AEM 101-47: Overriding Components with Resource Merger – A Comprehensive Guide

Adobe Experience Manager (AEM)
Adobe Experience Manager (AEM)


Welcome back to our AEM 101 series! Adobe Experience Manager (AEM) stands as a pivotal framework in the world of digital asset management and web content management. At the heart of its versatility and power are the AEM components – modular units of content that form the building blocks of your web pages.

In this installment, we delve into an advanced yet essential topic: the Resource Merger in AEM. This functionality is critical for developers seeking to customize and extend the capabilities of their AEM components without altering the original source code, thus maintaining the integrity and upgradability of their applications.

Understanding the necessity of overriding components in AEM is paramount for any developer aiming to tailor the platform to specific organizational needs. It’s not just about changing a layout or tweaking a design; it’s about empowering your AEM projects to meet and exceed the unique demands of your business and its audience.

As part of our ongoing AEM 101 series, this blog post aims to provide you with a comprehensive understanding of the Resource Merger and how it facilitates the overriding of components. Whether you’re a seasoned AEM developer or just starting out, this guide is designed to help you navigate the complexities of AEM customization with ease and confidence. Let’s dive into the intricacies of AEM’s Resource Merger and uncover how it can revolutionize your AEM projects.

1: Understanding AEM’s Resource Merger

1.1 What is Resource Merger and Its Role in AEM?

In the realm of Adobe Experience Manager, the Resource Merger is a powerful feature designed to streamline the process of customizing and managing components without altering the base code. Essentially, Resource Merger allows developers to overlay existing components with custom features, thereby extending functionality while preserving the original structures.

The primary role of the Resource Merger in AEM is to provide a systematic approach for merging resources from various sources within the AEM environment. This is particularly crucial when dealing with updates or modifications to AEM instances, as it ensures that custom changes are retained without conflicting with the core AEM updates.

1.2 Architecture and Workflow of Resource Merger

The architecture of AEM’s Resource Merger is built on the concept of overlaying and overriding. It operates on a layered approach, where the base layer (usually the out-of-the-box components) can be overlaid with custom layers (developer-created or modified components). This structure ensures that customizations are separate from the original components, facilitating easier updates and maintenance.

The workflow begins with the AEM system identifying the resources to be merged – this includes the original resource and the overlay resource. The Resource Merger then combines these resources, prioritizing the overlay resource’s properties and functionalities, resulting in a merged resource that is deployed to the site.

1.3 Importance of Resource Merger in AEM Development

The importance of Resource Merger in AEM development cannot be overstated. It enables developers to customize components in a manageable and non-destructive manner, ensuring that customizations can be easily updated or rolled back without affecting the base component code.

Furthermore, the Resource Merger simplifies the development process by allowing for the reuse of existing components with added custom features. This not only reduces the time and effort required for development but also enhances the consistency and reliability of the AEM sites.

2: The Basics of Component Overriding in AEM

2.1 Introduction to Component Overriding and Its Necessity

Component overriding in Adobe Experience Manager (AEM) refers to the process of customizing the out-of-the-box (OOTB) components to fit specific business requirements without altering the original code. This process is essential in situations where the default functionality of AEM components does not meet the unique needs of your project or when you wish to add new features or styles to existing components.

Overriding components is necessary to maintain the upgradability of AEM instances. By customizing components through overriding, developers can ensure that their customizations remain intact and do not conflict with future AEM updates or patches. This method promotes a cleaner, more manageable approach to AEM customization, allowing for greater flexibility and control over the application’s functionality and appearance.

2.2 The Difference Between Overriding and Extending Components

It’s crucial to distinguish between “overriding” and “extending” components in AEM. Overriding involves replacing the original component with a custom one, ensuring that the AEM instance uses the new version instead of the default. This approach is used when the changes are substantial or when the original functionality needs to be entirely reworked.

On the other hand, extending involves creating a new component that inherits properties and behaviors from an existing one, adding or modifying functionalities as needed. Extending is preferred when the required changes are minimal or when developers wish to preserve the original component’s functionality while adding new features.

2.3 Basic Steps Involved in Overriding a Component Using Resource Merger

Overriding a component in AEM using the Resource Merger involves several key steps:

  1. Identify the Component to Override: Determine which OOTB component requires customization. Understanding the component’s structure and functionality is crucial before proceeding with the override.
  2. Create the Overlay Structure: In your project’s codebase, create a new structure that mirrors the path of the original component within the /apps folder. This path tells AEM to use your custom component instead of the default.
  3. Copy the Original Component: Copy the entire structure and content of the original component into your new overlay structure. This serves as the starting point for your customization.
  4. Modify the Component: Apply your custom changes to the copied component in the overlay structure. This could include altering the dialog, changing the HTML output, adding new CSS or JavaScript, or modifying the backend logic.
  5. Deploy and Test: After making the necessary changes, deploy your code to the AEM server. Thoroughly test the overridden component to ensure that it behaves as expected and that all customizations are applied correctly.

