Apache Sling

Sling turns content into a request handler

Apache Sling is the layer between the repository and the HTTP request. Its founding idea is inverted from most web frameworks: instead of a route table that maps URLs to controllers, Sling maps the URL to a resource in the repository, then asks the resource what should render it. Content is king, and the content decides its own renderer.

Request URL → resolve to a resource → read its sling:resourceType → find a script or servlet registered for that type → render.

URL decomposition

A Sling URL carries more structure than it looks. Take:

/content/wknd/us/en/home.print.a4.html/archive/2024?debug=true
\__________________________/ \_______/ \__/ \____________/ \________/
        resource path         selectors  ext     suffix       query

• Resource path — the node Sling resolves to: /content/wknd/us/en/home.
• Selectors — dot-separated tokens (print, a4) that let one resource render several ways. Cacheable, unlike query parameters.
• Extension — html, json, csv. Chooses the output format.
• Suffix — everything after the extension, passed to your code as extra path information. Also cacheable.
• Query string — the only part the Dispatcher will usually refuse to cache.

Resource types: content pointing at code

Every renderable node carries a sling:resourceType property, whose value is a path (usually relative) to a component: wknd/components/title. Sling looks for that under /apps first, then /libs. That search order is exactly what makes the overlay mechanism work — your /apps copy wins over Adobe's /libs original.

sling:resourceSuperType declares inheritance. A component that sets sling:resourceSuperType = "core/wcm/components/title/v3/title" inherits that component's scripts and behaviour, and can override just the pieces it wants. This is the mechanism behind the Core Components proxy pattern you will meet on day 13.

Script resolution

Given resource type wknd/components/title and a request for .html, Sling looks for a script in /apps/wknd/components/title/ in roughly this order of specificity:

• title.html — matches the component name (the usual case)
• html.html — matches the extension
• With selectors, more specific names win: a request for .print.html prefers print.html over title.html
• HTTP method matters too: POST.html handles POSTs
• If nothing matches, Sling walks up the sling:resourceSuperType chain

Servlets can also register for a resource type in Java, via @SlingServletResourceTypes. Registering a servlet by resource type is strongly preferred over registering by path, because path-bound servlets bypass the repository's access control — a real security consideration you will revisit on day 32.

Resource vs Node

Sling wraps JCR nodes in a Resource abstraction. You mostly work with Resource, ResourceResolver and ValueMap rather than raw JCR APIs — they are simpler and not every resource is backed by a node. resource.getValueMap() gives you typed access to properties with defaults, which is what Sling Models use under the hood.