OSGi is a module system for Java that handles lifecycles of individual bundles, provides class loading isolation between bundles, and manages dependencies and versioning. It treats each deployable unit as a bundle, which is a JAR file with additional metadata. OSGi allows restricting visibility of classes on a per-bundle basis and loading classes from different bundles through separate classloaders. Bundles in OSGi go through lifecycle states like installation, resolution, starting, stopping, and updating. OSGi also supports a service registry for bundles to publish and discover services. Popular implementations of OSGi include Apache Felix and Eclipse Equinox.

1. OSGi - Basics
Runtime modular Java
Lifecycle handling of
individual bundles
Class-loading
Versioning
Depency management
Been around since 1999
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2. Java class visibility
jar (A) jar (B)
com.rl.log.api com.rl.app
com.rl.log.impl
com.rl.util
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3. OSGi visibility
bundle (A) Bundle-Version: 1.2.3 bundle (B) Import-Package:
Export: com.rl.api;v=1.2.3 com.rl.api;v=[1,2)
com.rl.log.api com.rl.app
com.rl.log.impl
com.rl.util
OSGi adds restrictions on a module-level basis to each deployable unit (bundle).
A bundle however is nothing more than a JAR with added meta-data in MANIFEST.MF
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4. Traditional vs OSGi classloading
Traditional (extremely simplified...) OSGi
ClassLoader
ClassLoader A ClassLoader A C.1
(Loads class A) (Loads class A) (Loads class
Sees: A, C.1)
Sees: C.1,
Sees: A (C.1) (D)
ClassLoader D
ClassLoader B
(Loads class
(Loads class B)
D)
Sees: A,B Sees: D
ClassLoader
ClassLoader X ClassLoader Y ClassLoader B C.2
(Loads class X) (Loads class Y) (Loads class B) (Loads class
C.2)
Sees: A,B,X Sees: B, Sees: C.2,
Sees: A,B,Y
(C.2) (D)
* - Imported bundles classes are actually
loaded from delegated classloader
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5. Bundle lifecycle
osgi:install
deploys bundle
osgi:start
osgi:stop
osgi:update
uninstall → install →
resolve → restart
osgi:refresh
rewire package dep.
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6. Services
get() register()
Consumer Service Implementer
Bundle Bundle
listen()
Listener
Bundle
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7. Implementations
Apache Felix (ASF)
ServiceMix / Karaf
Equinox (Eclipse)
Knoplerfish
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8. The ServiceMix runtime
ServiceMix / Fuse ESB
CXF Camel ActiveMQ
Karaf
Shell Security Provisioning Karaf features
Logging ConfigAdmin Deploy
Spring DM Dependency injection
Blueprint
(Spring OSGi (optional)
(Apache Aries)
ext)
Equinox Felix OSGi container
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