Modular Java - OSGi

Modular Java
An Introduction to OSGi

Craig Walls

Who Am I?

E-mail: craig@habuma.com Blog: http://www.springinaction.com Twitter: @habuma Slides: http://www.slideshare.net/habuma

Who Am I?
Java, Spring, and OSGi fanatic
Principal Consultant with .
Author
XDoclet in Action (Manning)

Spring in Action (Manning)

Modular Java (Pragmatic Bookshelf)


OSGi is...

A modularity speciﬁcation for Java

An intra-VM SOA


OSGi is NOT...

New
Heavyweight
Difficult
Only for Eclipse
EJB all over again
An acronym
Named very cool


Modularity is...
• High cohesion
• Modules are focused in purpose
• Low coupling
• Modules have minimal/no direct dependency on
each other
• Not a new idea...
“A well-defined segmentation of the project effort ensures system modularity.
Each task forms a separate, distinct program module. At implementation time
each module and its inputs and outputs are well-defined, there is no confusion
in the intended interface with other system modules. At checkout time the integrity
of the module is tested independently; there are few scheduling problems in
synchronizing the completion of several tasks before checkout can begin. Finally,
the system is maintained in modular fashion; system errors and deficiencies can
be traced to specific system modules, thus limiting the scope of detailed error
searching.”
Designing Systems Programs, Richard Gauthier and Stephen Pont, 1970.


Modularity promotes...


• Testability


• Testability
• Comprehensibility


• Testability
• Flexibility


• Testability
• Flexibility
• Reusability


• Testability
• Flexibility
• Reusability
• Plugability

Modularity in Java?


Modularity in Java?

• Java lacks some essential features to
support modular development


Modularity in Java?

• Classes encapsulate data and functionality


Modularity in Java?

• They’re too ﬁne-grained for practical modularity


Modularity in Java?


• Packages only contain classes


Modularity in Java?


• They’re only an organizational mechanism


Modularity in Java?


• They’re only an organizational mechanism

• JAR ﬁles only contain packages/classes


JAR Files: A false sense of modularity


Where’s the modularity?


public class MyAwesomeClass {
public void doSomething() {
FooInternal foo = new FooInternal();
// ...
}
}


public class MyAwesomeClass {
public void doSomething() {
FooInternal foo = new FooInternal();
// ...
}
}

/**
* Intended for INTERNAL USE ONLY!!!
* Do not use directly!!!
*/
class FooInternal { ... }

JAR Hell


JAR/Classpath Hell


No boundaries


Building walls around JARs


Delegating classloading


Add a service registry


Publish services


Consume services


Only import service API


Only import service API

OSGi!

The solution...OSGi

• Dynamic Runtime
• Modules can be installed, started, stopped,
updated, and uninstalled...in a live program

• Ends JAR Hell
• Multiple versions of a class can reside in OSGi
simultaneously


OSGi implementations



• Open Source
• Eclipse Equinox (http://www.eclipse.org/equinox)

• Apache Felix (http://felix.apache.org)

• Knopflerfish (http://www.knopflerfish.org)

• Concierge (http://concierge.sourceforge.net/)



• Open Source
• Eclipse Equinox (http://www.eclipse.org/equinox)

• Apache Felix (http://felix.apache.org)

• Knopflerfish (http://www.knopflerfish.org)

• Concierge (http://concierge.sourceforge.net/)

• Commercial
• Makewave Knopflerfish Pro (http://www.makewave.com)

• ProSyst mBedded Server (http://www.prosyst.com)

• Samsung OSGi R4 Solution
• HitachiSoft SuperJ Engine Framework

Bundles


Bundles

• Just JAR ﬁles


Bundles

• Just JAR ﬁles
• Contain special metadata in MANIFEST.MF


Bundles

• Just JAR ﬁles
• All content is private by default
• May export packages to be imported by other
bundles

• May publish services


Bundles

• Just JAR ﬁles
• All content is private by default
• May export packages to be imported by other
bundles

• May publish services
• May be versioned

Bundle Lifecycle


Fragments


Fragments

• Just JAR ﬁles, like bundles, but...


Fragments

• Must be hosted by another bundle


Fragments

• Physically separate, logically united


Fragments

• Physically separate, logically united
• Used to add content (classes, resources,
etc) to a hosting bundle


OSGi Manifest

• Deﬁnes the content of a bundle
Manifest-Version: 1.0
Built-By: wallsc
Created-By: Apache Maven Bundle Plugin
Bundle-Activator: com.osgiknowhow.hello.consumer.internal.HelloConsumerActivator
Import-Package: com.osgiknowhow.hello.service;version="1.0.0.SNAPSHOT",
org.osgi.framework,org.osgi.util.tracker
Bnd-LastModified: 1236686261405
Bundle-Version: 1.0.0.SNAPSHOT
Ignore-Package: com.osgiknowhow.hello.consumer.internal
Bundle-Name: com.osgiknowhow.hello.consumer
Bundle-Description: Generated using Pax-Construct
Build-Jdk: 1.5.0_16
Private-Package: com.osgiknowhow.hello.consumer.internal
Bundle-ManifestVersion: 2
Bundle-SymbolicName: com.osgiknowhow.hello.consumer
Tool: Bnd-0.0.255

• Don’t ever write this ﬁle yourself
• Generate it (see Bnd, Bundlor, etc)

