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?
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.
