How to handle different @Reference in OSGi DS Component

Question

I'm having a problem with the following situation: The Server is waiting for one or more Functions. When a Function is bound the bindFunction is called. It needs to call doSomething() of any SpecificSystem.

When there is no SpecificSystem in my OSGi Container nothing happens which is good because the System Reference is not satisfied. The problem occurs when I add a SpecificSystem to my container. In that case the bindFunction is called before the System Reference is set leading to a NullPointerException inside bindFunction.

Is there any OSGi-way to make sure the System Reference is set when the bindFunction is executed so that I can safely call system.doSomething() inside the bindFunction?

Answer 1

You're treading in dangerous water here :-) You require ordering. Your code assumes the bindFunction reference is called after the system reference.

The OSGi specification guarantees that injection takes place in the lexical order of the reference name. (Of course, this is only true for the available services.)

The cheap way is to name your references so that the system reference's name is lexically lower than the name of the bindFunction reference, for example asystem or _system. The injection takes place in the lexical order.

This is ugly if course. A way to handle this is just inject the Function services and use them when needed instead of actively doing something in your bind function. This makes things more lazy which is almost always good.