Stop Writing Messy AEM Code—HTL Is the Only Templating Language That Gets It Right

In this blog post, we’ll take a deep dive into HTL, exploring its core features, best practices, and how you can write efficient and maintainable code using this language. Whether you’re new to AEM development or looking to refine your skills, this guide will help you harness the full potential of HTL.

What is HTL?

HTL is a server-side templating language specifically designed for AEM. It allows developers to create reusable HTML templates with embedded logic, separating the presentation layer from business logic. Unlike JSP, which often mixes Java code directly into the markup, HTL enforces a cleaner separation of concerns, making it easier to maintain and debug.

Key features of HTL include:

• Security by default: Automatic XSS protection ensures that output is sanitized unless explicitly overridden.
• Readable syntax: HTL uses familiar HTML-like syntax, reducing the learning curve for front-end developers.
• Integration with Sling Models: HTL works seamlessly with Sling Models, enabling clean data binding between Java objects and templates.
• Performance optimization: HTL compiles into Java classes during runtime, ensuring high performance without sacrificing flexibility.

Best Practices for Writing HTL Components

HTML Template Language (HTL) is a game-changer for building dynamic, maintainable, and scalable web experiences in Adobe Experience Manager (AEM). But like any powerful tool, its true potential is unlocked only when used thoughtfully.

Naming Conventions

When naming variables, models, and resources, aim for consistency and descriptiveness. For example, instead of using generic names like item or data, adopt prefixes that reflect the component or context. A good rule of thumb is to use a format like componentName-property. This will make your code more readable and also help avoid naming conflicts, especially in larger projects.

<sly data-sly-use.hero="com.example.models.HeroComponent">
    <h1>${hero.title}</h1>
</sly>

Here, hero clearly indicates that the variable belongs to the HeroComponent. This small step can save hours of debugging and confusion down the line.

Avoid Inline Logic: Keep Your Templates Clean

One of the most common pitfalls in HTL development is embedding complex logic directly into templates. While it might seem convenient to include a quick calculation or condition within your HTL file, this approach quickly leads to cluttered and hard-to-maintain code.

Instead, delegate complex logic to Sling Models. These Java-based classes are designed to handle business logic and expose data to your HTL templates in a clean, reusable way. By keeping your HTL files focused on rendering, you ensure that they remain lightweight and easy to read.

For instance, instead of writing:

<p>${properties.showBanner ? 'Banner Visible' : 'Banner Hidden'}</p>

You could move the logic to a Sling Model:

@Model(adaptables = Resource.class)
public class BannerComponent {
    @ValueMapValue
    private boolean showBanner;

    public String getBannerStatus() {
        return showBanner ? "Banner Visible" : "Banner Hidden";
    }
}

And then reference it in your HTL:

<sly data-sly-use.banner="com.example.models.BannerComponent">
    <p>${banner.bannerStatus}</p>
</sly>

This separation of concerns improves readability and also makes your code easier to test and debug.

Optimize Loops and Iterations: Preprocess Data Early

Loops are a common feature in HTL, especially when working with collections like lists or arrays. However, iterating over large datasets directly in your templates can lead to performance bottlenecks. To avoid this, preprocess your data in Sling Models before passing it to your HTL files.

For example, if you’re displaying a list of products, filter and sort the dataset in your model:

@Model(adaptables = Resource.class)
public class ProductListComponent {
    @ValueMapValue
    private List<Product> allProducts;

    public List<Product> getFilteredProducts() {
        return allProducts.stream()
                          .filter(product -> product.isActive())
                          .sorted(Comparator.comparing(Product::getName))
                          .collect(Collectors.toList());
    }
}

Then, iterate over the preprocessed list in your HTL:

<ul data-sly-list.product="${productList.filteredProducts}">
    <li>${product.name}</li>
</ul>

This approach minimizes runtime overhead and keeps your HTL templates clean and focused on rendering.

Use Descriptive Variable Names: Context Matters

Imagine trying to decipher what item refers to in a loop. Is it a product? A blog post? A user profile? Without context, even simple code can become confusing. That’s why using descriptive variable names is crucial.

Instead of defaulting to generic names as item, choose names that reflect the context of your data. For example:

<ul data-sly-list.tag="${model.tags}">
    <li>${tag.title}</li>
</ul>

Here, tag immediately conveys that the variable represents a tag object. This small change can make a big difference in how quickly others (and the future you) can understand your code.

Expression Language Guidelines: Writing Cleaner and Safer HTL Code

When working with HTML Template Language (HTL), mastering the expression language is key to writing efficient, secure, and maintainable code. Expressions in HTL allow you to dynamically inject data into your templates, but how you use them can make a significant difference in the quality of your code. Below, we’ll explore some of the important guidelines for using HTL’s expression language effectively.

