In general, this is a pretty well known issue. Well enough known that OSGi has a special section of their spec to cover it. Well, at least for the Parser cases .... I don't recall seeing anything for Transformer, but I think the issues and principles are the same. So, if you didn't know that, you might find interesting the 'XML Parser Service Specification' section of the Compendium Specification http://www.osgi.org/download/r4v42/r4.cmpn.pdf
Also, you might study through bugs and code in Equinox related to this (again, I know of parser cases .. would be similar for Transformer, I suspect, if Transformer was spec'd). Such as see https://bugs.eclipse.org/bugs/show_bug.cgi?id=285505 At its heart, equinox provides the context classloader solution (again, for parsers). The bugzilla entry also provides links to some other xml commons bugs that are interesting from an OSGi point of view, though again, not directly related to solving your problem. Also be aware that Eclipse Orbit offers some "OSGi'ed" versions of xmlcommons libraries, but do not modify code or provide a service. Such as see http://download.eclipse.org/tools/orbit/downloads/drops/R20100519200754/ I'm not saying these would solve your problem ... but, if you create your own bundles, beware of "conflicts" with other bundles out in the wild. I am no OSGi expert, but I'd think you could provide a service to create your new instance, any way you want, and that'd be preferable to changing someone else's "newInstance" code ... but then application code would have to know to use your service (not sure if you were ruling that out, or not). I'm not sure the problem can be solved, without application code knowing how/which service to call, but best of luck. From: Lindsay Smith <[email protected]> To: "[email protected]" <[email protected]> Date: 10/11/2010 04:33 PM Subject: [osgi-dev] XML libraries - TransformerFactory.newInstance and friends Sent by: [email protected] I've run into some issues with the standard XML libraries. Calls like this: TransformerFactory.newInstance() are particularly unsuited to the OSGI environment. The above call, for example, attempts to identify the name of the class to instantiate from a number of locations. These are: 1) The value of a system property 2) The presence of some properties file in java.home 3) The presence of a resource under META-INF/services accessed through the context ClassLoader Once the name of the class is found, the context ClassLoader is used to instantiate it. The first two locations are especially nasty in an OSGI environment, as it can mean that the factory looks up the name of the class from a location completely unrelated to the class space in which the class will be instantiated. The third location can be made to work - as the context ClassLoader can be set to the current bundle ClassLoader prior to calling, and frameworks (like spring dm) often set it this way too. The product that I develop has recently moved to using OSGI for class isolation and modularity, and it's a great fit. One of the advantages we see in using OSGI is the ability to isolate your application from its environment. This is particularly important when you don't have control over the deployment environment e.g. the application is to be installed on a client site where the java or servlet container environment managed by another IT department. OSGI has definitely delivered benefits to us in keeping our code isolated from the environment but patterns such as the TransformerFactory.newInstance() situation still give us problems. In some situations the web container or even a sibling application sets the associated system property, and our application is then broken because the named class is not visible to us. In our own code we use direct constructor of the xml library we wish to instantiate to prevent this happening. However sometimes libraries we include new the newInstance() method (spring dm is the main offender here - the spring beans parser), and we can't change them. One thing we have done to standardise the xml libraries used inside our application is to remove all the javax.xml.* packages from the system packages list, and to include apache xmlcommons, resolver, and xerces bundles found in the springsource repository e.g: http://www.springsource.com/repository/app/bundle/version/detail?name=com.springsource.org.apache.xmlcommons&version=1.3.4 This standardises the xml apis across all jvms, but the newInstance() problem remains. I was thinking about modifying the newInstance() methods in the xmlcommons bundle to be correct for an OSGI environment. In the first instance I was simply going to change the search order so that the META-INF/services step comes first, as this can be controlled in our bundles. However I was wondering how I could go about making a full OSGI implementation of this system. It could do something like: 1) Return an instance of a TransformerFactory service if one exists (I guess the highest version available if multiple) 2) Use the META-INF/services location of the context ClassLoader 3) Read the OSGI framework properties (rather than system properties) So, some questions: Has anyone else had this problem and overcome it without requiring changes to the application environment? Would an OSGI version of xmlcommons be useful to anyone else? How should it be designed? Is the above search order sensible? Anything else that would make sense within an OSGI runtime? Regards Lindsay Smith _______________________________________________ OSGi Developer Mail List [email protected] https://mail.osgi.org/mailman/listinfo/osgi-dev
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