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.
Resources have a number of essentiall properties:
|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 (
|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
|Metadata||Resources in general support
For a complete description of the
Resource interface, please refer to the
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
||The value of the
|File System||File based resources are of type
|Bundle||File based resources are of type
|Servlet||The absolute path of the resource appended with the suffix
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 mostly depends on the Resource Provider providing the resource. For example all JCR node based resources always adapt to
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
AbstractResource which guarantees the Resource implementation to extend from
SlingAdaptable and thus supporting Adapter Factories.
To get at Resources, you need a
ResourceResolver. This interface defines four kinds of methods to access resources:
- Absolute Path Mapping Resource Resolution: The
resolve(String)methods are called to apply some implementation specific path matching algorithm to find a Resource. These methods are mainly used to map external paths - such as path components of request URLs - to Resources. To support creating external paths usable in an URL a third method
map(String)is defined, which allows for round-tripping.
- Absolute or Relative Path Resolution (including search path): The
getResource(Resource base, String path)methods may be used to access a resource with an absolute path directly. If it can't be found the path is assumed to be relative and the search path retrieved from
getSearchPath()is used to retrieve the resource. This mechanism is similar to resolving a programm with the
PATHenvironment variable in your favourite operating system.
- Resource Enumeration: To enumerate resources and thus iterate the resource tree, the
listChildren(Resource)method may be used. This method returns an
Iterator<Resource>listing all resources whose path prefix is the path of the given Resource. This method will of course also cross boundaries of registered
ResourceProviderinstances to enable iterating the complete resource tree.
- Resource Querying: Querying resources is currently only supported for JCR Resources through the
findResources(String query, String language)and
queryResources(String query, String language)methods. For more information see the section on Querying Resources below.
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:
HttpServletRequest.getScheme(), .getServerName(), getServerPortto get an absolute path out of the request URL: [scheme]/[host].[port][path] (
resolve(HttpServletRequest, String)method only, which)
- Check whether any virtual path matches the absolute path. If such a match exists, the next step is entered with the match.
- 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.
- If no mapping created a mapped path addressing an existing Resource, the method fails and returns a
resolve(HttpServletRequest,String)) or null (for the
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.
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.
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.
For convenience the
ResourceResolver provides two Resource querying methods
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:
Iteratory<Resource>of all Resources matching the query. This method is comparable to calling
QueryResultreturned from executing the JCR query.
Iterator<Map<String, Object>>. Each entry in the iterator is a
Map<String, Objectrepresenting a JCR result
RowIteratorreturned from executing the JCR query. The map is indexed by the column name and the value of each entry is the value of the named column as a Java Object.
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.
The virtual Resource tree to which the the Resource accessor methods
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:
- JCR Resource provider as
- Resource provider R1 as
- Resource provider R2 as
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 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.
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).
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 are registered by the Servlet Resolver bundle for Servlets registered as OSGi services. See Servlet Resolution for information on how Servlet Resources are provided.
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.
The merged resource provider exposes a view on merged resources from multiple locations.
For details see Resource Merger.
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.
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.
To be notified whenever certain resources or their properties have been modified/added/removed there are different possibilities
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 to be notified about local changes. To be also notified about external changes (i.e. changes triggered by another Sling instance leveraging a clustered repository make sure that your service implementation also implements the marker interface
org.apache.sling.api.resource.observation.ExternalResourceChangeListener. The interface
ExternalResourceChangeListener is not supposed to be registered with OSGi though. Certain properties can be used to restrict subscription to only a subset of events.
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
You receive events no matter whether they originate from the local repository or from a remote clustered repository. You can check though in your event listener for the event attribute
event.application, which is only set in case the event was triggered from an external resource modification (compare with
DEAConstants and try to reuse the constant from there).
The OSGi event handlers may be blacklisted by Apache Felix in case the processing takes too long. Therefore dispatch all long-lasting operations to a new thread or start a new Sling Job.
The Sling API provides an easy way to wrap or decorate a resource before returning. Details see Wrap or Decorate Resources.