Best Practices for Writing OSGi Services Using R7/R8 Annotations

AEM 3 Minutes

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 Adobe’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 “try-with-resource” 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.

Published by Kiran Sg

My name is Kiran SG. I am currently working as an AEM architect and handling site revamp and migration. I try to learn new tricks and techniques every day to improve my coding and deliver value-added projects. All the tricks and techniques sometimes seem to be straightforward working code with best practices. But sometimes it won't satisfy the developer's needs. But most of the time code is either lagging to meet the requirement or doesn't follow best practices. Hence I am taking the initiative to put all my learnings and tricks in my blog series to share the working code with best practices. Hoping to help other developers like me. I have been working on AEM for 8 years now. I started my career as a Java developer. I have delivered many projects with a core customer-centric and strong focus on infrastructure and architecture. If you have any queries related to my blog topics please do connect with me directly on my email and also you can connect with me over LinkedIn and Facebook.

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