Home » Documentation » The Sling Engine


What is a Resource

The Resource is one of the central parts of Sling. Extending from JCR's Everything is Content, Sling assumes Everthing is a Resource. Thus Sling is maintaining a virtual tree of resources, which is a merger of the actual contents in the JCR Repository and resources provided by so called resource providers. By doing this Sling fits very well in the paradigma of the REST architecture.

Resource Properties

Resources have a number of essentiall properties:

Property Description
Path Resources are part of a Resource Tree. As such each Resource has a path which is formed by concatenating the names of all Resources along the root to the Resource separated by a slash. Ok, really, this is much like a URL path or a file system path where the slash (/) is the separator character.
Name The name of the Resource is the last element (or segment) in the path.
Resource Type Each resource has a resource type which is used by the Servlet and Script resolver to find the appropriate Servlet or Script to handle the request for the Resource.
Resource Super Type The (optional explicit) super type of the Resource. See the section Resource Types below for more details.
Adapters Resources are always Adaptable and therefore can be adapted to a different view. See the section Resource Adapters below for more details.
Metadata Resources in general support link text providing access to values such as the length of a binary resource (which can be streamed) or the Resource's content type.

For a complete description of the Resource interface, please refer to the link text JavaDoc.

Resource Types

The exact method of setting the resource type for a Resource depends on the actual Resource Provider. For the four main Resource Provider implementations provided by Sling, the assignments are as follows:

Provider Resource Type Resource Super Type
JCR The value of the sling:resourceType property or the primary node type if the property is not set (a namespace separator colon is replaced by a slash, e.g. the nt:file primary node type is mapped to the nt/file resource type The value of the sling:resourceSuperType of the Resource node or resource super type of the resource pointed to by the resource type (when accessed with ResourceResolver.getResource(String)
File System File based resources are of type nt/file; folder based resources are of type nt/folder corresponding to the respective JCR primary node type none
Bundle File based resources are of type nt/file; folder based resources are of type nt/folder corresponding to the respective JCR primary node type none
Servlet The absolute path of the resource appended with the suffix .servlet sling/bundle/resource

Resource Types form a type hierarchy much like Java classes form a type hierarchy. Each resource type has a resource super type, either explicitly defined as for example for JCR or Servlet Resources or implicitly. The implicit Resource Super Type is at the same time the root Resource Type much like the java.lang.Object class is called sling/servlet/default (for historical reasons). The sling/servlet/default Resource Type is the only type without a super type.


The object types to which Resources may be adapted depend mostly depends on the Resource Provider providing the resource. For example all JCR node based resources always adapt to javax.jcr.Node objects.

If the actual Resource object class implementation extends from the SlingAdaptable class, then in addition all AdapterFactory services adapting Resource objects are considered when trying to adapt the Resource. In general Resource Providers are recommended to have their Resource implementation extend from link text which guarantees the Resource implementation to extend from SlingAdaptable and thus supporting Adapter Factories.

How to get a Resource

To get at Resources, you need a ResourceResolver. This interface defines four kinds of methods to access resources:

Absolute Path Mapping

As has been said, the absolute path mapping methods resolve(HttpServletRequest, String) and resolve(String) apply some implementation specific path matching algorithm to find a Resource. The difference between the two methods is that the former may take more properties of the HttpServletRequest into account when resolving the Resoure, while the latter just has an absolute path to work on.

The general algorithm of the two methods is as follows:

  1. Call HttpServletRequest.getScheme(), .getServerName(), getServerPort to get an absolute path out of the request URL: [scheme]()/[host].[port][path] (resolve(HttpServletRequest, String) method only, which)
  2. Check whether any virtual path matches the absolute path. If such a match exists, the next step is entered with the match.
  3. Apply a list of mappings in order to create a mapped path. The first mapped path resolving to a Resource is assumed success and the Resource found is returned.
  4. If no mapping created a mapped path addressing an existing Resource, the method fails and returns a NonExistingResource (for the
    resolve(String) and resolve(HttpServletRequest,String)) or null (for the getResource(String path) and getResource(Resource base, String path) methods).

The virtual path mapping may be used to create shortcut URLs for otherwise long and complicated URLs. An example of such an URL might be the main administrative page of a CMS system. So, administrators may access the root of the web application and directed to the main administrative page.

The path mapping functionality may be used to hide internal resource organization from the request URL space. For example to better control the structure of your repository, you might decide to store all accessible data inside a /content subtree. To hide this fact from the users, a mapping may be defined to prefix all incoming paths with /content to get at the actual Resource.

The map(String) applies the path mapping algorithm in the reverse order. That is, first the path mappings are reversed and then any virtual mappings are checked. So, a path /content/sample might be mapped /sample to revers the /content prefixing. Or the main administrative page - say /system/admin/main.html - may be mapped to the virtual URL /.

More details on mappings can be found at Mappings for Resource Resolution.

Relative Path Resolution

Sometimes it is required to resolve relative paths to Resources. An example of such a use case is Script and Servlet resolution which starts with a relative path consisting of the Resource type, optional selectors and the request extension or method name. By scanning a search path for these relative paths a system provided Resource may be overwritten with some user defined implementation.

