Best Practices for Writing OSGi Services Using R7/R8 Annotations

Problem Statement:

What is the best way to write an OSGi service?

Requirement:

What is the best way to write an OSGi service? What is the recommended way to register a service, as per R7/R8 annotations?

Introduction:

A Service Component is a Java class that can be optionally registered as an OSGi service and can optionally use OSGi services. The runtime for DS is called Service Component Runtime (SCR) and uses component descriptions in the bundle (in XML form) to instantiate and wire components and services together. The component descriptions are used by SCR as a performance choice so that SCR can avoid processing all classes in a bundle to search for annotations at runtime

As per Adobeā€™s best practices please start using OSGi R7/R8 annotations, avoid using felix based component annotations

OSGi R7/8

Create Services using Service Component

A Service Component is a Java class that can be optionally registered as an OSGi service and can optionally use OSGi services. The runtime for DS is called Service Component Runtime (SCR) and uses component descriptions in the bundle (in XML form) to instantiate and wire components and services together. The component descriptions are used by SCR as a performance choice so that SCR can avoid processing all classes in a bundle to search for annotations at runtime

Create Example Service – ExampleService

package com.mysite.core.services;

import java.util.Set;

public interface ExampleService {
    /**
     * Start generation content list
     *
     * @return result set
     */
    public Set<String> generateContentList(String path);
}

Create Example Service Object class definition – ExampleServiceConfig

  1. Creating a separate Interface annotation class will help in the long run when we want to introduce more fields
  2. Service config name and description
  3. Attribute name, description, and type
package com.mysite.core.services.config;

import org.osgi.service.metatype.annotations.AttributeDefinition;
import org.osgi.service.metatype.annotations.AttributeType;
import org.osgi.service.metatype.annotations.ObjectClassDefinition;

@ObjectClassDefinition(name = "Example Content list generator Service", description = "Example Content list generator Service generates Set")
public @interface ExampleServiceConfig {

    @AttributeDefinition(name = "Enabled Generator", description = "True, to generate the list", type = AttributeType.BOOLEAN)
    boolean isEnabled() default false;
}

Create Service Implementation – ExampleServiceImpl

  1. Get OCD based configs @Activate without any method
  2. You can also get config inside @Modfied with the method
  3. To get a resource resolver inside the service it’s recommended to use ā€œtry-with-resourceā€ which helps to never forget to close a resource(avoids unclosed resource resolver warnings)
package com.mysite.core.services.impl;

import com.mysite.core.services.ExampleService;
import com.mysite.core.services.config.ExampleServiceConfig;
import org.apache.sling.api.resource.LoginException;
import org.apache.sling.api.resource.ResourceResolver;
import org.apache.sling.api.resource.ResourceResolverFactory;
import org.osgi.service.component.annotations.Activate;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.component.annotations.Modified;
import org.osgi.service.component.annotations.Reference;
import org.osgi.service.metatype.annotations.Designate;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;

@Component(service = ExampleService.class, immediate = true, name = "Example Content Generator Service")
@Designate(ocd = ExampleServiceConfig.class)
public class ExampleServiceImpl implements ExampleService {

    private static final Logger LOGGER = LoggerFactory.getLogger(ExampleServiceImpl.class);
    /** Service User name */
    private static final String SERVICE_USER = "exampleUset";
    private boolean serviceEnabled;

    @Reference
    private ResourceResolverFactory resolverFactory;

    //R8 Annotation
    @Activate
    ExampleServiceConfig exampleServiceConfig;

    //R7 Annotation but captures and @Modified configs
    @Modified
    protected void activate(final ExampleServiceConfig exampleServiceConfig) {
        this.exampleServiceConfig = exampleServiceConfig;
    }

    @Override
    public Set<String> generateContentList(String path) {
        if(exampleServiceConfig.isEnabled()){
            try (ResourceResolver resourceResolver = resolverFactory.getServiceResourceResolver(Collections.singletonMap(ResourceResolverFactory.SUBSERVICE, SERVICE_USER))){

            } catch (LoginException e) {
                LOGGER.error("Error Occured during Login", e.getMessage());
            }
        }
        return new HashSet<>();
    }
}

Project Structure

Code structure for best practices

Note: Make sure to add the config XML’s for the services and please don’t depend on the OCD default values.

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