3: Step-by-Step Guide to Overriding AEM Components

Overriding components in Adobe Experience Manager (AEM) allows developers to tailor the platform to their specific needs. Below is a detailed, step-by-step guide to help you override AEM components effectively:

3.1 Detailed Instructions for Overriding Components in AEM:

  1. Identify the Component to Override: Begin by pinpointing the exact component you wish to override. Navigate through the AEM repository and find the path of the component within the /libs directory.
  2. Create Your Overlay Structure: Utilize the CRXDE Lite or your preferred IDE to replicate the structure of the component you intend to override within the /apps directory. Ensure the structure within /apps mirrors the original path from /libs to ensure AEM redirects to your custom component.
  3. Copy the Original Component: Transfer the entire content of the original component from /libs to your new structure in /apps. This includes all nodes and properties associated with the component.
  4. Apply Your Customizations: Modify the copied component as needed. This could involve editing the component’s dialog, HTML, CSS, JavaScript, or JSP files to meet your requirements.
  5. Deploy Your Changes: Once your customizations are complete, deploy the updated component to your AEM instance. Use Maven or another deployment tool suited to your project’s build process.
  6. Test Thoroughly: Test the overridden component extensively in various scenarios and browsers to ensure it functions correctly and as expected.

3.2 Tips and Best Practices for Successful Component Overriding:

  • Use a clear naming convention: Make sure your overlay structure clearly indicates that it’s a custom override to avoid confusion with the original component.
  • Minimize changes: Only make necessary changes to avoid potential conflicts during AEM upgrades.
  • Document changes: Keep a record of what was changed, why, and when to streamline future updates or troubleshooting.
  • Leverage version control: Use a version control system to track changes and facilitate collaboration among developers.
  • Test across environments: Ensure your custom component works well in all target environments (development, staging, production).

3.3 Common Pitfalls and How to Avoid Them:

  • Overriding too much: Avoid copying and overriding entire components when a few adjustments will suffice. This can lead to maintenance headaches and conflicts with future AEM updates.
  • Neglecting the base component’s updates: Regularly check for updates to the original component in /libs. If Adobe releases updates, you’ll need to integrate them into your custom component.
  • Poor testing practices: Failing to thoroughly test overridden components can lead to unexpected behavior or performance issues. Always perform comprehensive testing.
  • Ignoring best practices: AEM provides best practices for component development and overriding. Ignoring these can result in suboptimal or error-prone components.

4: Advanced Techniques and Best Practices

4.1 Advanced Strategies for AEM Component Customization:

  1. Granular Overrides: Instead of overriding entire components, focus on granular overrides. Target specific areas like dialog fields, design properties, or clientlibs. This approach minimizes the impact on overall functionality and simplifies updates.
  2. Client Library Overrides: Utilize client library categories to override CSS and JavaScript. By creating a clientlib with the same category as the original but placing it in the /apps directory, AEM will load your custom clientlib instead of the default.
  3. Sling Resource Merger for Deep Overrides: For more complex overrides that involve nested structures, leverage the Sling Resource Merger’s deep merging capabilities. This allows for more detailed customizations at different levels of the component hierarchy.
  4. Conditional Logic in JSPs: When customizing JSPs, incorporate conditional logic to handle different scenarios or configurations. This makes your components more flexible and adaptable to various use cases.

4.2 Best Practices for Using Resource Merger Effectively:

  1. Clear Layering: Maintain a clear distinction between layers in your project structure. Keep customizations separate from the original components to avoid confusion and facilitate easier updates.
  2. Consistent Testing: Regularly test merged resources, especially after updates to AEM or your customizations. Ensure that merged outcomes perform as expected across all relevant scenarios.
  3. Minimal Overrides: Only override what is necessary. Excessive use of Resource Merger can lead to performance issues and complicate future updates or maintenance.
  4. Documentation: Document all overrides thoroughly, including the rationale behind each customization. This aids in troubleshooting and streamlines future development efforts.

