only care that it is up-to-date when I invoke "getChildrenNames".

        • 1. Re: Use of getChildrenNames
          Bela Ban Master

          Bela wrote:

          Set children=new TreeSet(result), no need for a clone().


          this must happen within the same transaction as getChildrennames(). If not using transactional locking there's still a chance that the Constructor of TreeSet will throw ConncurrentModificationException too. (It makes use of the Iterator of the 'result').

          • 2. 3856675
            Jimmy Wilson Apprentice

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            • 3. 3856814
              Norbert von Truchsess Newbie

               

              "jiwils" wrote:
              I have some code that uses "getChildrenNames", and this works well except when other threads via the same VM (or a remote one upon replication) change the children a node has by removing or adding children.

              The issue is that to use a set, you need to use an iterator. When the underlying set is modified, the iterator properly throws a ConcurrentModificationException. To avoid this, I attempted to use the "toArray" method on the returned set, but this suffers from the same problem.

              In other code where I have used iterators, I have used the "clone" method to avoid this problem, but since the Set interface is returned by this method, I can not use that. I tried to see the type actually returned through "getClass().getName()", but it is an UnmodifiableSet returned from the Collections class, and it does not implement Cloneable either.

              Can anyone suggest a course of action to take? I do not care if what I get back remains up to date as changes are made to the cache node, I only care that it is up-to-date when I invoke "getChildrenNames".



              Although the set in unmodifiable for the caller, the cache itself can modify it. You should copy the set immediately, e.g.

              Set children=new TreeSet(result), no need for a clone().

              • 4. Re: Use of getChildrenNames
                Jimmy Wilson Apprentice

                Bela wrote:

                Set children=new TreeSet(result), no need for a clone().


                this must happen within the same transaction as getChildrennames(). If not using transactional locking there's still a chance that the Constructor of TreeSet will throw ConncurrentModificationException too. (It makes use of the Iterator of the 'result').

                • 5. Re: Use of getChildrenNames
                  Norbert von Truchsess Newbie

                  Although the methods of 'Node' are declared as public, this class is (currently) not intendet to be used directly by TreeCache-clients.

                  The only way to access TreeCache in a threadsafe manner is through TreeCache's methods (put(), get() etc...)

                  The methods of Node are not threadsafe. Whenever a method of Node is called the node is not locked in any way. Locking is being taken care of by the TreeCache itself.

                  So you might ask, 'why are these methods declared as public?' - The only reason, that these methods are declared public, is, that you cannot declare classes to be 'friends' in java and Node is being used by other (internal) packages of JBossCache. (This packaging design might need some rework, or at least be somewhat better documented in the javadoc....)

                  • 6. Re: Use of getChildrenNames
                    Norbert von Truchsess Newbie

                    You are absolutely right - I did not read your posting carefully enaugh. Node's 'getChildren()' is known to be not threadsafe, while TreeCache's 'getChildrenNames(Fqn)' is intendet to be used by clients.

                    The ConcurrentModificationException is a bug. I'll fix this in cvs-head later today, so it can be includet in JBossCache 1.2

                    I also evaluated whether this can be workarounded it in your code (by doing a 'clone()' on the keyset before getting the iterator, but unfortunally the Set that is returned by HashMap's keySet()-method is not clonable).

                    • 7. Re: Use of getChildrenNames
                      Bela Ban Master

                       

                      "jiwils" wrote:
                      I have some code that uses "getChildrenNames", and this works well except when other threads via the same VM (or a remote one upon replication) change the children a node has by removing or adding children.

                      The issue is that to use a set, you need to use an iterator. When the underlying set is modified, the iterator properly throws a ConcurrentModificationException. To avoid this, I attempted to use the "toArray" method on the returned set, but this suffers from the same problem.

                      In other code where I have used iterators, I have used the "clone" method to avoid this problem, but since the Set interface is returned by this method, I can not use that. I tried to see the type actually returned through "getClass().getName()", but it is an UnmodifiableSet returned from the Collections class, and it does not implement Cloneable either.

                      Can anyone suggest a course of action to take? I do not care if what I get back remains up to date as changes are made to the cache node, I only care that it is up-to-date when I invoke "getChildrenNames".



                      Although the set in unmodifiable for the caller, the cache itself can modify it. You should copy the set immediately, e.g.

                      Set children=new TreeSet(result), no need for a clone().

                      • 8. Re: Use of getChildrenNames
                        Norbert von Truchsess Newbie

                        Bela wrote:

                        Set children=new TreeSet(result), no need for a clone().


                        this must happen within the same transaction as getChildrennames(). If not using transactional locking there's still a chance that the Constructor of TreeSet will throw ConncurrentModificationException too. (It makes use of the Iterator of the 'result').

                        • 9. Re: Use of getChildrenNames
                          Bela Ban Master

                          Correction: can't remove Node from public exposure. Reason: interceptors in different packages use it, so the methods cannot be package-private.
                          Bela