I realise too that this decision affects ServiceUI.
In the case of ServiceUI, the ServiceAPI is not known to the client
directly, it is known to the ServiceUI, which is known to the Client, so
the Service API, must be visible to the ServiceUI, which is visible to
the client.
Since there may be any number of ServiceUI that are platform or client
specific, it is the client that knows which ServiceUI is suitable.
ServiceUI is an implementation detail, it can be displayed into a JFrame
or something similar depending on platform.
If the ServiceAPI only exists in the Proxy ClassLoader, then the
ServiceUI must also be loaded into the Proxy ClassLoader. Reading the
source for UIDescriptor, this is the intended behaviour for ServiceUI.
I had originally imagined the dumb proxy's could be loaded into the same
ClassLoader as the Service API, each proxy with its own
ProtectionDomain, as this would save memory. However the ServiceUI
classes cannot be loaded there.
In this case the ServiceUI will need it's own child ClassLoader.
Also there is the case of a dumb proxy (java.lang.reflect.Proxy), where
Service API in the client is old and the Proxy has been created with an
Extended Service API, that requires new classes. It will not be
possible to unmarshall this dumb proxy, unless it is given it's own
class loader in which to load any new Service API class files and the proxy.
It is very likely that over time, Service API will be extended, this
will cause earlier clients to consume additional memory, as they create
new ClassLoader's to accommodate additional interfaces required by dumb
proxy's.
It may instead be preferable to provide a single ClassLoader, a child of
the AppClassLoader that loads any additional Service API for dumb
proxy's and all dumb proxy's.
dumb proxy's with ServiceUI's will probably need a child classLoader of
this for each ServiceUI, in case of implementation variation's between
ServiceUI's
With these additional concerns, I provide a slightly revise class loader
tree.
_______________________________________________________________
| |
| AppClassLoader |
| |_________________________ |
| | | |
| | Extended Service API |
Extended Service API
| | Dumb Proxy ClassLoader | Only
visible to Proxy
| | | | and
ServiceUI.
| | Dumb Proxy ServiceUI |
| | ClassLoader's |
| | |
| | |
| Common Classloader (As Per Dennis' comments) |
| _________|_______________________________________ |
| | | | | |
| Service Imp Smart Proxy Parameter Impl Application | Child
Implementation
| ClassLoader's ClassLoader's ClassLoader's ClassLoader |
ClassLoader's
|_______________________________________________________________|
AppClassLoader - Contains the main() class of the container. Main-Class
in manifest points to com.sun.jini.start.ServiceStarter
Classpath: boot.jar, start.jar, jsk-platform.jar, service-api.jar
(mulitple Service API jar's allowed, can be untrusted)
Codebase: none
CommonClassLoader - Contains the common Rio and Jini technology classes
(and other declared common platform JARs) to be made available to its
children.
Classpath: Common JARs such as rio.jar
Codebase: Context dependent. The codebase returned is the codebase of
the specific child CL that is the current context of the request. (Not
Sure about the last sentence please explain?)
Child ClassLoader's - Contains the service specific implementation
classes and client application classes.
Classpath: serviceImpl.jar
Codebase: "serviceX-dl.jar rio-dl.jar jsk-lib-dl.jar"
Extended Service API ClassLoader - Contains any Extended Service API not
present locally to enable unmarshalling of dumb proxy's that require
extended interfaces. Dumb proxy's can all be loaded into this ClassLoader.
Extended Service API is not visible to the client.
Codebase: service-api.jar (only classes missing locally will be loaded).
Dumb Proxy ServiceUI ClassLoader's - Contains ServiceUI's for dumb
proxy's, implementation's may vary and are essentially independent.
Codebase: serviceUI.jar.
Note: ServiceUI for smart proxy's will be loaded into the Smart Proxy
ClassLoader with the smart proxy.
If you can see problems I've overlooked with the above approach, please
indicate, but also see if you can help with a solution or alternate
arrangement.
Cheers,
Peter.
Peter Firmstone wrote:
Note this is static Service API, it is not dynamically loaded.
I remember now, why I had initially thought about dynamically loadable
Service API, it was for cases where the Service API was extended as it
evolved, to avoid Proxy unmarshalling errors. However as suggested at
the bottom of the previous message, this can be worked around by
including the Service API in the proxy's *-dl.jar, in addition to
including it in the client. The Service API in the client will be
used in preference, however if anythings missing, the proxy can still
be unmarshalled.