Consider for example, the system would provide a Servlet to render Resources of type nt:file. This Servlet would be registered under the path /libs/nt/file/html. For a certain web application, this default HTML rendering might not be appropriate, so a Script is created as /apps/nt/file/html.jsp with a customized HTML rendering. By defining the search path to be [/apps,/libs] the Servlet resolver would call the ResourceResolver.getResource(String) method with the relative path nt/file/html and be provided with the first matching resource - /apps/nt/file/html.jsp in this example.

Of course the search path is not used for absolute path arguments.

Querying Resources

For convenience the ResourceResolver provides two Resource querying methods findResources and queryResources both methods take as arguments a JCR query string and a query language name. These parameters match the parameter definition of the QueryManager.createQuery(String statement, String language) method of the JCR API.

The return value of these two methods differ in the use case:

These methods are convenience methods to more easily post queries to the repository and to handle results in very straight forward way using only standard Java functionality.

Please note, that Resource querying is currently only supported for repository based Resources. These query methods are not reflected in the ResourceProvider interface used to inject non-repository Resources into the Resource tree.

Providing Resources

The virtual Resource tree to which the the Resource accessor methods resolve and getResource provide access is implemented by a collection of registered ResourceProvider instances. The main Resource provider is of course the repository based JcrResourceProvider which supports Node and Property based resources. This Resource provider is always available in Sling. Further Resource providers may or may not exist.

Each Resource provider is registered as an OSGi service with a required service registration property provider.roots. This is a multi-value String property listing the absolute paths Resource tree entries serving as roots to provided subtrees. For example, if a Resource provider is registered with the service registration property provider.roots set to /some/root, all paths starting with /some/root are first looked up in the given Resource Provider.

When looking up a Resource in the registered Resource providers, the ResourceResolver applies a longest prefix matching algorithm to find the best match. For example consider three Resource provider registered as follows:

When accessing a Resource with path /some/path/resource the Resource provider R2 is first asked. If that cannot provide the resource, Resource provider R1 is asked and finally the JCR Resource provider is asked. The first Resource provider having a Resource with the requested path will be used.

JCR-based Resources

JCR-based Resources are provided with the default JcrResourceProvider. This Resource provider is always available and is always asked last. That is Resources provided by other Resource providers may never be overruled by repository based Resources.

Bundle-based Resources

Resources may by provided by OSGi bundles. Providing bundles have a Bundle manifest header Sling-Bundle-Resources containing a list of absolute paths provided by the bundle. The path are separated by comma or whitespace (SP, TAB, VTAB, CR, LF).

The BundleResourceProvider supporting bundle-based Resources provides directories as Resources of type nt:folder and files as Resources of type nt:file. This matches the default primary node types intended to be used for directories and files in JCR repositories.

For details see Bundle Resource.

Servlet Resources

Servlet Resources are registered by the Servlet Resolver bundle for Servlets registered as OSGi services. See Servlet Resolution for information on how Servlet Resources are provided.

File System Resources

The Filesystem Resource Provider provides access to the operating system's filesystem through the Sling ResourceResolver. Multiple locations may be mapped into the resource tree by configuring the filesystem location and the resource tree root path for each location to be mapped.

For details see File System Resources.

Merged Resources

The merged resource provider exposes a view on merged resources from multiple locations.

For details see Resource Merger.

Custom Resource providers

Custom ResourceProvider services can be used to integrate your own custom resources in the Sling resource tree.

For a simple example of that, see the PlanetResourceProvider used in our integration tests.

Writeable Resources

Sling now supports full CRUD functionality on Resources, without necessarily having to go through the JCR API.

The advantage is that this works for any ResourceProvider that supports the required operations.

See the testSimpleCRUD method in WriteableResourcesTest for a basic example of how that works. More details can be found at Sling API CRUD Support.

Resource Observation

To be notified whenever certain resources or their properties have been modified/added/removed there are different possibilities

Resource Observation API (ResourceChangeListener)

This API is only available since Sling API 2.11.0 (SLING-4751).

Register an OSGi service for org.apache.sling.api.resource.observation.ResourceChangeListener or org.apache.sling.api.resource.observation.ExternalResourceChangeListener to be notified about changes. Certain properties can be used to restrict subscription to only a subset of events.

OSGi Event Admin

Resource events are sent out via the OSGi Event Admin. You can subscribe to those event by registering an OSGi service for org.osgi.service.event.EventHandler. Several properties should be used to restrict the subscription to only the relevant event. The event topics which are used for resources are listed as constants in org.apache.sling.api.SlingConstants starting with the prefix TOPIC_.

Wrap/Decorate Resources

The Sling API provides an easy way to wrap or decorate a resource before returning. Details see Wrap or Decorate Resources.

Rev. 1784234 by kwin on Fri, 24 Feb 2017 08:00:34 +0000
Apache Sling, Sling, Apache, the Apache feather logo, and the Apache Sling project logo are trademarks of The Apache Software Foundation. All other marks mentioned may be trademarks or registered trademarks of their respective owners.