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From Gregg Wonderly <gr...@wonderly.org>
Subject Re: CodebaseAccessClassLoader replacement for RMIClassLoader
Date Sun, 12 Aug 2012 03:26:16 GMT
My initial focus, was on just striping away the dependency on RMIClassLoaderSPI 
as the "only" way to change class loading behavior.  The impetus, of course, was 
that Netbeans does not have a trivial way (let alone functional) way to specify 
a classpath change (required for the static initialization of RMIClassLoaderSPI) 
at startup, without reengineering netbeans itself.  So, I decided that I wanted 
to just make it possible to completely avoid RMIClassLoaderSPI.  The 
optimizations and odd calling paths that you mentioned, were things that I 
looked at, but felt it might be better to go ahead and get some feed back from 
everyone before working on all the changes and optimizations to do all of that.

However, I am sure that PreferredClassProvider really should not be a 
CodebaseClassAccess instance.  There are two usage patterns at play here.

There is the service/client apps, who will assert some CodebaseClassAccess 
instance, and then there is downloaded code, which might use RMI, or which might 
otherwise call PreferredClassProvider.loadClass().  Those  codebases also need 
to be affected by the assertion of a CodebaseClassAccess instance.

I don't remember the exact details of what I experienced when I was putting this 
together, but it seems like I experienced an unexpected call to 

Note, that my old changes to PreferredClassProvider, to provide deferred 
downloading based on "alwaysPreferred" classes, has been moved into my own 
implementation of CodebaseClassAccess et.al.  I used to have lots of hooks into 
different parts of PreferredClassProvider, including loadClass(), so it may be 
that I'm speaking of past issues which don't exist any longer.

I need to drag all the Netbeans stuff back out and see where things stand it 

Gregg Wonderly

On 8/11/2012 7:05 PM, Peter Firmstone wrote:
> Peter Firmstone wrote:
>> Just thought I'd get a little clarification on how CodebaseAccessClassLoader 
>> should work and whether it needs any further refinements or tweaks.  I'm 
>> currently patching CodebaseAccessClassLoader back into the main trunk, so it 
>> can make the next release.
>> All references to RMIClassLoader static method calls (except for 
>> RMIClassLoader.getDefaultProviderInstance()) in the platform and supporting 
>> service implementations were replaced with equivalent method calls to 
>> CodebaseAccessClassLoader, I'm now going through the test kits, replacing all 
>> similar method calls.
>> The relationship between PreferredClassProvider, RMIClassLoader and 
>> CodebaseAccessClassLoader appear circular, so I'm finding it a little 
>> confusing how it should be applied in Netbeans or an OSGi environment.
>> Observations:
>>   1. CodebaseAccessClassLoader is the replacement for RMIClassLoader,
>>      it has identical static methods (except for
>>      getDefaultProviderInstance()) and three additional methods
>>      (identical to CodebaseClassAccess).
>>   2. CodebaseAccessClassLoader providers must implement the
>>      CodebaseClassAccess interface, which it delegates to.
>>   3. CodebaseAccessClassLoader has a static method to change the
>>      provider, guarded with a security check.
>>   4. CodebaseClassAccess has identical methods to RMIClassLoader
>>      (except for getDefaultProviderInstance()) and three additional
>>      methods:
>>         1. createClassLoader(URL[] urls, ClassLoader parentLoader,
>>            boolean requiredDlperm, AccessControlContext ctx)
>>         2. getParentContextClassLoader()
>>         3. getSystemContextLoader(ClassLoader defaultLoader).
>>   5. RMIClassLoaderCodebaseAccess is a wrapper around RMIClassLoader
>>      that implements CodebaseClassAccess
>>   6. RMIClassLoaderCodebaseAccess is the default provider for
>>      CodebaseAccessClassLoader
>>   7. PreferredClassProvider doesn't implement CodebaseClassAccess.
>>   8. PreferredClassProvider now calls CodebaseAccessClassLoader to get
>>      the context ClassLoader (which may now be something other than the
>>      call Thread's context ClassLoader) and also calls
>>      CodebaseAccessClassLoader.createClassLoader instead of creating a
>>      PreferredClassLoader directly.
>>   9. Call path CodebaseAccessClassLoader -->
>>      RMIClassLoaderCodebaseAccess --> RMIClassLoader -->
>>      PreferredClassProvider --> CodebaseAccessClassLoader -->
>>      RMIClassLoaderCodebaseAccess
>>  10. The interface CodebaseClassAccess includes deprecated methods from
>>      RMIClassLoader
>> A flaw with the original RMIClassLoaderSPI mechanism is you don't get a 
>> choice of provider, like you do for encryption or other providers, you get 
>> the first loaded provider.  The ServiceProvider mechanism in Java 6 is more 
>> flexible than RMIClassLoaderSPI, allowing loading from child ClassLoaders, 
>> not just the system loader.
>> I understand and appreciate that Gregg has created this to allow development 
>> using Netbeans, a task which the code has proven successful, I also 
>> understand that Chris used it with OSGi. Lets make sure we get it right prior 
>> to release.
>> Some Questions:
>> Should CodebaseAccessClassLoader be used to replace RMIClassLoaderSPI?
>> Shouldn't PreferredClassProvider also implement CodebaseClassAccess?  So it 
>> can be used directly as a provider without using RMIClassLoader or 
>> RMIClassLoaderCodebaseAccess?
>> Shouldn't PreferredClassProvider provide its own methods for creating 
>> ClassLoaders and finding the parent ClassLoader rather than relying on 
>> CodebaseAccessClassLoader, which might be delegating to a different provider.
>> Should we drop the deprecated RMIClassLoader methods?
> My mistake, it doesn't contain any deprecated RMIClassLoader methods.
>> Should we have additional mechanisms for loading CodebaseAccessClassLoader 
>> providers other than the static setter method?  Eg configuration or 
>> ServiceProvider?
>> Should we have more than one provider available?
>> Regards,
>> Peter.

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