java ee | modular ejbs for enterprise osgi | tim ward

PowerPoint-Prsentation

Modular EJBs in OSGi

Tim Ward

IBM

21 Sep 2011

OSGi Alliance Marketing 2008-2010 . All Rights Reserved

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Agenda

A Quick EJB refresher

How EJBs might fit into OSGi

A case study in implementing Modular EJB

Proof that it really works

Terminology

EJB Enterprise Java Bean

EJB JAR A JAR packaging EJBs

Modular EJB An EJB running in OSGi

EJB Bundle An OSGi bundle packaging Modular EJBs

EJBs the basics

EJBs are managed objects, the container injects their dependencies

Session EJBs define one or more business views

These proxy the real EJB object(s)

The same view object may delegate to the same, or a different, EJB for successive calls

Different EJB types have different delegation styles

The EJB runtime adds declarative transactions, security and other services when a business method is called

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EJB Views and OSGi services

EJB ViewOSGi Service

CardinalityMay proxy one or many EJB objectsOne per registration or one per client bundle

LocationStored in JNDIStored in the OSGi service registry

InterfaceOne business interface per EJB viewOne service may expose many interfaces

LifecycleRelatively static once created, no reinjection, no notifications from JNDIDynamic, may be removed or modified and underlying dependencies may change

An EJB view shares a number of concepts with an OSGi service.

Integrating EJBs with OSGi

The service registry is the integration point in OSGi

Expose modular EJBs as OSGi services

Register one service per EJB view

Remote EJB views should be Remoteable Servicesservice.exported.interfaces = *

EJB services only work with the right lookup lifecycle

Stateless are an interchangable pool

Singleton is like a normal service

Stateful EJBs are one per lookup

Identifying EJB Bundles in OSGi

Requirement

EJB Bundles should be able to be valid EJB JARs

Fit with existing OSGi module types (e.g. WABs)

ProposalAdd a new header Export-EJB:

Identifies a bundle as an EJB-Bundle

Defines which EJBs are exposed as OSGi services

Pioneered by Glassfish Application ServerKnown to work good basis for a standard?

Running Modular EJBs in Aries

Apache Aries provides pieces of an OSGi container

Integrate with existing projects where possible

OpenEJB has been packaged as an OSGi bundle for a couple of releases

Some tentative OSGi support, little true integration

Mission StatementIntegrate OpenEJB with existing OSGi standards and Aries features to provide support for Modular EJBs

Issues Running EJBs in OSGi

Locating EJBs in OSGi bundles

OSGi class loading semantics

Building EJB proxy stubs

Transaction Manager integration

JPA integration

Security integration

Miscellaneous

Working with EJB Bundles
Finding the EJBs

EJBs are defined in one of two ways

via annotations @Stateless

via XML ...

Requirement 1 says that EJB Bundles should put XML in META-INF/ejb-jar.xml (just like Java EE)

Finding the XML is non-trivial for OpenEJB in OSGi

Use the extender pattern to help OpenEJB out

Recognise bundles using Export-EJB header

Working with EJB Bundles
Finding the EJBs (2)

Finding Annotated EJBs much harder than for XML

Typically a OpenEJB scans the classpath by listing files on the file system (using file: or jar: URLs)

In OSGi there is no guarantee of the bundle being on the filesystem (or in its original layout)

Typical scanning breaks at this point, so either:Try to scan the raw bundle bytes (if they exist!)No fragments, imports or bundle classpath

Implement an OSGi aware scanner

Apache Aries
Locating META-INF/ejb-jar.xml

OpenEJB allows us to build our own EJBModule representing the EJB Bundle

An EJBModule allows us to supply a URL to the EJB XML deployment descriptor

This is parsed and processed by OpenEJB

Aries makes all .xml files in META-INF available

Covers other Java EE specs

Covers OpenEJB config files

Apache Aries
Writing an OSGi aware scanner

In OSGi 4.3 a new core API method was added

BundleWiring.listResources(String, String, int)

We can use this to build an OSGi specific Xbean scanner for our EJBModule

for(String resource : bundle.adapt(BundleWiring.class). listResources("/", "*.class", LISTRESOURCES_RECURSE)) { URL u = bundle.getResource(resource) readClassDef(u.openStream()); }






JPA integration


Miscellaneous

Java EE vs OSGi class loading

One of the key differences between OSGi and Java EE is how they load classes

Java EE has a hierarchy of ClassLoader instances

EJB JAR Application EJB Container Java

OSGi has a ClassLoader network...

EJB BundleEJB APILoggerBusinessAPI????

Apache Aries
Classloading for EJB Bundles

Clearly the OpenEJB EJBModule ClassLoader should be the EJB Bundle ClassLoader

OpenEJB relies on the fact that its internals are visible from the EJB JAR ClassLoader

No requirement for EJB Bundles to import OpenEJB

Make OpenEJB visible from Application ClassLoader

EJB BundleEJB APIBusinessAPI

EJB JAR ClassLoaderOpen EJB

Application ClassLoader

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JPA integration


Miscellaneous

Implementing EJBs in OSGi

EJBs are usually implemented using proxy stubs

Implement a single business interface

May implement other container specific interfaces

Often implement javax.naming.Referencable

Stubs may be dynamic proxys or generated classes

In either case they must be loaded as classes

In OSGi each bundle has a different ClassLoader

There may not be any one bundle that can see all the interfaces on the proxy!

