What is HTL (Sightly)? How is it different from JSP?

HTL (HTML Template Language), formerly known as Sightly, is Adobe Experience Manager's (AEM) preferred templating system for HTML. It's designed to replace JSP (JavaServer Pages) as the standard way to create the presentation layer of AEM components.

Key Features and Principles of HTL:

Logic-less: HTL emphasizes keeping logic out of the template itself. Ideally, the template focuses on presenting content, while any necessary logic is handled in separate Java or JavaScript code.
HTML-centric: HTL is designed to work seamlessly with HTML. It uses HTML attributes and elements to define how content should be displayed, making templates easier to read and maintain.
Security: HTL has built-in security features that help prevent cross-site scripting (XSS) vulnerabilities. It automatically escapes data to ensure that user-generated content is displayed safely.
Maintainability: By separating presentation from logic, HTL makes it easier to maintain and update templates. Changes to the HTML structure can be made without affecting the underlying code.
Testability: HTL templates can be easily tested, as they don't contain complex logic. This helps ensure that the presentation layer works as expected.

How is HTL different from JSP?

Feature	HTL (Sightly)	JSP (JavaServer Pages)
Logic	Logic-less, encourages separation of concerns	Allows embedding Java code directly in the template
Syntax	HTML-centric, uses HTML attributes and elements	XML-based, uses special tags and scriptlets
Security	Built-in XSS protection	Requires manual escaping to prevent XSS
Maintainability	Easier to maintain and update	Can be difficult to maintain due to embedded logic
Testability	Easier to test	Can be harder to test due to embedded logic
Performance	Can be more performant in some cases	Performance can vary depending on implementation

Why did AEM switch to HTL?

Improved Security: HTL's built-in XSS protection helps to create more secure AEM applications.
Enhanced Maintainability: Separating logic from presentation makes it easier to maintain and update templates.
Increased Productivity: HTL's simpler syntax and focus on HTML can make developers more productive.
Better Performance: In some scenarios, HTL can offer performance advantages over JSP.