Sightly vs JSP: Migrating Legacy AEM Components to HTL

Adobe Experience Manager projects often carry years of component decisions, custom tag libraries and JSP files that still render reliably. That stability can make a migration feel risky, particularly when a site supports several brands, publishing regions or integrations. Yet legacy presentation logic tends to become harder to test, secure and hand over as the platform evolves.

For Australian teams, the work also sits within a practical delivery environment: Sydney and Melbourne agencies frequently support clients across several time zones, while Brisbane, Perth and Adelaide teams may share release duties remotely. A migration needs to respect those operating patterns, existing author habits and the commercial reality of keeping a live website available while components are modernised.

Why legacy JSP components become difficult to maintain

JSP allowed developers to mix markup, Java scriptlets, tag libraries and business decisions in one file. In an early AEM implementation, that approach could be quick and familiar. Over time, a component might acquire permission checks, URL construction, analytics attributes, fallback content and calls to services, all embedded in a template that is difficult to reason about.

HTL, formerly known as Sightly, establishes a clearer boundary. The template describes the intended HTML, while Java code prepares the data through Sling Models, Use-API classes or other approved services. This separation improves readability and reduces the chance that a content authoring change will require someone to edit presentation code.

The difference is especially valuable for teams working under Australian privacy and accessibility expectations. Keeping output logic predictable makes it easier to review consent-related attributes, keyboard-friendly markup and content supplied to public-sector or regulated clients.

How HTL changes the component model

JSP developers often begin by translating each expression into an HTL expression. That is rarely enough. HTL is deliberately limited: it does not provide a general-purpose programming language inside the template. Conditions, iteration, escaping and element attributes are available, but substantial processing belongs in a model or service.

A JSP expression such as ${properties.title} may become ${model.title} in HTL, with a Sling Model exposing a typed and validated value. Repeated conditions should be simplified before they reach the template. For example, a model can provide hasImage, ctaUrl and accessibleLabel, allowing the HTL file to remain focused on rendering rather than implementation detail.

This pattern also makes ownership clearer. Front-end developers can work on HTML and CSS without navigating Java code, while Java developers can test mapping, defaults and service interactions independently. Teams looking for broader developer-community context can compare approaches through CIRCUIT recordings, which document technical discussions around AEM architecture and implementation.

Map the JSP before changing the markup

A safe migration starts with an inventory rather than a rewrite. Record each component’s resource type, selectors, dialog fields, client libraries, edit configurations, inherited behaviours and references from other components. Search for JSP includes, tag files, custom tags, scriptlets and calls to services. This often reveals hidden dependencies that are absent from the component dialog.

Render samples should be captured for meaningful content states: empty fields, long headings, missing images, multiple child items, unpublished references and authoring mode. Include mobile variations and pages with unusual content policies. For Australian sites, test real patterns such as suburb and state fields, Australian phone numbers, postal addresses and dates displayed in the expected local format.

The mapping exercise should identify whether a JSP is a simple view or a facade for several responsibilities. A component that appears to be a card may also select DAM renditions, build tracking parameters and apply business rules. Splitting those concerns before migration is usually safer than reproducing the old file line by line.

Move business logic into Sling Models

Sling Models are a common destination for logic extracted from JSP. Use injection carefully: explicit @ValueMapValue, @ChildResource and @OSGiService declarations are easier to understand than broad, permissive injection. Define optional fields where content may legitimately be absent, and provide sensible defaults without hiding authoring errors.

Models should expose presentation-ready data rather than reproduce repository structure. A model might return a resolved image URL, a safe link target, a label for screen readers and a list of child items. It should not force HTL to know which DAM path, node property or service call produced those values.

External integrations deserve particular scrutiny. If a legacy component obtains assets or metadata from another platform, isolate the client in an OSGi service, add timeouts and define a fallback. A useful reference for this kind of design is the guide to REST DAM integration, particularly when a migration exposes old assumptions about repository-local assets.

Rebuild dialogs and authoring behaviour

A visual rewrite can fail if the dialog remains confusing. Review field names, defaults, required states, validation and help text as part of the component migration. Avoid changing stored property names unless there is a clear benefit; a compatibility layer or model mapping can preserve existing content while new authoring options are introduced.

HTL also changes how empty values behave. A missing property can result in no element, an empty attribute or an unwanted wrapper, depending on the expression. Use data-sly-test, data-sly-list and data-sly-resource deliberately, and inspect the generated HTML rather than assuming the result matches JSP.

Authoring teams should be included early. A content producer in Parramatta or Geelong may use a component differently from the developer who built it. Short review sessions with authors can uncover practical issues, such as a title field that needs plain-language guidance or a link field that must support documents as well as pages. In everyday Australian workplace language, a quick “show us how this works in the editor” session can save a fair bit of rework.

Protect security, performance and accessibility

HTL’s context-aware escaping is a major advantage over manually assembled JSP output, but it does not make unsafe data safe in every context. Check URL, HTML, attribute, style and script contexts separately. Avoid allowing author-controlled values into JavaScript or inline styles without strict validation and an appropriate design.

Performance testing should compare more than server response time. Measure model execution, repository queries, remote calls, cache behaviour and the number of rendered DOM nodes. A migration is a good opportunity to remove repeated queries from loops, request only required data and replace remote calls during page rendering with cached or author-time processes where appropriate.

Accessibility needs rendered-output testing. Check heading hierarchy, link purpose, image alternatives, focus order and announcements for dynamic content. Australian organisations commonly refer to WCAG when procuring digital work, so accessibility should be treated as a release requirement rather than a final visual check. Test with keyboard navigation and assistive technology, not just automated tooling.

Roll out the migration in controlled stages

Avoid replacing every JSP component in one release. Select a component family with clear boundaries, build an HTL version under a new resource type or controlled variation, and compare its output against approved fixtures. Where content compatibility is strong, a model can support both old and new properties during a transition period.

Automated tests should cover Sling Models, service failures, escaping and representative HTML. Browser tests can verify authoring mode, responsive behaviour and analytics attributes. Run the comparison against pages with genuine production complexity, including campaign landing pages and older content that has passed through several migrations.

Deployment planning matters for distributed Australian operations. Schedule authoring freezes and validation windows so teams in Perth are not forced into an awkward overnight handover with colleagues on the east coast. Keep rollback straightforward, monitor error logs and agree who owns triage after release. A small pilot, followed by measured adoption, is often more reliable than a large cutover.

A migration can also be paired with professional development. Developers who want to explore adjacent tooling, templating and language technologies may find relevant community material through Xtext developer resources, while AEM specialists can use conference archives to compare architectural decisions and implementation trade-offs.

Measure the result after launch

Success is not simply the disappearance of JSP files. Track rendering errors, author support requests, page performance, accessibility defects and the time required to change a component. Ask whether a new developer can understand the model and template without reconstructing years of project history.

Review the migration after several publishing cycles. Some issues only appear when authors create unusual content, when a remote service is unavailable or when a campaign introduces a new combination of fields. Use those findings to improve component standards, model conventions, test fixtures and code review guidance.

The strongest HTL implementations are intentionally ordinary: small templates, explicit models, predictable output and limited hidden behaviour. That simplicity gives teams room to evolve integrations, improve front-end experiences and maintain AEM sites across agencies, vendors and internal departments.

Start with one representative legacy component, document its current behaviour and create a tested HTL version beside it. Involve authors, front-end specialists and Java developers from the first review, then use the evidence from that pilot to shape the wider AEM migration.