[jira] Commented: (CONFIGURATION-264) SubnodeConfiguration does not see reloads of its parent configuration

["Oliver Heger (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CONFIGURATION-264?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12489707 ] 

Oliver Heger commented on CONFIGURATION-264:
--------------------------------------------

To make a SubnodeConfiguration compatible with reloads of its parent configuration two things need to be changed:

- An access of the subnode configuration needs to trigger the parent configuration, so that it can check if a reload is required.
- When the parent configuration is reloaded, its set of nodes is completely replaced. So the subnode configuration has to be reconstructed.

To achieve this, it is necessary to store in the subnode configuration the expression that was used to construct it. Then, when the subnode configuration's root node is queried (which happens for every property access), the parent can be asked to re-evaluate the expression and return the new root node. This will also trigger a reload of the parent if necessary.

This solution has the following drawbacks:
- It impacts the performance of SubnodeConfiguration because for each property access an additional query has to be performed on the parent.
- If the expression engine is changed on the parent configuration, we are lost.
- Some updates of the parent configuration may cause unexpected results. As an example consider that the parent configuration contains a list of properties and the subnode configuration points to a single element of this list. If now a list element before this element is removed, the subnode configuration will point to a different list element when it is reconstructed. This is because the expression the subnode configuration is based on cannot be updated according to changes on the parent's properties.
- The parent configuration could be changed in a way that the subnode configuration becomes invalid (e.g. if the root node of the subnode configuration is removed).

Because of these drawbacks I would like to make this solution optional: I would add a new configurationAt() method to HierarchicalConfiguration that takes an additional boolean argument. If this flag is set to true, the subnode configuration will do the described checks and thus will be aware of reloads of its parent. Otherwise (which is the default) the subnode configuration will behave as it does now (and miss the reloads).

> SubnodeConfiguration does not see reloads of its parent configuration
> ---------------------------------------------------------------------
>
>                 Key: CONFIGURATION-264
>                 URL: https://issues.apache.org/jira/browse/CONFIGURATION-264
>             Project: Commons Configuration
>          Issue Type: Bug
>    Affects Versions: 1.4
>            Reporter: Oliver Heger
>         Assigned To: Oliver Heger
>             Fix For: 1.5
>
>
> The problem can be reproduced as follows:
> 1 config = new XMLConfiguration("c:\\conf.xml");
> 2 config.setReloadingStrategy(new FileChangedReloadingStrategy());
> 3 SubnodeConfiguration parentItemConfig = config.configurationAt("parent-item");
> 4 String ss2 = parentItemConfig.getString("sub-item");
> 5 //Now, <sub-item> node value changes to "new_value"
> 6 ss2 = parentItemConfig.getString("sub-item"); // still returns old_value

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