问题
I would like to know what is "Activate this plug-in when one of its classes is loaded" check-box in Eclipse manifest editor useful for.
I thought Eclipse always use "lazy initialization" approach. Does have this option a relation to the BundleActivator class of the plugin? is initialization something different to activation?
Here is a similar question, but I don't understand it entirely.
回答1:
Ticking the box causes the following header to be set in the manifest:
Bundle-ActivationPolicy: lazy
I'll start with how "pure" OSGi deals with this. IF the bundle is started with the START_ACTIVATION_POLICY flag then the bundle enters the STARTING state but the activator's start() method is not invoked and a ClassLoader is not allocated for the bundle. The bundle stays in STARTING until, for whatever reason, a class needs to be loaded from the bundle. At that point a ClassLoader is allocated and the activator (if any) is instantiated and its start() method is invoked before the requested class is loaded.
However Eclipse layers additional semantics on the top. As background, Eclipse always tries to avoid starting bundles in order to keep its start-up time minimal. A very small core set of bundles is started by default (the list is in configuration/config.ini) and one of these is called the p2 "simpleconfigurator". The simpleconfigurator looks for bundles that have the Bundle-ActivationPolicy:lazy header and it starts them with the START_ACTIVATION_POLICY flag... therefore these bundles will be "lazily" started as described above.
The important point is that all other bundles that do not contain the header will not be started at all under Eclipse. They will stay in RESOLVED state, their activators will not be invoked, and if they contain any Declarative Services component they will not be loaded. This is because Declarative Services only ever looks at bundles that are in ACTIVE or STARTING state.
Therefore the main reason to use the header is if we want to write a bundle containing Declarative Services components that need to work under Eclipse.
In other environments there is no need to use the header. Most normal OSGi apps simply start all bundles, rather than trying to selectively start a subset of bundles. Note that this doesn't mean OSGi apps don't worry about lazy loading! Declarative Services already supports lazy loading without messing around with bundle class loading triggers. In my opinion Eclipse gets this wrong and has added unnecessary complexity to the bundle lifecycle. Nevertheless if you are running in Eclipse then you have no choice but to understand and work with its limitation.
来源：https://stackoverflow.com/questions/17590464/lazy-activation-of-eclipse-plugins