Chapter 2 of AEM Learning Series — Understanding the engine that runs AEM.

🚀 What is OSGi?

OSGi = The engine that runs AEM

AEM is not a single huge Java application.
It is made of hundreds of small modules called Bundles.

OSGi is the framework that:

• Loads bundles

• Starts bundles

• Stops bundles

• Connects bundles together

AEM uses the Apache Felix implementation of OSGi.

📱 Smartphone Analogy

Phone OS        → OSGi
Mobile Apps     → Bundles

• Install/uninstall apps without restart
• Apps talk to each other
• OS manages lifecycle

🤯 Why AEM Uses OSGi

Without OSGi (Traditional Java App)

ONE BIG APPLICATION
Change one thing → Redeploy everything
Restart → 15+ min downtime 😬

With OSGi (AEM)

Bundle A (Assets)
Bundle B (Workflow)
Bundle C (Search)
Bundle D (Auth)
Bundle E (Editor)
Your Custom Bundle

👉 Update one bundle → No restart required

This is called Hot Deployment 🔥

🧩 Key OSGi Concepts

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1️⃣ Bundle (The Deployment Unit)

A Bundle = JAR file + OSGi metadata

Each bundle contains:

META-INF/MANIFEST.MF

This file tells OSGi:

• Bundle name

• Version

• Exported packages

• Imported packages

• Dependencies

👉 Smallest deployable unit in AEM.

2️⃣ Bundle Lifecycle

INSTALLED → RESOLVED → STARTING → ACTIVE
                                     │
                                     ▼
                                  STOPPING → UNINSTALLED

💡 Most common AEM issue:
Bundle stuck in INSTALLED → Missing dependencies.

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3️⃣ OSGi Services (Plug & Socket Model)

Service = Interface + Implementation

Interface  → Socket
Implementation → Plug

Any bundle can request a service, and OSGi injects it automatically.

Example Code

// Interface
public interface GreetingService {
    String getGreeting(String name);
}

// Implementation
@Component(service = GreetingService.class)
public class GreetingServiceImpl implements GreetingService {
    public String getGreeting(String name) {
        return "Hello " + name;
    }
}

// Injecting in Sling Model
@Model(adaptables = Resource.class)
public class HeaderModel {
    @OSGiService
    private GreetingService greetingService;
}

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4️⃣ OSGi Configurations (No Hardcoding!)

Instead of hardcoding values:

smtp.gmail.com

Use configuration:

mail.smtp.host = smtp.gmail.com
mail.smtp.port = 587

Different environments can have different configs:

👉 No code changes required.

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🔎 Where to See OSGi in AEM

This is called the Felix Web Console.

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🧠 OSGi in AEM Architecture

AEM
 └── OSGi (Felix)
      ├── Sling Bundles
      ├── Oak Bundles
      ├── Workflow Bundles
      └── Your Custom Bundles

All communicate via OSGi Service Registry.

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📝 Things to Remember

• OSGi = modular runtime used by AEM

• Bundle = deployable JAR

• Service = interface + implementation

• Hot deployment = no restart required

• Felix Console = /system/console

• Bundle not ACTIVE? → Dependency issue

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🎯 AEM Interview Questions — OSGi (30 Q&A)

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🟢 Section A — Basics

Q1. What is OSGi?
Modular Java framework used by AEM.

Q2. OSGi implementation in AEM?
Apache Felix.

Q3. Why OSGi?
Modularity • Hot deployment • Dependency management.

Q4. What is a Bundle?
JAR with OSGi metadata.

Q5. What is MANIFEST.MF?
Bundle metadata file.

Q6. Felix Web Console?
Admin UI at /system/console.

Q7. Default credentials?
admin/admin (dev only).

Q8. Bundles console URL?
/system/console/bundles

Q9. Configurations console URL?
/system/console/configMgr

Q10. Components console URL?
/system/console/components

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🔵 Section B — Lifecycle

Q11. Bundle states?
Installed → Resolved → Active → Stopping → Uninstalled.

Q12. Bundle stuck in INSTALLED?
Missing dependencies.

Q13. Debug steps?
Check bundle → imported packages → install dependency.

Q14. ACTIVE → RESOLVED?
Yes, when stopped or dependency removed.

Q15. Fragment bundle?
Extends another bundle.

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🟣 Section C — Services

Q16. OSGi Service?
Java object registered in Service Registry.

Q17. Component vs Service?
Service = component exposed via interface.

Q18. Register service annotation?
@Component(service=MyInterface.class)

Q19. @Reference?
Inject service in OSGi component.

Q20. @Reference vs @OSGiService?
Component vs Sling Model injection.

Q21. Service Registry?
Runtime service lookup system.

Q22. service.ranking?
Priority for multiple implementations.

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🟠 Section D — Configuration

Q23. OSGi Config?
Externalized properties.

Q24. Create configs?
Console or repository config files.

Q25. Run-mode configs?
config.author, config.publish.

Q26. Factory config?
Multiple instances of same component.

Q27. PID?
Unique config identifier.

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🔴 Section E — Scenarios

Q28. Sling Model not working?
Check bundle, components, Sling Models console.

Q29. DS vs SCR?
DS = modern, SCR = deprecated.

Q30. Two bundles exporting same package?
Yes — version resolution.

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🎉 Final Summary

Understanding OSGi = Understanding how AEM actually runs.

Everything you build in AEM becomes an OSGi bundle.