Hot deployment in BOF – yes or no?

One of the primary benefits of BOF is hot deployment of updates. But do you get hot deployment for all BOF elements? The answer is no, and you need to understand when you do and when you don’t.

If you look at the BOF class loaders you will see that all module-specific loaders share a common parent – the BOF shared loader. The shared loader is primarily intended to contain module interfaces. There is a second supported case though – non-sandboxed Java libraries. These are used to share implementation jars across modules. This is not a recommended practice, but it does avoid potential duplication of class files in memory (the only reason that it is supported).

It is fatal to update interfaces in a running VM in many cases and ill advised in others, so the expectation is that classes in the shared loader are not dynamic. Anything that is loaded by a module-specific loader is subject to hot deployment, but anything loaded by the shared loader does not enjoy hot deployment. The main reason for this is that invalidating this class loader would mean invalidating all existing module-specific class loaders, possibly resulting in duplicates for some time as modules are garbage collected which could be a significant memory cost. Regardless of the reason, it is a fact.

The take home lesson is that if you use non-sandboxed Java libraries you will not get hot deployment for those libraries. At the cost of some class file duplication you can get all the BOF benefits including hot deployment if you sandbox your Java libraries, and we strongly recommend this. Note that an update to a sandboxed Java library is propagated to all consuming modules via hot deployment.

As an aside, be careful to avoid caching module instances because these instances are immune to hot deployment.