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Apache Sling Scripting Bundle Tracker


Note that this code has moved to the o.a.s.servlets.resolver module - as I write this, discussions are ongoing on the Sling dev list on whether this module needs to be deprecated or repurposed.


The org.apache.sling.scripting.bundle.tracker bundle represents an add-on module that can be deployed on a Sling 10 instance or newer to enhance the scripting resolving capabilities. Being an add-on, the bundle hooks into Sling's current mechanisms for content-to-servlet resolution and essentially acts as a service registration broker for bundles providing scripting capabilities (more details in the “How” section).


Although traditionally scripts are deployed as content stored in the search paths of a Sling instance, this leaves very little room for script evolution in a backwards compatible way. Furthermore, versioning scripts is a difficult process if the only mechanism to do this is the sling:resourceType property, since consumers (content nodes or other resource types) have then to explicitly mention the version expected to be executed.

Scripts should not be considered content, since their only purpose is to actually generate the rendering for a certain content structure. They are not consumed by users, but rather by the Sling Engine itself and have very little meaning outside this context. As such, scripts should be handled like code:

  1. they provide an HTTP API;
  2. they can evolve in a semantical [1] way;
  3. they have a developer audience.


Being built around a BundleTrackerCustomizer [2], the Scripting Bundle Tracker monitors the instance's bundles wired to itself and scans the ones providing a sling.resourceType capability [3]. The wiring is created by placing a Require-Capability header in the bundles that provide the sling.resourceType capability:


A sling.resourceType capability has the following attributes:

  1. sling.resourceType:String - mandatory; defines the provided resource type; its value is a valid Java package identifier, though it does not need to correspond to one;
  2. sling.resourceType.selectors:List - optional; defines the list of selectors that this resource type can handle;
  3. sling.resourceType.extensions:List - optional; defines the list of extensions that this resource type can handle;
  4. version:Version - mandatory; defines the version of the provided resourceType;
  5. extends:String - optional; defines which resource type it extends; the version range of the extended resource type is defined in a Require-Capability.

The BundleTrackerCustomizer will register a Sling Servlet with the appropriate sling.servlet properties for each sling.resourceType capability. The servlets will be registered using the bundle context of the bundle providing the sling.resourceType capability, making sure to expose the different versions of a resource type as part of the registered servlet's properties. On top of this, a plain resource type bound servlet will also be registered, which will be automatically wired to the highest version of resourceType. All the mentioned service registrations are managed automatically by the BundleTrackerCustomizer.

So how do I deploy my scripts?

Short answer: exactly like you deploy your code, preferably right next to it. Pack your scripts using the following conventions:

  1. create a javax.script folder in your bundle;
  2. each folder under javax.script will identify a resourceType; the folder name should preferably be a valid Java package identifier, but it does not need to refer to an existing one;
  3. inside each resourceType folder create a Version folder; this has to follow the Semantic Versioning constraints described at [1];
  4. add your scripts, using the same naming conventions that you were used to from before [4];
  5. manually define your provide and require capabilities; just kidding; add the scriptingbundle-maven-plugin to your build section and add its required properties in the maven-bundle-plugin's instructions (check this example);
  6. mvn clean sling:install.

Integration Tests

The integration tests are provided by the org.apache.sling.scripting.bundle.tracker.it project. To run the integration tests execute the following command:

mvn clean verify -Pit


To play around with a Sling instance on localhost port 8080 (override with -Dhttp.port=) that has the examples installed run:

mvn clean verify -Pexample

from the org.apache.sling.scripting.bundle.tracker.it project.


[1] - https://semver.org/
[2] - https://osgi.org/javadoc/r6/core/org/osgi/util/tracker/BundleTrackerCustomizer.html
[3] - https://osgi.org/download/r6/osgi.core-6.0.0.pdf, Page 41, section 3.3.3 “Bundle Capabilities”
[4] - https://sling.apache.org/documentation/the-sling-engine/url-to-script-resolution.html