Java Tech & Tools | OSGi Best Practices | Emily Jiang
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The document discusses OSGi best practices for modular application development. It covers: 1) Using Import-Package instead of Require-Bundle to avoid tight coupling between bundles. 2) Avoiding split packages to prevent classpath issues. 3) Versioning bundles and packages to allow compatibility checks. 4) Separating APIs from implementations for flexibility.
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Java Tech & Tools | OSGi Best Practices | Emily Jiang
- 1. OSGi Best Practices Emily Jiang, IBM OSGi Developer, Apache Aries Committer [email_address]
- 2. AGENDA Why OSGi? What is OSGi? How to best use OSGi?
- 3. Jars have no modularization characteristics No “jar scoped” access modifiers.
- 4. No means for a jar to declare its dependencies.
- 5. No versioning. Modularization in Java Jar Package Class Class Class Package Class Class Class Package Class Class Class
- 6. Problems with Global Java ClassPath Java VM log4j barcode4j axis batik commons derby fop ezmorph freemarker httpunit jakarta jcl json jdbm jdom jenks jpos18 jython looks lucene mail mx4j naming jetty poi resolver rome serializer servlets tomcat velocity ws-commons xalan wsdl4j xerces xmlgraphics xmlrpc xmlapis .. geronimo bsh bsf guiapp hhfacility manufact. marketing minerva accounting assetmaint base bi catalina common oagis order ebay content datafile ecommerce entity googlebase ofbiz widget minilang party pos. product workeffort workflow … sunjce_prov. plugin jsse jce rt dnsns .. … Class Not Found Exception Begin Here
- 7. Problems with EARs/WARs Enterprise Applications have isolated classpaths but… No Sharing Common libraries/frameworks in apps and memory Version conflicts webA.war WEB-INF/classes/servletA.class WEB-INF/lib/spring.jar WEB-INF/lib/commons-logging.jar WEB-INF/lib/junit.jar … webB.war WEB-INF/classes/servletB.class WEB-INF/lib/spring.jar WEB-INF/lib/commons-logging.jar WEB-INF/lib/junit.jar … webC.war WEB-INF/classes/servletC.class WEB-INF/lib/spring.jar WEB-INF/lib/commons-logging.jar WEB-INF/lib/junit.jar … plankton.v1 plankton.v2
- 8. AGENDA Why OSGi? What is OSGi? How to best use OSGi?
- 9. What is OSGi? “ The dynamic module system for Java” Mature 10-year old technology
- 10. Governed by OSGi Alliance: http://www.osgi.org
- 11. Used inside just about all Java-based middleware IBM WebSphere, Oracle WebLogic, Red Hat JBoss, Sun GlassFish, Paremus Service Fabric, Eclipse Platform, Apache Geronimo, (non-exhaustive list) http://www.osgi.org/wiki/uploads/News/2008_09_16_worldwide_market.pdf Jar Jar Explicit exports Explicit dependencies Package Class Class Class Class Class Class Package Class Class Class Class Class Class
- 12. OSGi Bundles and Class Loading OSGi Bundle – A jar containing: Classes and resources.
- 13. OSGi Bundle manifest. What’s in the manifest: Bundle-Version: Multiple versions of bundles can live concurrently.
- 14. Import-Package: What packages from other bundles does this bundle depend upon?
- 15. Export-Package: What packages from this bundle are visible and reusable outside of the bundle? Class Loading Each bundle has its own loader.
- 16. No flat or monolithic classpath.
- 17. Class sharing and visibility decided by declarative dependencies, not by class loader hierarchies.
- 18. OSGi framework works out the dependencies including versions. Manifest-Version: 1.0 Bundle-ManifestVersion: 2 Bundle-Name: MyService bundle Bundle-SymbolicName: com.sample.myservice Bundle-Version : 1.0.0 Bundle-Activator : com.sample.myservice.Activator Import-Package: com.something.i.need;version="1.1.2" Export-Package: com.myservice.api;version="1.0.0" Bundle
- 19. OSGi Bundle Bundles have a dynamic lifecycle Can come and go independently Bundle
- 20. OSGi Service Registry Service An object associated with a list of classes (usually interfaces) it provides
- 21. Dynamic (can come and go), framed by bundle lifecycle
- 22. Services are the primary means of collaboration between bundles. S
- 23. AGENDA Why OSGi? What is OSGi? How to best use OSGi?
