How to build a plugin architecture using CDI - I'm using Wildfly 10

Question

I want to build a JEE plugin based architecture. The main idea is do something similar to what eclipse is, but in the context of JEE. My goal is to have a minimum of modules as the core, and allow others modules extend its functionality. To this, I have implemented a test using 4 modules:

gauges: Defines and implements a gaugesregistry service, also defines a gauge POJO.
cashgauges: implements a gauge producer using CDI. this is a plugin mock.
othergauges: implements a gauge producer using CDI. this is a second plugin mock.
gauges-web: Contains a basic JSF view to query the gauges registry.

dependencies are as follows:

cashgauges --> gauges
othergauges --> gauges
gauges-web --> gauges

This is done by using jboss-deployment-structure.xml on each deployed file.

The deployment is done as individual files:

gauges.jar 
cashgauges.jar 
othergauges.jar 
gauges-web.war

All services start, but what I see is, my gaugesregistry is instantiated several times. I started wildfly in debug mode and what I see is each module has its own instance of gaugesregistry: cashgauges and othergauges call same method (addGauge) on registry, but instances of this registry are not the same.

This happens in both cases, using @ApplicationScoped and @Singleton annotations. What am I doing wrong?

Source code is available on https://github.com/hatit/research

After a couple of days, I'm considering using a ServiceLocator pattern and remote references instead of CDI. Any suggestions?

Answer 1

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After some hours understanding differences between CDI Beans and EJBs lifecycle when used as independent modules (JBoss Modules), i found:

Singleton CDI Beans are instantiated one time per module, not really singleton among all modules.

To avoid this i had to create Registry as a Singleton Enterprise Session Bean. This cames with new problems, CDI injection doesn't works among modules, so i had to package a CDI producer (i don't care if it's singleton or not, its only a producer) which can be instantiated by any module. Main responsibility of this producer is to lookup Registry EJB, this to avoid hardcoding jndi path each time i need access the Registry.

I changed my trivial example to support JSF plugins also, this is an example of what i am using currently.

Module facelets:

Registry interface:

public interface FaceletsModuleRegistry {
    void registerModule(String module);

    List<String> getRegisteredModules();
}

Registry implementation:

@Local(FaceletsModuleRegistry.class)
@Singleton(name="FaceletsModuleRegistry")
@TransactionAttribute(TransactionAttributeType.NOT_SUPPORTED)
@Vetoed
public class FaceletsModuleRegistryImpl implements FaceletsModuleRegistry {

    private Set<String> registeredModuleNames = new TreeSet<>();

    @Override
    public void registerModule(String module) {
        registeredModuleNames.add(module);
    }

    @Override
    public List<String> getRegisteredModules() {
        return Collections.unmodifiableList(new ArrayList<>(registeredModuleNames));
    }

}

Registry producer:

@ApplicationScoped
public class FaceletsModuleRegistryBuilder {

    @EJB(lookup="java:global/facelets/FaceletsModuleRegistry!co.hatit.enterprise.facelets.services.FaceletsModuleRegistry")
    protected FaceletsModuleRegistry faceletsModuleRegistry;

    @Produces
    public FaceletsModuleRegistry getFaceletsModuleRegistry(){
        return faceletsModuleRegistry;
    }
}

Any other module that i want to plugin implements this code (please see @Inject can be used on any module requiring access the Registry singleton instance):

@ApplicationScoped
public class InmueblesActivator {

    @Inject
    private FaceletsModuleRegistry faceletsModuleRegistry;

    public void init(@Observes @Initialized(ApplicationScoped.class) Object init){
        String moduleName = Module.getCallerModule().getIdentifier().getName();
        String name = StringUtils.substringBetween(moduleName, "deployment.", ".jar");
        faceletsModuleRegistry.registerModule(name);
    }

}

Then i can reference Registry from any module as a really singleton instance (solved my problem having multiple instances of same class when used CDI singleton beans among several modules).

Now, i can plugin JEE modules, not just java code, but facelets resources also:

public class FaceletsResourceHandler extends ResourceHandlerWrapper {

    Logger logger = LoggerFactory.getLogger(FaceletsResourceHandler.class);

    @Inject
    FaceletsModuleRegistry faceletsModuleRegistry;

    private ResourceHandler wrapped;

    public FaceletsResourceHandler(ResourceHandler wrapped) {
        this.wrapped = wrapped;
    }

    @Override
    public ViewResource createViewResource(FacesContext context, final String name) {
        ViewResource resource = super.createViewResource(context, name);

        if (resource == null) {
            resource = new ViewResource() {
                @Override
                public URL getURL() {
                    try {
                        //iterates over plugins to find the required resource.
                        for(String module : faceletsModuleRegistry.getRegisteredModules()){
                            URL resource = Module.getCallerModule().getModuleLoader()
                                    .loadModule(ModuleIdentifier.create("deployment." + module + ".jar"))
                                    .getExportedResource("META-INF/resources" + name);
                            if (resource != null) return resource;
                        }
                    } catch (ModuleLoadException e) {
                        throw new FacesException(e);
                    }

                    return null;
                }
            };
        }

        return resource;
    }

    @Override
    public ResourceHandler getWrapped() {
        return wrapped;
    }

}