Peter Firmstone wrote:
You know something I realised about Evolving code, Interfaces and
Lookup?
It's a little difficult to explain, Jini veterans are probably well
aware of it.
I'll give you a hint,
1. It makes Versioning unnecessary.
2. It enables easy backward compatibility.
3. Implementation is isolated from API.
4. API is easily extended.
Well it's Jini Lookup semantics.
Clients lookup Service instances, based on their Service Interface
(API), so they only discover compatible Services. Ok that probably
sounds obvious and "So What", well the Service implementation is free
to change, it can extend the Service Interface, old clients continue
to lookup with the earlier interface while new clients will find
extended service interfaces with added functionality.
The Service knows nothing of the Client, the Client know's nothing of
the Service's implementation. But even better, if we use Interfaces
for return values and Parameters, neither the client or the service
need to know anything about each others implementation objects.
The service is free to change and the client is free to change. All
interfaces can be extended, so the service API has flexibility in all
directions, new methods can be added by extending existing
interfaces, return values and parameters can change and implement new
interfaces too.
This is an extremely flexible communication contract point. So not
only can the protocols change but so can the API be extended, while
remaining fully backward compatible.
Back to the humble isolated JVM for a moment, I guess the problem
with using classes without interfaces is, a class has an API and an
implementation.
Suddenly the flexibility is gone.
Take String as a case in point, new methods were added in Java 5,
this means later code that utilise the new methods, can't run on Java
1.4.
So we're forced to use versioning to work around these issues of
incompatibility.
But Service Interfaces don't need to be versioned, they have many
degrees of freedom in which to maneuver to accommodate change,
developers use versioning when their implementation would be
compromised by maintaining complete backward compatibility.
It gets better though, all implementations in Services, Proxy's and
Clients can be isolated in their own ClassLoader's and so not step on
each other's classes if we structure the ClassLoader tree properly.
Versioning is a concern of implementations.
We must prevent any implementation from being visible to another -
seeing the other's classes while running in the same JVM.
So there is something that concerns me about Application code,
running in a ClassLoader that is parent to a proxy or a Service.
Applications could depend on classes, while not part of Service API,
may be present in other implementation code, eg, the Proxy, but being
free to vary as implementation classes are, if implementations vary
(has another version!) and an incompatible class is loaded into a
Parent ClassLoader, the wrong class will be used, causing runtime
errors.
So without fleshing out all the details and leaving as much out as
possible to allow others to advise of their needs and concerns, I'd
like to suggest that the following ClassLoader relationship based on
the Rio ClassLoader structure provided by Dennis, invisibility
between implementations is fundamental.
_______________________________________________________________
| |
| AppClassLoader |
| | |
| Common Classloader (As Per Dennis' comments) |
| _________|_______________________________________ |
| | | | | |
| Service Imp Smart Proxy Parameter Impl Application |
| ClassLoader's ClassLoader's ClassLoader's ClassLoader |
|_______________________________________________________________|
AppClassLoader - Contains the main() class of the container.
Main-Class in manifest points to com.sun.jini.start.ServiceStarter
Classpath: boot.jar, start.jar, jsk-platform.jar, service-api.jar
(mulitple Service API jar's allowed, can be untrusted)
Codebase: none
CommonClassLoader - Contains the common Rio and Jini technology
classes (and other declared common platform JARs) to be made
available to its children.
Classpath: Common JARs such as rio.jar
Codebase: Context dependent. The codebase returned is the codebase of
the specific child CL that is the current context of the request.
(Not Sure about the last sentence please explain?)
Child ClassLoader's - Contains the service specific implementation
classes and client application classes.
Classpath: serviceImpl.jar
Codebase: "serviceX-dl.jar rio-dl.jar jsk-lib-dl.jar"
Evolution of Service API is important and to avoid unmarshalling
errors, due to classes missing from the client's Static Service API,
when a proxy uses a Service API extending one present in the client.
For example, when a proxy contains additional Service API, the client
lacks, it should be loaded into the Proxy's ClassLoader. I think
this is why Service API might exist in downloadable archives, for
safety reasons.
So perhaps as a protection against unmarshalling error's the proxy's
codebase should contain the Service API too, however the Service API
present in the client is given preference due to ClassLoader hierarchy.
Thoughts?
Cheers,
Peter.
