Web applications and web services - OSGi in Action - page 492

OSGi and JNDI
Retrieving the OSGi bundle context from the ServletContext is the most direct way
to interact with the OSGi environment. Many existing servlets use JNDI to discover
Java EE services—wouldn't it be great if a bridge existed between these two worlds?
Such a bridge does exist in the R4.2 Enterprise specification, so rest assured that
you can use this mechanism to access services. The technical details of how this
interaction works are beyond the scope of this topic; refer to the specification for
more information ( www.osgi.org/Download/Release4V42 ).

15.1.3 Standard WARs: the Web URL Handler

As a convenience for users who wish to migrate web applications to OSG i but don't wish
to undertake the effort of converting a WAR file to a WAB , the Web Applications speci-
fication provides a utility mechanism to convert a WAR file to a WAB at execution time:
the Web URL Handler. It uses the OSG i URL Handlers Service to turn WAR s into WAB s.

URL Handlers Service specification
The OSGi URL Handlers Service specification provides a service-based approach for
bundles to offer custom stream and content handlers associated with URLs. The normal
approach for dealing with stream and content handlers in Java is to set a URLStream-
HandlerFactory and/or a ContentHandlerFactory on URL and URLConnection , re-
spectively. Unfortunately, these are singletons, so they can't be shared and they aren't
dynamic. The URL Handlers specification addresses both of these issues.

The URL Handlers Service works by setting the URLStreamHandlerFactory and
ContentHandlerFactory objects once; it then uses the whiteboard pattern to dis-
cover services offered by bundles implementing custom stream and content han-
dlers. When the URL Handlers service receives a request for a specific protocol or
content type, it delegates the request to the appropriate underlying service to perform
the processing.

To u s e t h e We b URL Handler, all you need to do is prefix any existing URL pointing to
a WAR file with the webbundle protocol when installing the WAR file into the frame-
work. For example, you could use your shell's install command like this:

install webbundle:http://www.acme.com/acme.war?Bundle-SymoblicName=
com.example&Web-ContextPath=acme
The Web URL Handler converts the referenced WAR file into a WAB on the fly prior to
the OSG i framework installing. The Web URL Handler makes a best-effort attempt to
convert a WAR to a WAB , but in certain circumstances you may have to give it extra
hints to help the process go smoothly. In this example above, you specify a Bundle-
SymbolicName as a parameter in the query portion of the URL . The Web URL Handler
also supports a number of other parameters that affect the outcome of the conver-
sion; these parameters are listed in table 15.2.