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Date: Wed, 27 Mar 2013 14:45:16 +0000 (UTC)
From: "Rick Curtis (JIRA)" <jira@apache.org>
To: dev@openjpa.apache.org
Message-ID: <JIRA.12526684.1318349356435.63819.1364395516084@arcas>
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Subject: [jira] [Commented] (OPENJPA-2057) Rethinking ClassLoading
 architecture
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    [ https://issues.apache.org/jira/browse/OPENJPA-2057?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13615330#comment-13615330 ] 

Rick Curtis commented on OPENJPA-2057:
--------------------------------------

Assigning back to Pinaki.... hopefully we can get to this piece of work in the near future.
                
> Rethinking ClassLoading architecture 
> -------------------------------------
>
>                 Key: OPENJPA-2057
>                 URL: https://issues.apache.org/jira/browse/OPENJPA-2057
>             Project: OpenJPA
>          Issue Type: Improvement
>          Components: kernel
>            Reporter: Pinaki Poddar
>            Assignee: Pinaki Poddar
>
> This issue proposes an overhaul of the classloading architecture.
> Background:
> ------------------
> OpenJPA runtime needs to load classes and resources at various time points in its life cycle and employs various classloading strategies at different parts of its code base. 
> These are few broad categories of classes/resources OpenJPA needs to load
> 1. Resources: user-specified resources such as persistence.xml or orm.xml
> 2. Persistent Domain classes
> 3. Native Plug-ins: Implementation of interfaces e.g. UpdateManager that are supplied by OpenJPA and packaged in its own distribution
> 4. User Plug-ins: Implementation of interfaces e.g. MappingStrategy or ValueHandlers that the user has supplied via configuration and packaged in deployed units
> 5. Temporary classloader for java agent or weaving code loading domain classes to enhance them prior to their use
> To load these different artifacts by their name, OpenJPA at different places employ available classloaders such as 
>    i) the current thread's context class loader 
>    ii) the clasloader that loaded OpenJPA native classes/interfaces 
>   iii) the classloader that loaded a deployed application which can vary based on the container (Spring, OSGi, JEE) environment
>   iv) system classloader   
> The problem is the decision about which classloader is appropriate in a given context is quite scattered. This weakness appears in numerous places where a method is supplied with a ClassLoader and if the supplied loader is null, the method chooses a classloader (often the context classloader) or a class has its own classforname() method that tries a series of classloaders etc. 
> This is a perennial problem and manifested in several reported bugs whose resolutions often introduced further localized logic to account for the point defects, thereby  accentuating the same trends that I believe is the root cause of the problem itself. 
> Proposed solution/design:
> -------------------------------------
> Unify classloading decision in a singular abstraction. 
> Allow that abstraction to cope with classloading regimes of different containers (Spring, OSGi, JEE etc). 
> The natural candidate for unifying classloading is existing Configuration object. This object is a per persistence unit singleton and available throughout the runtime. 
> However, certain class/resource loading must take place even before a Configuration instance is instantiated. For example, to locate and load the persistence.xml itself. 
> Also note that the persistence.xml or orm.xml may contain fully-qualified names of persistent domain classes or  plug-in names (both native and custom/user variety) and they can occur either by their  fully-qualified class name or registered alias. The specialized parsers often has to load the class given their parsed string names or aliases. 
> The bootstrap sequence of OpenJPA runtime is to construct a specific ConfigurationProvider and a series of specialized parsers to parse meta-data of various sorts (annotations, mapping xml, persistence.xml). These ConfigurationProviders are responsibilities of ProductDerivation -- the core mechanics that contributes their individual aspects to build up a Configuration. 
> Given this existing well-designed bootstrap strategy, here is a proposal
>   1. Let each ProductDerivation make their decision on how they will load whatever they need to load using whatever classloader they may need. For example, a OSGi ProductDerivation will use a bundle classloader to load its relevant resources. This phase occurs *before* a persistence unit (i.e. EntityManagerFactory) is constructed.
>   2. Once the ProductDerivations have finished their loading using their own ConfigurationProvider, they transfer the accumulated information to a Configuration instance which essentially becomes the holder of entire runtime information for an EntityManagerFactory. During this transfer phase, let the ProductDerivations set the classloader as well into the Configuration instance.
>   3. Once the Configuration instance has the classloader, this classloader is used throughout the codebase.
> But what kind of classloader is used by the Configuration that will suit complex needs of class/resource loading? 
> OpenJPA already has a powerful abstraction called MultiClassLoader which can employ an ordered series of (possibly unrelated) classloaders. So that MultiClassLoader is the correct classloader for Configuration instance. The ProductDerivation or ConfigurationProvider can add/remove their contributions. 
> I understand that a change of this nature could be destabilizing in short-term. Also the change is difficult to validate across various container environments. I hope the community users will help by suggesting and testing such an overhaul to streamline OpenJPA classloading for long-term sustainability and maintenance. 
>    
>   

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