如何在没有ClassCastException的情况下从另一个OSGi捆绑包公开服务

Question

I have been using ServiceMix on and off for a couple of years now and still haven't managed to share a service implementation between my bundles successfully - it always results in a ClassCastException due to a proxy implementation being used.

I have two bundles and an embedded jar.

• The embedded jar contains the interface(s) that my service class implements.
• Bundle 'A' has the service class implementation and exports the package that the impl lives within.

• Bundle 'B' imports the package exposed by bundle 'A'. Within Bundle 'B' the following code successfully gets the service;

    IScenarioService scenarioService = null; try { ServiceReference<?>[] servRefs = context.getServiceReferences(IScenarioService.class.getName(), null); if (servRefs == null || servRefs.length == 0) { LOGGER.error("Found no service references for " + IScenarioService.class.getName()); return false; } else { LOGGER.info("Services: " + servRefs.length); boolean assign = servRefs[0].isAssignableTo(context.getBundle(), IScenarioService.class.getName()); LOGGER.info("Assign: " + assign); scenarioService = (IScenarioService) context.getService(servRefs[0]); } } catch (InvalidSyntaxException e) { LOGGER.error(e.getMessage()); e.printStackTrace(); return false; } 

My log shows that 1 service is found and that the service 'isAssignable', but on line scenarioService = (IScenarioService) context.getService(servRefs[0]); i get

java.lang.ClassCastException: Proxy511e3d1b_93b7_4de1_835f_3e5df19040b4 cannot be cast to xx.x.xx.IScenarioService

I've tried to inject the service via Blueprint, accessing in code as above, changing the import/ export relationships in the pom's maven-bundle-plugin as well as having all of the interfaces stored within Bundle 'A' and exporting the relevant package.... All to no avail.

Could someone please provide an answer to put me out of my misery? Thanks.

Answer 1

I assume that the problem is that the bundle providing the service and the bundle using the service see different instances of the class IScenarioService. This typically is caused by embedding the interface in both bundles or having two bundles that provide the interface.

The easiest way to avoid this is to put the interface package in a third bundle that exports the package and import the package in both service provider and service consumer bundle.

The maven bundle plugin will do this automatically when you use defaults.

As an example see this tutorial . The model bundle contains the service interface, the persistence bundle contains the service provider and the ui bundle contains the service consumer.

As you can see in the code the maven bundle plugin is only set up in the parent and needs virtually no additinal OSGi setup in the individual bundles.

So as you see providing and using a service is very simple if you do it correctly.