Apache Aries
OSGi safe proxy classes

Aries contains an OSGi aware proxy implementation

Supports dynamic interface implementation generation for one to N interfaces

The proxy allows a parent bundle to be specified

The proxy understands that not all interfaces may be visible to the bundle!

Aries replaces the default OpenEJB Proxy factory

EJB stubs can use any mixture of interfaces

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JPA integration


Miscellaneous

EJBs and Transactions

EJBs have an extremely strong link with transactions

All invocations use a global transaction by default

More complex interactions can be configured

EJBs can control their own transactions too

In OSGi we use the JTA Service to get hold of a TransactionManager

OpenEJB knows nothing about this...

Tx Client

Apache Aries
JTA integration

Aries contains a JTA Service implementation that uses Geronimo's transaction manager

Also provides a TransactionSynchronizationRegistry

Aries overrides the OpenEJB transaction manager

Use the JTA Service

Provide the Tx Manager and Tx Registry

This is a clean and well used plug point






JPA integration


Miscellaneous

EJBs and JPA

JPA replaced Entity Beans as the persistence strategy for Java EE

EJBs have tight integration with JPA

Injection via Annotations @PersistenceUnit

Injection via XML ...

JNDI lookup in java:comp/env

EJBs may use JPA in two ways

Application Managed

Container Managed

EJBs and JPA Application Managed

In Application managed JPA the EJB manages lifecycle

Responsible for creating and closing EntityManagers

Responsible for joining any active transactions

Adds a dependency on a named persistence unit

Injects or looks up an EntityManagerFactory

OpenEJB expects to find, create, and manage any persistence units in persistence.xml

ClassLoader problems make this impossible in OSGi

Apache Aries
Updates to Aries JPA container

Aries JPA normally uses the Meta-Persistence header to locate persistence bundles in the framework

Java EE also defines rules for finding persistence.xml

WARs in WEB-INF/classes, or in WEB-INF/lib

EJB JARs in META-INF

Aries JPA already checks for Web-ContextPath

Add support for the Export-EJB header too

Apache Aries
JPA (Application Managed)

Hide META-INF/persistence.xml from OpenEJB

Don't put the URL in the EJBModule

Override the OpenEJB validation failure, Aries JPA will provide the missing EntityManagerFactory!

Listen for the registration of OSGi persistence units

If the unit is used by an EJB then bind it into the right place in java:comp/env

EJBjava:comp

Extender

EJBs and JPA Container Managed

In Container managed JPA the container manages everything!

Tx integration

Creating and closing EntityManagers

More importantly, the container propagates context

Different EJBs that use the same persistence unit in a transaction will get the same EntityManager

Aries JPA already supports this mode of operation for blueprint beans and OSGi service lookups

Apache Aries
JPA (Container Managed)

Replace the existing OpenEJB JPA context registry

Check for Aries JPA contexts and OpenEJB contexts

Cross register any created contexts so both agree!

Listen for the registration of OSGi persistence units

If the unit is used as a managed context in an EJB then create an OpenEJB managed EntityManagerRegister this EntityManager in the relevant part of java:comp/env

EJBjava:comp

Extender






JPA integration


Miscellaneous

Java EE Security

Java EE supports role-based authorization

On Servlet methods

On EJB method calls

OpenEJB provides a Security Service plugpoint

Allows third party authentication/authorization engines to be used

Aries has no security component - any volunteers?

OpenEJB can cope without a Security Service

No integration at this time






JPA integration


Miscellaneous

Issues with OpenEJB in OSGi

OpenEJB makes extensive use of XBean internally to build things

This has the option of providing a ClassLoaderOpenEJB typically provides none

OpenEjbVersion throws an ExceptionInInitializerError

Attempts to classpath scan for properties

To work around these Aries has to extensively set the Thread Context ClassLoader when starting OpenEJB

New JAXB code in OpenEJB needs delegating to a 2.1 JAXB implementation every time we build an app

Summary

There are a few rough edges

Some can easily be remedied in OpenEJB internals

Some support is clearly missing, but could be added

Security, Messaging, EJB lite

Broadly speaking, it works

And I can prove it!Apache Aries Blog sample with an EJB implemented comment service!

References

Apache Aries: http://aries.apache.org/

Tim Ward: @TimothyWard [email protected]

OSGi and JPA on YouTube: http://www.youtube.com/user/EnterpriseOSGi

For more information on Enterprise OSGi take a look at Enterprise OSGi in Action : http://www.manning.com/cummins

01.11.11

OSGi Alliance Marketing 2008-2011 . All Rights Reserved,

IBM Corp. 2011

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java ee | modular ejbs for enterprise osgi | tim ward

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