Set Display Context Only When Necessary

One of the standout features of HTL is its ability to automatically determine the appropriate display context for expressions. This means that in most cases, you don’t need to explicitly set a display context. For example:

<p>${teaser.title}</p>

Here, HTL will automatically infer that teaser.title should be treated as plain text. Explicitly setting a display context is only necessary when the default behaviour doesn’t align with your needs. For instance, if you’re injecting HTML content, you might specify the html context:

<div>${teaser.htmlContent @ context='html'}</div>

By avoiding unnecessary context declarations, you keep your code cleaner and easier to read.

Use the Safest Possible Display Context

Security should always be a priority when working with dynamic content. HTL provides a range of display contexts to ensure your output is properly escaped and sanitized. To maximize security, always choose the safest context that fits your scenario. Here’s a quick breakdown of common contexts:

• number : For numeric values.
• uri : For URLs or paths.
• text : For plain text content.
• html : For injecting safe HTML markup.

For example, if you’re injecting a URL, use the uri context:

<a href="${link.url @ context='uri'}">Visit Link</a>

Choosing the wrong context can expose your application to vulnerabilities like Cross-Site Scripting (XSS). Always err on the side of caution and select the most restrictive context that meets your requirements (full list of Display Context).

Avoid Unnecessary Expressions for Literals

It might seem obvious, but redundant expressions can clutter your code and make it harder to maintain. For example, consider this snippet:

<p>${"Hello World"}</p>

This is functionally identical to:

<p>Hello World</p>

By removing unnecessary expressions, you simplify your templates and reduce cognitive load for anyone reading your code. Reserve expressions for dynamic data, not static content.

Simplify Code with Logical Operators

HTL supports logical operators, which can help you write cleaner and more concise code. Instead of relying on ternary operators, you can often achieve the same result with logical || (OR) or && (AND) operators.

For example, instead of writing:

<p>${properties.subtitle ? properties.subtitle : 'No subtitle available'}</p>

You can simplify it to:

<p>${properties.subtitle || 'No subtitle available'}</p>

This approach reduces verbosity and improves readability. Logical operators are particularly useful for providing fallback values or handling optional properties.

Utilize Native URI Manipulation Features

Building URLs manually can be error-prone and time-consuming. Fortunately, HTL includes built-in URI manipulation capabilities that simplify this process. For example, you can append query parameters or modify file extensions without hardcoding strings:

<a href="${link.url @ extension='html'}">Read More</a>

Using these native features ensures consistency and reduces the risk of mistakes. Rolling your own URI builder or hardcoding URLs is discouraged because it introduces unnecessary complexity and the potential for errors.

Block Statements Best Practices: Writing Clean and Maintainable HTL Code

Block statements are the backbone of HTML Template Language (HTL), enabling developers to inject logic, iterate over data, and structure templates efficiently. However, using block statements effectively requires adherence to best practices that prioritize clarity, performance, and maintainability. Below, we’ll explore some important guidelines for working with block statements in HTL.

Use sly Tag for Non-Markup Elements

The sly tag is a powerful tool in HTL that allows you to include logic or functionality without cluttering your final HTML output. Any element of the sly tag is automatically unwrapped during rendering, meaning it won’t appear in the final markup.

For example:

<sly data-sly-use.teaser="com.example.models.TeaserComponent">
    <h1>${teaser.title}</h1>
</sly>

The sly tag ensures that only the <h1> element appears in the rendered HTML. This keeps your templates clean and avoids unnecessary elements in the DOM.

Note: If you’re using HTL 1.0 (AEM 6.0), you’ll need to explicitly add the data-sly-unwrap attribute to achieve the same behaviour.

Organize data-sly-use Statements at the Top Level

When working with data-sly-use, it’s crucial to place these statements at the top level of your template. Since identifiers created with data-sly-use are global, organizing them at the top makes it easier to spot naming conflicts and prevents redundant initializations.

For example:

<sly data-sly-use.teaser="com.example.models.TeaserComponent"
     data-sly-use.navigation="com.example.models.NavigationComponent">
    <h1>${teaser.title}</h1>
    <nav>${navigation.links}</nav>
</sly>

This approach ensures that all dependencies are declared upfront, improving readability and maintainability.

Follow lowerCamelCase Naming Convention

Consistency in naming conventions improves code readability and reduces confusion. In HTL, identifiers should follow the lowerCamelCase convention, where the first word is lowercase, and subsequent words are capitalized (e.g., sampleIdentifierName).

For example:

<sly data-sly-use.teaserComponent="com.example.models.TeaserComponent">
    <h1>${teaserComponent.title}</h1>
</sly>

This standard aligns with common programming practices and ensures compatibility with HTL’s internal handling of identifiers.

Reuse Expressions with Identifiers

Reusing expressions not only optimizes performance but also enhances code clarity. Instead of repeating the same expression multiple times, define an identifier with data-sly-set and reuse it:

<sly data-sly-set.fullName="${user.firstName + ' ' + user.lastName}">
    <p>Welcome, ${fullName}!</p>
    <p>Your profile is ready, ${fullName}.</p>
</sly>

This approach allows the HTL compiler to cache the result, reducing redundant evaluations and improving efficiency.