4.3 How to Troubleshoot Common Issues Encountered During Component Overriding:

  1. Component Not Overriding: Check the overlay structure and paths to ensure they exactly match those of the original component. Verify that the AEM instance is referencing your customized component instead of the default.
  2. Broken Functionality: When functionality breaks after an override, revert to the original component to isolate the issue. Incrementally apply changes to identify the specific customization causing the problem.
  3. Performance Degradation: If the override leads to performance issues, review the customization for inefficient code or resource-heavy operations. Optimize scripts, queries, and other elements that may impact performance.
  4. Update Conflicts: Following an AEM update, if your overrides stop working, compare your customized components with the new version of the original components. Integrate any necessary changes or improvements introduced in the update.

5: AEM Component Overriding: What’s Better, Overriding or Extending?

5.1 Pros and Cons of Overriding vs. Extending Components:

Overriding Components:

  • Pros:
    • Directly modifies existing components, ensuring immediate compatibility with current templates and pages.
    • Simplifies the process when changes are minimal or highly specific.
    • Useful when the original component is almost perfect for your needs but requires minor tweaks.
  • Cons:
    • Makes future updates more challenging, especially if the original component receives updates or bug fixes.
    • Can lead to a cluttered codebase if overrides are not managed properly.
    • Risk of losing context or breaking functionality if not done with thorough understanding.

Extending Components:

  • Pros:
    • Maintains the integrity of the original component, allowing for safer updates and maintenance.
    • Encourages reuse and modularity by leveraging existing functionality while adding new features.
    • Reduces potential conflicts during AEM upgrades since original components remain unaltered.
  • Cons:
    • Requires more initial setup and understanding of AEM inheritance.
    • Can be overkill for minor changes, leading to unnecessary complexity.
    • Extended components might not seamlessly fit into existing page templates without additional adjustments.

5.2 Guidelines on Choosing the Right Approach for Different Scenarios:

  1. Nature of Changes: If the changes are minor or cosmetic, overriding might be the simpler route. For substantial modifications or additions, extending is often more robust and future-proof.
  2. Future Maintenance: Consider the likelihood of future updates to the original component. If updates are frequent or critical, extending might preserve update compatibility better than overriding.
  3. Project Scope: In smaller projects or those with a tight deadline, overriding might provide the quick results needed. Larger, longer-term projects might benefit from the scalability and maintainability of extending.
  4. Compatibility and Integration: Evaluate how the modified component needs to integrate with the rest of the system. Overriding is beneficial for seamless integration with existing templates and workflows. If the component introduces new functionalities that diverge significantly from the original design, extending may be more appropriate.
  5. Skillset and Resources: Consider the technical skills available within your team. Overriding can be more straightforward but requires careful handling to avoid breaking changes. Extending requires a deeper understanding of AEM’s component hierarchy and inheritance but is generally safer and more scalable.

6: Maximizing AEM Customization Through Resource Merger

6.1 Exploring Further Customization Options in AEM Beyond Component Overriding:

Adobe Experience Manager (AEM) offers a vast landscape for customization beyond the basic overriding of components. Utilizing the Resource Merger, developers can explore additional avenues such as:

  • Theme Customization: Tailor the look and feel of your AEM sites by customizing themes and stylesheets, ensuring consistency across all components and pages.
  • Content Fragment Enhancements: Enhance your content management by customizing content fragments and associated models to suit diverse content requirements.
  • Workflow Customizations: Adapt AEM workflows to fit your business processes by creating or modifying workflow models, scripts, and actions.
  • Template Adjustments: Beyond individual components, leverage the Resource Merger to customize page templates, enabling new layouts and structures for your digital content.

6.2 Tips for Maintaining and Managing Customized Components Effectively:

  1. Regular Audits: Periodically review your customized components to ensure they are still necessary and functioning as intended. Remove or update obsolete customizations.
  2. Documentation: Maintain comprehensive documentation for each customization, including the purpose, implementation details, and any dependencies.
  3. Version Control: Utilize version control systems to manage changes to customized components, facilitating easier rollbacks and collaboration.
  4. Testing Strategies: Implement rigorous testing strategies, including automated tests, to ensure customizations do not adversely affect site functionality or performance.
  5. Community Engagement: Engage with the AEM community to stay updated on best practices and common customization patterns.


Throughout this post, we’ve explored the intricacies of AEM’s Resource Merger and the art of component overriding and extending. From understanding the basics to diving into advanced customization techniques, AEM offers a powerful platform for delivering personalized and dynamic digital experiences.

I encourage you to experiment with component overriding and extending within your AEM projects. Explore the balance between maintaining the core integrity of AEM components while injecting your unique business requirements and creative flair.

I invite you to share your experiences, challenges, or questions about AEM customization in the comments below. Whether you’re a seasoned AEM veteran or new to the platform, your insights and inquiries contribute to a richer understanding of AEM’s capabilities for all of us. Let’s learn from each other and push the boundaries of what’s possible with AEM!

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