Manifest headers
Bundle-SymbolicName

Bundle-Version

Import-Package

Export-Package

Bundle-Activator


Versioning


Versioning

• Bundles and packages can be versioned
• Can even be versioned independent from each
other


Versioning

other

• Multiple versions can be available
simultaneously


Versioning

other

• Multiple versions can be available
simultaneously

• Packages can be imported by specifying a
speciﬁc version, a version range, or no
version at all (implying an inﬁnite version
range)


Services


Services
• Bundle functionality encapsulated behind
services


Services
services

• Bundles can publish/consume services
• Identiﬁed by their interface(s) and optional
parameters

• Publish programatically using bundle context
• Consume programatically using bundle context
and service tracker


Services
services

• Bundles can publish/consume services
• Identiﬁed by their interface(s) and optional
parameters

• Publish programatically using bundle context
• Consume programatically using bundle context
and service tracker

• Can be published/consumed declaratively
• Declarative Services, iPOJO, Spring-DM

OSGi Tools Disclaimer

• My OSGi toolbox includes
• Maven
• Pax Construct
• BND
• TextMate (and Eclipse)
• You don’t have to use what I use
• There are a lot of options to choose from
• Ant with BND also works ﬁne

Just enough Maven...

• For the most part, you’ll only ever need...
• mvn install
• Occasionally you may need...
• mvn test
• mvn clean


Pax Construct

• Rails/Grails/Roo-ish model
• Scripts backed by Maven archetypes
• Key Pax Construct scripts:
• pax-create-project
• pax-create-bundle
• pax-provision
• pax-import-bundle
• pax-add-repository

BND
• A giant in a small JAR
• Can be used as an Ant task
• Can be used as an Eclipse plugin
• Can be used at the command line
• Automatically infers much of a manifest
• Can be coerced, if necessary
• BND instructions can take wildcards!
• It’s the magic underneath the Felix Maven
Bundle Plugin
• Which is some of the magic underneath Pax
Construct

Example 1

Simple Service/Consumer using a
Bundle Activator

In the beginning (circa 2006)

OSGi Declarative Services

Spring Dynamic Modules

iPOJO


Publishing a Service
In OSGI-INF/blueprint/*.xml:
<?xml version="1.0" encoding="UTF-8"?>

<blueprint xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0"
xmlns:ext="http://geronimo.apache.org/blueprint/xmlns/
blueprint-ext/v1.0.0"
default-activation="lazy">

<bean id="numberService" class=
"com.habuma.numbers.internal.NumberToEnglishServiceImpl" />

<service ref="numberService"
interface="com.habuma.numbers.NumberToEnglishService" />

</blueprint>


Consuming a Service
In OSGI-INF/blueprint/*.xml:
<?xml version="1.0" encoding="UTF-8"?>

<blueprint xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0"
xmlns:ext="http://geronimo.apache.org/blueprint/xmlns/
blueprint-ext/v1.0.0"
default-activation="lazy">

<bean id="numberService"
class="com.habuma.numbers.ui.NumberConverterUI" />

<reference id="numberService"
interface="com.habuma.numbers.NumberToEnglishService" />

</blueprint>


Blueprint vs. Spring-DM


Spring-DM implements Blueprint Services



Spring-DM isn’t the only implementation




If you want/need the power of Spring, then
use Spring-DM




If you want/need the power of Spring, then
use Spring-DM

Otherwise, any implementation of Blueprint
Services will do

Example 2

Service/Consumer using OSGi
Blueprint Services

Levels of Testing

Unit
Integration
User Acceptance
Functional


Levels of Testing

Unit
Integration
Bundle
User Acceptance
Functional


Bundle Testing


Bundle Testing
• Does the bundle export what it’s supposed
to?


Bundle Testing
to?

• Does the bundle publish the services that
it’s supposed to?


Bundle Testing
to?

it’s supposed to?

• Does the bundle’s service(s) behave like
they’re supposed to?


Bundle Testing
to?

it’s supposed to?

• Does the bundle’s service(s) behave like
they’re supposed to?

• Does the bundle get along with other
bundles?

Bundle Testing Options



Spring-DM



Spring-DM

Pax Exam



Spring-DM

Pax Exam

DA-Testing
(dynamicjava.org)


Pax-Exam/Spring-DM Testing


Written as JUnit Test



JUnit 3 for Spring-DM; JUnit 4 for Pax Exam




Test setup identiﬁes one or more bundles and
an OSGi framework to deploy to





Test methods make assertions against
bundles and services





Test methods make assertions against
bundles and services

When run, test becomes an on-the-ﬂy bundle
running within the framework

Example 3

Testing OSGi Services with Pax Exam

Strategy


Strategy

1. Convert full app into one large bundle
a. Embed JARs (Use Bundle-ClassPath)
b. Add Bundle-SymbolicName


Strategy

1. Convert full app into one large bundle
a. Embed JARs (Use Bundle-ClassPath)
b. Add Bundle-SymbolicName

2. Peel off dependencies one-by-one
a. Start with 3rd party libraries
b. Pull out proprietary modules


Converting Big Apps


Dealing with Legacy JARs



• BND



• BND
• Pax Construct:
• pax-wrap-jar or pax-embed-jar



• BND
• Pax Construct:
• pax-wrap-jar or pax-embed-jar
• Find them in a bundle repository
• OBR
• http://www.osgi.org/Repository

• SpringSource Enterprise Bundle Repository
• http://www.springsource.com/repository


Modular Java - OSGi

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