Use Descriptive Variable Names in Lists

When iterating over lists, avoid using default variable names like item. Instead, choose names that reflect the context of the data. For example:

<ul data-sly-list.tag="${model.tags}">
    <li>${tag.title}</li>
</ul>

Here, tag provides meaningful context, making the code easier to read and understand.

Prioritize Block Statements Over Regular Attributes

Block statements should always come before regular HTML attributes. This ensures that variables declared via data-sly-use or other block statements are available when needed. Additionally, block statements often determine whether an element appears at all (data-sly-test) or multiple times (data-sly-repeat), making their placement critical.

<div data-sly-test="${properties.showBanner}" class="banner">
    <p>This banner is visible!</p>
</div>

Placing data-sly-test before the class attribute ensures logical consistency and avoids potential issues.

Leverage Existing HTML Elements for Block Statements

Whenever possible, attach block statements to existing HTML elements instead of creating additional wrappers. This keeps your templates concise and avoids unnecessary complexity.

For example:

<h1 data-sly-text="${teaser.title}"></h1>

This approach eliminates the need for extra <sly> tags or divs, resulting in cleaner code.

Avoid Certain Block Statement Types

Certain block statements, such as element, attribute, and text, can make your code harder to read and maintain. Instead, opt for more explicit and straightforward alternatives.

For example, instead of:

<div data-sly-element="${headlineElement}">${event.year}</div>
<span data-sly-text="${event.year}"></span>
<a data-sly-attribute.href="${event.link}" href="#"></a>

Use:

<h2>${event.year}</h2>
<p>${event.year}</p>
<a href="${event.link}"></a>

This simplifies your templates and avoids unnecessary abstraction.

Separate Template Definitions

Templates should be defined in separate files to keep your codebase organized and modular. This approach makes it easier to reuse templates across components and improves maintainability.

For example:

<!-- teaser.html -->
<template data-sly-template.teaser="${@ title, text}">
    <h1>${title}</h1>
    <p>${text}</p>
</template>

<!-- main.html -->
<sly data-sly-use.teaserModel="com.example.models.TeaserComponent"
     data-sly-call="${teaser @ title=teaserModel.title, text=teaserModel.text}"></sly>

By separating concerns, you create a cleaner and more scalable architecture.

Prefer data-sly-set Over data-sly-test

While data-sly-test can be used to set variable bindings, it’s better to use data-sly-set for this purpose. Using data-sly-test for variable assignments can unintentionally hide elements if the expression evaluates to false, leading to debugging challenges.

For example, instead of:

<sly data-sly-test.fullName="${user.firstName + ' ' + user.lastName}">
    <p>Welcome, ${fullName}!</p>
</sly>

Use:

<sly data-sly-set.fullName="${user.firstName + ' ' + user.lastName}">
    <p>Welcome, ${fullName}!</p>
</sly>

This approach avoids unintentionally hiding elements if the expression evaluates to false. It also ensures predictable behaviour and makes debugging easier, especially when multiple data-sly-test statements are involved. By using data-sly-set, you clearly define variables without affecting the visibility of elements.

Minimize Unnecessary sly Tags

Avoid wrapping elements with sly tags unless absolutely necessary. Instead, attach block statements directly to relevant HTML elements. This reduces visual noise and makes your intentions clearer.

For example:

<h1 data-sly-text="${teaser.title}"></h1>

This is cleaner than:

<sly data-sly-text="${teaser.title}"></sly>

Use Explicit End Tags for sly

Finally, always use explicit end tags for sly elements. Unlike void or foreign elements, sly must be closed with an explicit </sly> tag. Self-closing tags are not allowed and will result in errors.

For example:

<sly data-sly-use.teaser="com.example.models.TeaserComponent">
    <h1>${teaser.title}</h1>
</sly>

This ensures compliance with HTL syntax and avoids rendering issues.

Final Thoughts

In summary, HTL stands out as a powerful tool that not only enhances security and efficiency in component development but also enforces a clear separation between presentation and business logic. By following best practices — such as adhering to proper naming conventions, modularizing components, and leveraging built-in mechanisms like the Use-API — developers can build robust, maintainable, and scalable applications in AEM.

Remember, while HTL simplifies many aspects of component rendering, the true value lies in how you structure and organize your code. Investing time in establishing consistent coding standards and reusing expressions where possible can significantly reduce technical debt and streamline future development efforts.

As you refine your HTL components, consider auditing your existing code to ensure it adheres to these best practices. To keep your templates clean, embrace the use of Sling Models and externalized logic, and always strive for a balance between simplicity and functionality.

Remember that great HTL code tells a story. When another developer looks at your components, they should understand not just what the code does but why it’s structured that way. This approach to writing clear, purposeful code is what sets apart exceptional AEM implementations.

Additional Resources and References

• Adobe Getting Started with HTL
• HTML Template Language Specification
• HTL Developer Tools
• AEM HTL Style Guide
• Adobe Code Samples