To Declare Or Not to Declare

A few weeks ago I described the “hibernate” problem, how to let multiple bundles independently use a Hibernate Session Factory without requiring a priori knowledge of each other. These postings are not just finding practical solutions to today’s problems with today’s specifications. These are my research subjects; I am trying to keep future generations of the OSGi specification clean and in the right spirit so I am playing with options. Next time I will try to be more pragmatic.

The solution I found to share the Session Factory was service based, it allowed bundles to contribute Hibernate domain classes, and it allowed bundles to use a collective of domain classes. However, in this solution there still lurked a class loading problem caused by a peculiar habit of Hibernate. In the solution this problem was solved with a simple Require-Bundle. However, though this solution works, it is a hack that can easily fall apart if the required classes are refactored into another bundle. So for today, this solution is good enough, for tomorrow we need to understand the root problems and come up with solutions to this root problem. So lets dive deeper into the problem and analyze what is going on.

When Hibernate creates a Session Factory, it actually creates a dynamic class proxy to its implementation class. This in itself is not a problem was it not for the fact that it uses the client’s class loader to create the proxy. The client therefore must have visibility to classes that it does not use. That is, Hibernate assumes all the classes itself sees are visible to the client. In a modular system, this is obviously not the case. A simple fix is to let the client bundle use Require-Bundle for the Hibernate bundle, however, this Require-Bundle has its own set of problems, see the OSGi R4.1 specification.

It is easy to take the high ground and tell Hibernate to get its act together. However, this is not a very productive stance for most of us. Despite the increasing popularity of OSGi Technology, the majorities of JARs out there are not prepared for proper modularity.

These type of class loading problem are endemic because so many developers have (ab)used class loaders for home grown extension mechanisms, due to a lack of a proper extension mechanisms in Java á la the OSGi service registry. All of these home grown mechanisms just fall apart when it becomes necessary to share the VM with multiple versions of the same package. They also can not provide class space consistency, which is important for applications that want to work reliably. Therefore, the best solution would be if the OSGi specifications addressed these issues directly.

The OSGi CPEG asked me to come up with an OSGi RFP about this problem so I Skyped my favorite OSGi Invited Researcher: Richard S. Hall. We did some brainstorming and in the past few weeks we did some prototyping to better understand the problems. Richard modified Apache Felix to support our experiments and I used these modifications with a simple web based notes taking application that uses Hibernate.

The first approach we took was the declarative “implicit-wire” directive. We came to this solution because the deep structure of the problem is that when you import a package, the consequence of this import is that you should import additional packages. That is, in the Hibernate example, when you import org.hibernate.cfg, you should also import net.sf.cglib.*, even if you have no direct dependency on these packages. Richard therefore created an x-implicitwire directive on an Export-Package clause. The value of this directive is a list of packages that should be added to the import of any bundle importing the package. For example:

Export-Package: org.hibernate.cfg;x-implicitwire:=”net.sf.cglib.proxy,net.
sf.cglib.core,net.sf.cglib.reflect, org.hibernate.proxy”

If Apache Felix has to wire to the org.hibernate.cfg package, it will now automatically add wires to the packages from the x-implicitwire directive as well, thereby ensuring that the importing bundle can see the proper packages when Hibernate attempts to create a proxy for the Session Factory.

The declarative approach is quite simple and it did solve the problem. The prototype code, a simple web based notes program, worked fine. The only change that was necessary was the bnd file that created the hibernate bundle. However, we were not convinced that this solution could solve world hunger; there are many cases where the declarative approach with x-implicitwire is not feasible because the set of required implicit wires is not known ahead of time. A crucial example is generic Aspect Oriented Programming, where the set of additional classes is completely open-ended. That is, an AspectJ program can weave any possible class in another class.
The only solution to this problem is allowing another bundle to add imports during runtime. After some discussions Richard found the courage, and time, to add an addRequire function to the Bundle Context object. Imports added this way are treated as DynamicImport-Package clauses. That is, they are consulted last and always until the import is found.

I created a simple extender bundle that inspects all bundles when they become resolved. I chose the import of the org.hibernate.cfg package as trigger. That is, when a bundle imported this package, it would automatically get the net.sf.cglib.* packages. As can be expected, also this approach worked fine and did not require anything special from the client bundle, no changes were necessary. However, unfortunately there is a race condition to which we do no have a good solution. If the extender bundle gets started after the client bundle, we have a problem. The resolved client bundle could get started and create a Session Factory before the extender has a change to add the imports. Alternatively, the additional imports could be added persistent, in that case the install event could trigger the adding of the imports. In that case, the window for the race condition is much smaller. This problem is prevented with the declarative approach of x-implicitwire.

So what is the best solution? I am really not sure. I like the simplicity of the declarative approach. It allows a clean and very simple solution for problems where the implicit imports are known ahead of time. A very important advantage of this approach is that it does not have race conditions. However, when these imports are dynamic, the problem is no longer usable. The procedural approach of dynamically adding an import in runtime has the required flexibility but has the problem of the race condition. It will require deployers to use the start level to ensure that the extender bundle is started before the client bundles are installed.

What do you think?

Peter Kriens