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Java Web User Group
Impala Framework Update
Modular class loaders and OSGi
Phil Zoio
LondonDecember 2, 2008
Why is modularity a good thing?
Static modularity decouples parts of a system
less complexity
easier to deploy in part
easier to configure
easier to test
Dynamic modularity allows partial loads/updates
developer productivity
operational benefits (e.g. hot fixes)
Example – fulfilment system
main
hibernate dao import web
orderprocessing
warehousemanageme
nt
Contains main domain model classes plus interfaces shared by modules
Tools of the trade Service registry
decouples service providers from users
Proxies
allow static references to dynamic services
Dependency injection (Spring)
provides transparent programming model
Class loaders
limits visibility of classes across modules
allows “reloading” by replacing class loaders
Class loader options Single class loader
Hierarchical class loader
Graph-based class loader
OSGi
All
Single class loader All modules share same class loader
main
hibernate dao import web
orderprocessing
warehousemanageme
nt
Single class space – all public classes are visible to all other classes
No runtime enforcement of modularity boundaries
No partial reloading
Hiearchical class loader Visibility inherited from parents
main
hibernate dao import web
orderprocessing
warehousemanageme
nt
Modules can see classes within module plus those visible to parents
Supports reloading of module subtree
e.g. dao and import cannot see each other's classes
Graph-based class loader Visibility inherited from multiple dependencies
main
hibernate dao import web
orderprocessing
warehousemanageme
nt
Unlike hierarchical, allows importing from more than one module
Supports reloading of subgraph
web import
OSGi class loader Versioning and enhanced visibility controls
main
hibernate dao import web
orderprocessing
warehousemanageme
nt
More powerful than graph-based class loader:- Either package imports and exports or explicit module dependencies- Multiple versions of same package- “Private” packages- Visibility not necessarily inherited - Applies to 3rd party and app classes
web imports
Graph relationships are largely implicit
Pros and cons Single class loader
simplest BUT but no enforcement or reloading
Hierarchical
simplest which supports dynamic reloading BUT has limitations
Graph-based
more complex BUT more powerful and flexible than hierarchical
OSGi
most powerful BUT most complex and requires extra runtime support (OSGi container)
Usage scenarios Single class loader
multiple class loaders not an option
quick running test suites
Hierarchical
simpler apps requiring dynamic modularity
Graph-based
complex apps requiring dynamic modularity
OSGi
must address 3rd party library conflicts or dynamic reloading requirements
need dynamic integration of 3rd party modules
Impala support Single class loader
present in current version (1.0 M3)
Hierarchical
present in 1.0 M3
Graph-based
in SNAPSHOT and next version (1.0 M4)
OSGi
standalone support in SNAPSHOT & 1.0 M4
web support planned for 1.0 M5
Why use Impala with OSGi? Eases transition into OSGi
Allows switching between environments
Adds framework for managing application modules
Leverages Spring and Spring DM
Resources
Home: http://impala.googlecode.com/
Blog: http://impalablog.blogspot.com/
Me: philzoio@realsolve.co.uk
Q & A
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