Ticket #1934 (closed defect: fixed)

I was able to reproduce this bug by changing imagej.Main to:

ImageJ ij = new ImageJ(ModuleService.class, MenuService.class);

And the same NPE can be witnessed.

This bug is a symptom of a race condition, introduced by some changes to how services manage their event handling. It used to be that each service would subscribe itself to events when finished initializing. But that logic was generalized, such that all "contextual" objects (including services) now subscribe themselves as soon as their context is assigned. So now, MenuService subscribes itself to e.g. ModuleAddedEvents before it is finished initializing.

I believe I have fixed the problem in scijava-common: services now defer event handler registration until the end of initialization. If a service overrides the initialize() method, it is important for it to call super.initialize(), or else its event handlers will not be registered. Relevant changeset is:  b019a06a.

I will cut a new release of scijava-common soon and update ImageJ2 to use it; will close this bug when that is done, tested and working.

It really bothered me that calling super.initialize() was now required for any services overriding initialize(). I think that requiring such a thing is very suboptimal and would result in many future programming errors. So I reworked the fix slightly to avoid that need:  2f08d695.

The change does introduce another Service API method, which breaks downstream Service implementations (though anything extending AbstractService is in the clear). But I think the pain is worth it to fix this critical bug.

I've released SciJava Common 1.5.0 which includes this fix, and updated ImageJ2's master branch to use it:  a229f9ad.