The Java EE Servlet model provides the backbone of Web based applications written in Java. With the release of the OSGi enterprise there is a Web application specification which provides a seamless experience for deploying existing and new Web applications to Servlet containers operating on the OSGi service platform as well as means of taking a traditional Web archive and componentizing it into several classic bundles.

1. Public
Migration of existing web
applications to OSGi
ESE 2010
Diyan Yordanov
SAP Labs Bulgaria

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Agenda
1. OSGi Web Container
2. Migrating Existing Web Applications to OSGi
3. Challenges That We Overcome
4. Conclusions

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Why web applications on OSGi?
Last years trends of software development
A good choice for product evolving
Encapsulated modules
Versioning
Class space isolation
Declarative dependencies
Dynamic refreshing and updating
Ongoing work to make OSGi platform more appealing to the enterprise
world
Eclipse Gemini – Enterprise Modules project
Eclipse Virgo – Dynamic Enterprise Application Platform project

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OSGi Web Container
Based on Web Application Specification part of OSGi Enterprise
Specification
Provides support for Web Applications written to
Servlet 2.5 or later
JSP 2.1 or later
Java EE OSGi
WAR

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Available OSGi specific features
Main Features in OSGi Web Container
Installation of WAR files
Installation of Web Application Bundles (WABs)
Control of configuration properties using URL parameters
Multiple applications with single context path
Web application lifecycle managed by OSGi bundle lifecycle
Facilitates interoperability between web application with other OSGi
components
Modularization – one big blob WAR to be broken into smaller modules
Dynamic deployment, and fast update of individual modules

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Agenda
1. OSGi Web Container
2. Migrating Existing Web Applications to OSGi
3. Challenges That We Overcome
4. Conclusions

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Migrating Web Applications to OSGi
• deploying standard WAR
• removing JAR files from WAR and declaring dependencies on
corresponding OSGi bundles
• separating web artifacts from services they depend on
Web Application
WAR
Services
CatalogService
PaymentService
Libraries
Resources
Images CSS
Spring
Hibernate
Services
CatalogService
PaymentService
How to modify the Web application in order to benefit from OSGi?

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Agenda
1. OSGi Web Container
2. Migrating Existing Web Applications to OSGi
3. Challenges That We Overcome
4. Conclusions

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Certain APIs and JARs are missing
Web application assumes that Java EE environment provides a certain API
or JARs in the classpath
In OSGi environment:
Manifest – single management point for the bundle
Export-Package
Import-Package
Require-Bundle
Do not rely on automated add for standard Java EE packages
Manual scan and update import packages for required ones
Or use tools like BND, Eclipse PDE, Bundlor STS
Be careful with
org.osgi.framework.bootdelagation
osgi.compatibility.bootdelagation (Eclipse Equinox specific)

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Support for context.xml
Context.xml is widely adopted by web applications used to run on Tomcat
Support for web application context configuration was contributed to
Gemini Web
The configuration files are located as follow:
 $SERVER_HOME/config/context.xml is the default context configuration file for all
web applications.
 $SERVER_HOME/config/[enginename]/[hostname] directory contains:
o The default context configuration for all web applications of a given virtual host.
o Individual web applications’ context configuration files.
[enginename] is the name of Tomcat’s engine (“Catalina” by default) and [hostname]
names a virtual host, both of which are configured in tomcat-server.xml.
Note that the following context configuration features are not supported in Gemini Web:
• Custom class loaders.
• Specifying the context path. This is specified using the Web-ContextPath header in web
application’s MANIFEST.MF file.
• Specifying the document base directory.

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WAR extraction
Web application assume it’s extracted on the file system
widely use file system operations
search for resources, configurations, customizations
WAB is installed as bundle
web application is run from the archive
file system operations do not work as expected
Problem
Application cannot access its configurations
Valid for web apps like Orbeon Forms, Jira,
or Confluence wiki
Solution
Implemented support for deploying web application
in exploded form from a file system directory,
following the format of the WAR file

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Class and resource loading problems – Part I
Known problems for standard bundles are still valid for WAB
java.lang.ClassNotFoundException
Because of:
 Missing import or export package
 Import version mismatch
 Forgotten/missing component (really missing!)
 Wrong boot class delegation property
 Missing file system permissions (can’t read the JARs)
java.lang.NoClassDefFoundError
Because of:
 Exception in static block
 Missing imported class (ClassNotFoundException for it)
 Class version mismatch
Class definition is
found, but
instantiation failed!
Class definition
is missing!

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Class and resource loading problems – Part II
java.lang.ClassCastException
Because of:
 Additional copies (besides the system bundle) of the OSGi framework classes
 Undesired additional JARs in classpath
 Clash between RT.jar and application libs(previously used embedded jars)
 JavaSE 5 vs.Java SE 6 does make a difference… (a bunch of new packages
added (StAX, JDBC 4.0, JAXB 2.0, etc.)

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Class and resource loading problems – Part III
Classloader resource assumptions
Access resources as InputStream via the getResourceAsStream()
Access resource using URL object:
URL resourceURL = this.getClass().getResource(“agenda.txt”);
File resourceFile = new File(resourceURL.toURI());
The result is:
java.lang.IllegalArgumentException: URI scheme is not "file"
Because the URL for the resource is returned like this
bundleresource://46.fwk30983464/summit/agenda.txt
This is a limitation for Gemini Web, but is work around in Virgo
(see bug 325578).

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Cluster Features
Some applications may use setups where some cluster functionality is
needed
If Tomcat’s “cluster-howto” doc is used the result is:
java.lang.ClassNotFoundException: org.apache.catalina.ha.tcp.SimpleTcpCluster
at
org.eclipse.gemini.web.tomcat.internal.loading.ChainedClassLoader.doLoadClass(Cha
inedClassLoader.java:174)
at
org.eclipse.gemini.web.tomcat.internal.loading.ChainedClassLoader.loadClass(Chain
edClassLoader.java:157)
at
org.apache.tomcat.util.digester.ObjectCreateRule.begin(ObjectCreateRule.java:205)
Solution was provided in MS4 if Gemini Web based on fragments and:
Standard Apache Tomcat Cluster configuration is supported
Disabled by default but can be enabled when needed

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Automatic Tools
There is NO tool for migrating standard Java EE web application (WAR) to
Web Application Bundle (WAB)
 Eclipse PDE – can help with the MANIFEST.MF file
 SpringSource Tool Suite
Different tools for creating MANIFEST.MF
 BND tool
 Apache Felix Maven plugin
 Bundlor
OSGi Enterprise Tools (Eclipse proposal)
 Wizard for creating WAB
 Tools for converting existing Java EE Dynamic Web projects to WAB project
 Tools for editing the OSGi metadata – manifest headers
 Generic OSGi server adapter

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Agenda
1. OSGi Web Container
2. Migrating Existing Web Applications to OSGi
3. Challenges That We Overcome
4. Conclusions

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Takeaways
Learning curve
Know your application
Assumption for unique context root
Assumption for availability of default Java EE
lib jars
Know OSGi
How classloaders work
Classloader “hell”
ClassNotFoundException, NoClassDefFoundError, ClassCastException
Avoid Class.forName()
Know your wirings
Make sure that you understand the setup of your OSGi environment

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Conclusions
Web on OSGi does work.
Just don’t stumble over small defects.
Working together with the community is the key for success.

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Diyan Yordanov
diyan.yordanov@sap.com
Thank you!