- 24. BP1 - Use Import-Package not Require-Bundle Require-Bundle Tightly coupled with a particular bundle with the specified symbolic name and version
- 25. High coupling between bundles
- 27. Bundle version management Import-Package Can wire to any bundles exporting the specified package
- 28. Loose coupling between bundles
- 29. Only import the package you need
- 30. Package version management MANIFEST.MF ... Require-Bundle: com.ibm.ws.service;bundle-version=2.0.0 MANIFEST.MF ... Import-Package: com.ibm.ws.service.api;version=2.0.0
- 31. BP2 - Avoid split packages Split package A package is exported by two bundles at the same version and the set of classes provided by each bundle differs. Why? Leads to the use of Require-Bundle, compromising the extent to which systems using the bundles can be extended and maintained. How? Keep all of the classes from any one package in a single bundle
- 32. BP2 - Split Package examples API B Implementation C org.hal.a 1.0.0 Bundle C has to use 'Require-Bundle' to ensure that it has classes from both parts of the API Figure 1. Consumers of split packages need to use the Require-Bundle header API A org.hal .api 1.0.0 org.hal.a pi 1.0.0
- 33. BP2 - Split Package examples API A Implementation C org.hal.a 1.0.0 Figure 2. A complete package exported from a single bundle maintains high bundle cohesion org.hal.api 1.0.0
- 34. BP3 - Version bundles and packages What is semantic versioning? Uses a major.minor.micro.qualifier numbering scheme Major - Packages with versions that have different major parts are not compatible both for providers as well as consumers.
- 35. Minor – API enhancement, e.g. adding methods to an API
- 36. Micro – bug fixing
- 37. Qualifier – identifier such as timestamp
- 38. Example: 1.1.2.20101122am, 1.0.0, 2.0 Changes in major: a binary incompatible, minor: enhanced API, micro: no API changes Why? Clients can protect themselves against API changes that might break them.
- 39. BP3 - Version bundles and packages examples Figure 3: A client and an implementation can use either of two equivalently versioned packages Figure 4: How a client and implementation are affected differently by a minor API version change Implementation A Imports: org.hal.a [1.0, 1.1) API A org.hal.a 1.0.0 API B org.hal.a 1.0.0 Client A Imports: org.hal.a [1.0, 2.0) API B org.hal.a 1.1.0 API A org.hal.a 1.0.0 Client A Imports: org.hal.a [1.0, 2.0) Client B Imports: org.hal.a [1.1, 2.0) Implementation B Imports: org.hal.a [1.1, 1.2) Implementation A Imports: org.hal.a [1.0, 1.1)
- 40. BP3 - Version bundles and packages examples Figure 5. How clients and implementations are similarly affected by a major API version change API A org.hal.a 1.0.0 org.hal.a 2.0.0 API B Client A Imports: org.hal.a [1.0, 2.0) Client B Imports: org.hal.a [2.0, 3.0) Implementation B Imports: org.hal.a [2.0, 2.1) Implementation A Imports: org.hal.a [1.0, 1.1)
- 41. BP4 - Separate API from Implementations Why? Great flexibility
- 42. Many implementation bundles -> enable more services provided
- 43. Reduce package dependencies -> reduce circular dependencies How? Put API classes in one bundle
- 44. Put implementation classes in a separate bundle
- 45. BP4 - Separate API and implementation examples Figure 6. Badly designed provider bundle where the API and implementation classes are in the same bundle API + Implementation org.hal.myapi org.hal.a.impl Both API and implementation packages imported by client Client org.hal.b.client
- 46. BP5 - Share services not implementations Use the OSGi service registry to construct instances
- 47. Why? Able to obtain an instance of an implementation without knowing which one
- 48. Achieve a loosely coupling of client, API and implementation How? Register an instance of the API interface in the OSGi service registry
- 49. Register an implementation of the OSGi ServiceFactory interface in the OSGi service registry.
- 50. BP4 & 5 - examples org.hal.myapi API and implementation packages in different bundles but still imported by client Implementation org.hal.a.impl Figure 7. Badly designed provider bundles where the API and implementation classes have been separated API Client org.hal.b.client
- 51. BP4 & 5 - examples Figure 8: Well designed provider bundles where the API and implementation classes have been separated API org.hal.myapi Client org.hal.b.client Client and implementation in separate bundles, both import API. Client uses implementation through a service defined by the API. Implementation org.hal.a.impl
- 52. Using services can be hard! @Override public void serviceChanged(ServiceEvent event) { ServiceReference ref = event.getServiceReference(); if ( ls .get() == null && event.getType() == ServiceEvent. REGISTERED ) { ls .set((LogService) ctx .getService(ref)); } else if ( ls .get() != null && event.getType() == ServiceEvent. UNREGISTERING && ref == lr .get()) { ref = ctx .getServiceReference(LogService. class .getName()); if (ref != null ) { ls .set((LogService) ctx .getService(ref)); lr .set(ref); } } } private BundleContext ctx ; private AtomicReference<LogService> ls = new AtomicReference<LogService>(); private AtomicReference<ServiceReference> lr = new AtomicReference<ServiceReference>(); public void start(BundleContext ctx) throws InvalidSyntaxException { this . ctx = ctx; ctx.addServiceListener( this , "(objectClass=org.osgi.service.log.LogService)" ); ServiceReference ref = ctx.getServiceReference(LogService. class .getName()); if (ref != null ) { ls .set((LogService) ctx.getService(ref)); lr .set(ref); } }
- 53. BP6 - Use Blueprint Specifies a Dependency Injection container, standardizing established Spring conventions
- 54. Configuration and dependencies declared in XML “module blueprint”, which is a standardization of Spring “application context” XML. Extended for OSGi: publishes and consumes components as OSGi services Simplifies unit test outside either Java EE or OSGi r/t.
