This document discusses modularity improvements in OSGi Framework R3, RFC 70, and RFC 79. It defines modularity as isolation between system components that can be independently examined, modified, and maintained. OSGi aims to provide modularity through class loading boundaries and package sharing between bundles. The examples show how different versions of packages can be exported and imported between bundles to satisfy various dependency and compatibility needs.

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OSGi Framework Modularity Improvements
Glyn Normington, IBM UK Limited

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Contents
• OSGi Framework
• What is Modularity?
• Requirements and Use Cases
• Evolution: OSGi R3, RFC 70, RFC 79
• Simple Examples

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OSGi Framework
Hardware
Driver Driver Driver
Operating System
Java
OSGi
Fram
ew
ork
Bundle

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What is Modularity?
• “(Desirable) property of a system, such that individual
components can be examined, modified and maintained
independently of the remainder of the system. Objective
is that changes in one part of a system should not lead to
unexpected behavior in other parts.”
www.maths.bath.ac.uk/~jap/MATH0015/glossary.html
• For the purposes of this discussion we are focusing on
the Java execution environment
• Class loading is the primary tool to enforce modularity
– This provides for isolation between modules except for defined
interactions
– Customer class loaders not easy for application developers

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Modularity Requirements
• Static analysis
– e.g. reasoning about how a module will behave when
combined with other modules
• Intra-module static optimisations
– based on strictly-enforced module boundaries
– e.g. a class private to a module and with no
subclasses may be treated as final
• Existing code to be modularised without change
– e.g. no need to change the package of a class

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Use Case: Module Privacy
Module A
Module B
Module
Class
Key:
X
X
Both module A and
module B need their
own, distinct versions
of a class named X.

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Use Case: Compatible Version Sharing
Module C v1.1
Module BModule A
v1 v1.1
Note that a simple case of
this scenario is where
module A and module B
require the same version
of module C
Module
Class
Key:

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Use Case: Incompatible Version
Separation
Module C v1 Module C v2
Module BModule A
v1 v2
Module
Class
Key:

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OSGi Release 3
• Modularity features
– Java package sharing between bundles
• One bundle “exports”; other bundles “import”
– Anonymous sharing
– Single version of package may be shared
– Simple version constraint (>=)
• Generally suitable for embedded devices with
newly developed bundles

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RFC 70
• Used by Eclipse 3.0
– RFC 70 is a post-R3 design effort to support modularity concepts of
Eclipse plug-ins
– Eclipse 3.0 includes an OSGi framework which implements OSGi
R3 spec plus RFC 70 design
• New modularity features
– Grouped Java package sharing between bundles
• Multiple bundles “provide”; other bundles “require”
– Named sharing
– Multiple versions of a package may be shared
– Version range constraints using interval notation e.g. [minv, maxv]
• Suitable for larger systems with legacy code and
implementation dependencies

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• Opposite extremes of package sharing
– R3: loose coupling to shared exported version
– RFC 70: tight coupling to particular bundle
• Syntax/semantics not unified
• Some interesting intermediate cases are
not addressed
– e.g. loose coupling to matching exported
version
Release 3 + RFC 70 Sufficient?

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RFC 79 – Work in Progress
• Modularity features of RFC 79
– Unified metadata encompassing R3 and RFC
70 needs as well as broader requirements
– Targeted for OSGi Release 4
• Prototyping in Eclipse Technology Project

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R3 Interface Sharing Example
module A
export javax.servlet 2.1.0
import javax.servlet 2.1.0
module B
export javax.servlet 2.2.0
import javax.servlet 2.2.0
module C
import javax.servlet 2.1.0

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Implementation Sharing Example
module A
export org.foo 1.0
module B
export org.foo 2.0
module C
import org.foo [1.0, 1.0]
module D
import org.foo 2.0

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Attribute Matching Example
module A
export org.foo myAttr=x
module B
export org.foo myAttr=y
module C
import org.foo myAttr=x