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Sling Capabilities Module

This module is part of the Apache Sling project.

The servlet provided by this module allows for creating Capabilities HTTP endpoints on a Sling instance: Resources that provide information on which services are available, version levels etc.

To avoid exposing more information than strictly needed, this module provides two mechanisms, described below, to control where resources that expose capabilities can be found in the Sling resource tree, as well as which CapabilitiesSource services are considered by those resources.

The example configuration below shows the JSON vocabulary used to expose capabilities.

For now, we have one module which provides a CapabilitiesSource, that's the sling-org-apache-sling-capabilities-jcr module which provides information on the JCR repository.

CapabilitiesSource services

Services that implement the CapabilitiesSource interface provide capabilities.

Each service must have its own unique namespace, used to split the capabilities in categories that can be provided separately (see below):

public interface CapabilitiesSource {

    /** @return the namespace to use to group our capabilities.
     *  That name must be unique in a given Sling instance.
    String getNamespace();

    /** @return zero to N capabilities, each being represented by
     *      a key/value pair.
     * @throws Exception if the capabilities could not be computed.
    Map<String, Object> getCapabilities() throws Exception;

The sling/capabilities resource type

To act as an endpoint for capabilities a resource must have the sling/capabilities resource type.

A required property named namespace_patterns must be present, containing 1..N Java regexp patterns to select which capabilities namespaces are exposed by this resource.

Write access to such resources should be strictly controlled to avoid leaking unwanted information, along with the CapabilitiesServlet path restrictions described below.

CapabilitiesServlet configuration

To restrict access to capabilities, the CapabilitiesServlet requires a configuration (see example below) that specifies which paths patterns are acceptable for sling/capabilities resources.

The idea is to strictly control write access to these paths, so that even if users can create sling/capabilities resources elsewhere they will not expose capabilities data.

If the path of a sling/capabilities resource does not match any of the configured patterns, the servlet returns a 403 status code saying Invalid path.

Example configuration

This module does not provide any active CapabilitiesSource out of the box, but it provides a SlingServletsSource that can be used to exposes which Sling servlets are active, including their sling.servlet.* properties for reference.

With the example configuration below a sling/capabilities resource with namespace_patterns='org\.apache\.sling\.servlets\.test[A|B]' and a path that matches /var/capabilities/.* produces the following output at /var/capabilities/caps.json with the resource shown below:

  "": {
    "data": {
      "": {
        "GetAclServlet_89b2e8f3": {
          "sling.servlet.extensions": "json",
          "sling.servlet.methods": "GET",
          "sling.servlet.resourceTypes": "sling/servlet/default",
          "sling.servlet.selectors": [
      "": {
        "ChangeUserPasswordServlet_458ad2ce": {
          "sling.servlet.methods": "POST",
          "sling.servlet.resourceTypes": "sling/user",
          "sling.servlet.selectors": "changePassword"

The configured testC namespace is omitted due to the namespace_patterns property of the resource shown below.

Here's the required configuration, excerpted from /system/console/status-Configurations:

resourcePathPatterns = [/var/capabilities/.*]

Factory PID =
capabilitiesNamespaceSuffix = testA
servletsLdapFilter = (&(sling.servlet.extensions=json)(sling.servlet.selectors=acl))    

Factory PID =
capabilitiesNamespaceSuffix = testB
servletsLdapFilter = (&(sling.servlet.resourceTypes=sling/user)(sling.servlet.selectors=changePassword))

Factory PID =
capabilitiesNamespaceSuffix = testC
servletsLdapFilter = (sling.servlet.extensions=html)

And a resource that then generates the above output can be created with

curl -u admin:admin \
  -Fsling:resourceType=sling/capabilities \
  -Fnamespace_patterns='org\.apache\.sling\.servlets\.test[A|B]' \