- 55. The Blueprint DI container can be a part of the server runtime (compared to the Spring container which is part of the application.) dependencies injected publishes service consumes service A static assembly and configuration of components (POJOs) Blueprint bundle OSGI-INF/blueprint/ blueprint.xml
- 56. BP6 - Blueprint service-bundle examples public interface BillingService { void bill(Order o); } Billing < blueprint > < service ref = ”service” interface = ”org.example.bill.BillingService” /> < bean id = ”service” scope = ”prototype” class = ”org.example.bill.impl.BillingServiceImpl” /> </ blueprint > Billing service bundle “ prototype” scope indicates a new instance is created by the container for each use.
- 57. “ singleton” scope is the default.
- 58. BP6- Blueprint client-bundle examples public class ShopImpl { private BillingService billingService ; void setBillingService(BillingService srv) { billingService = srv; } void process(Order o) { billingService .bill(o); } } e-Commerce < blueprint > < bean id = ”shop” class = ”org.example.ecomm.ShopImpl” > < property name = ”billingService” ref = ”billingService” /> </ bean > < reference id = ”billingService” interface = ”org.example.bill.BillingService” /> </ blueprint > e-Commerce bundle injected service reference
- 59. service can change over time
- 60. can be temporarily absent without the bundle caring
- 61. managed by Blueprint container
- 62. BP7 – Make Bundles Loosely Coupled & Highly Cohesive 'Hairball' effect – one bundle has many package dependencies org.hal.log.impl Implementation org.hal.log.impl SomeotherAPI DBAPI TransactionsAPI FileSystemAPI JPAAPI org.hal.fslog.impl Figure 9. Poorly purposed system where a single bundle provides multiple implementations of the same API API org.hal.log.api
- 63. BP7 – Make Bundles Loosely Coupled & Highly Cohesive Implementation org.hal.DBlog.impl DBAPI API org.hal.log.api Implementation org.hal.fslog.impl FileSystemAPI Implementation org.hal.DBlog.impl DBAPI Implementation org.hal.DBlog.impl DBAPI Figure 10. A well purposed system where each implementation of an API is provided by a separate bundle
- 64. BP8 – Use bundle repositories
- 65. Summary – OSGi Best Practices BP1 - Use Import-Package instead of Require-Bundle
- 66. BP2 - Avoid split packages
- 67. BP3 - Version bundles and packages
- 68. BP4 - Separate API from implementations
- 69. BP5 - Share services not implementations
- 70. BP6 - Use Blueprint
- 71. BP7 - Make bundles loosely coupled & highly cohesive
- 72. BP8 - Use Bundle Repositories
- 73. Quiz – What best practices are used? Blogging Service Blog Persistence Service blog-servlet Web application bundle META-INF/ persistence.xml WEB-INF/ web.xml OSGI-INF/blueprint/ blueprint.xml OSGI-INF/blueprint/ blueprint.xml JNDI EM blog blog-persistence blog-api
- 74. Questions?
- 75. Additional References OSGi Best Practices www.ibm.com/developerworks/websphere/techjournal/1007_charters/1007_charters.html Enterprise OSGi YouTube Channel: www.youtube.com/user/EnterpriseOSGi
- 76. Copyright and Trademarks © IBM Corporation 2011. All Rights Reserved. IBM, the IBM logo, and ibm.com are trademarks or registered trademarks of International Business Machines Corp., registered in many jurisdictions worldwide. Other product and service names might be trademarks of IBM or other companies. A current list of IBM trademarks is available on the Web at “Copyright and trademark information” at www.ibm.com/legal/copytrade.shtml.
Editor's Notes
- #11: WSAD512_IntroToPortal.ppt Page of 32
- #24: .
- #27: On the provider side, the BillingServiceImpl is another POJO implementing a Java interface for which a service is registered by the Blueprint container when the Billing bundle is started. By default beans are created with “singleton” scope which means only a single instance is created by the container. If the bean maintains any state then it can be declared with “prototype” scope so that the container creates a new instance each time it needs to inject the dependency into a client.
- #28: Here is a simple example of an eCommerce bundle which contains a “shop” bean which uses a BillingService provided by another bundle. The Blueprint container locates the Billing Service provider and injects it into the shop bean at runtime. OSGi services are dynamic so if the service provider can be changed then the blueprint container is able to dynamically rewire the reference to a new service without impacting the shop bean instance.
- #33: What best practices used? BP4 - Separate API from implementations BP5 - Share services not implementations BP6 – Make bundles loosely coupled & highly cohesive BP7 - Use Blueprint If you show the source code, the following three best practices can be included: BP1 - Use Import-Package instead of Require-Bundle BP2 - Avoid split packages BP3 - Version bundles and packages