Using Sling Adapters

Sling offers an Adapter pattern to conveniently translate objects that implement the Adaptable interface. This interface provides a generic adaptTo() method that will translate the object to the class type being passed as the argument.

For example to translate a Resource object to the corresponding Node object, you can simply do:

Node node = resource.adaptTo(Node.class);

Use Cases

There are the following use cases:

  • Get implementation-specific objects.

    For example, a JCR-based implementation of the generic Resource interface provides access to the underlying JCR Node.

  • Shortcut creation of objects that require internal context objects to be passed.

    For example, the JCR-based ResourceResolver holds a reference to the request’s JCR Session, which in turn is needed for many objects that will work based on that request session, such as the PageManager or UserManager.

  • Shortcut to services.

    A rare case - sling.getService() is simple as well.

Null Return Value

adaptTo() can return null.

There are various reasons for this, including:

  • the implementation does not support the target type
  • an adapter factory handling this case is not active (eg. due to missing service references)
  • internal condition failed
  • service is not available

It is important that you handle the null case gracefully. For jsp renderings it might be acceptable to have the jsp fail if that will result in an empty piece of content.

Caching

To improve performance, implementations are free to cache the object returned from a obj.adaptTo() call. If the obj is the same, the returned object is the same.

This caching is performed for all AdapterFactory based cases.

However, there is no general rule - the object could be either a new instance or an existing one. This means that you cannot rely on either behavior. Hence it is important, especially inside AdapterFactory, that objects are reusable in this scenario.

How it works

There are various ways that Adaptable.adaptTo() can be implemented:

  • By the object itself; implementing the method itself and mapping to certain objects.

  • By an AdapterFactory, which can map arbitrary objects.

    The objects must still implement the Adaptable interface and must extend SlingAdaptable (which passes the adaptTo call to a central adapter manager).

    This allows hooks into the adaptTo mechanism for existing classes, such as Resource.

  • A combination of both.

For the first case, the javadocs can state what adaptTo-targets are possible. However, for specific subclasses such as the JCR-based Resource, often this is not possible. In the latter case, implementations of AdapterFactory are typically part of the private classes of a bundle and thus not exposed in a client API, nor listed in javadocs. Theoretically, it would be possible to access all AdapterFactory implementations from the OSGi service runtime and look at their “adaptables” (sources and targets) configurations, but not to map them to each other. In the end, this depends on the internal logic, which must be documented. Hence this reference.

Reference

Sling

Resource adapts to:

Node If this is a JCR-node-based resource or a JCR property referencing a node.
Property If this is a JCR-property-based resource.
Item If this is a JCR-based resource (node or property).
Map Returns a map of the properties, if this is a JCR-node-based resource (or other resource supporting value maps).
ValueMap Returns a convenient-to-use map of the properties, if this is a JCR-node-based resource (or other resource supporting value maps). Can also be achieved (more simply) by using
ResourceUtil.getValueMap(Resource) (handles null case, etc.).
InheritanceValueMap Extension of ValueMap which allows the hierarchy of resources to be taken into account when looking for properties.
ModifiableValueMap An extension of the ValueMap, which allows you to modify properties on that node.
InputStream Returns the binary content of a file resource (if this is a JCR-node-based resource and the node type is nt:file or nt:resource; if this is a bundle resource; file content if this is a file system resource) or the data of a binary JCR property resource.
URL Returns a URL to the resource (repository URL of this node if this is a JCR-node-based resource; jar bundle URL if this is a bundle resource; file URL if this is a file system resource).
File If this is a file system resource.
SlingScript If this resource is a script (e.g. jsp file) for which a script engine is registered with sling.
Servlet If this resource is a script (e.g. jsp file) for which a script engine is registered with sling or if this is a servlet resource.
String
Boolean
Long
Double
Calendar
Value
String[]
Boolean[]
Long[]
Calendar[]
Value[]		 Returns the value(s) if this is a JCR-property-based resource (and the value fits).
LabeledResource If this is a JCR-node-based resource.
Page If this is a JCR-node-based resource and the node is a cq:Page (or cq:PseudoPage).
Component If this is a cq:Component node resource.
Design If this is a design node (cq:Page).
Template If this is a cq:Template node resource.
Blueprint If this is a cq:Template node resource.
Asset If this is a dam:Asset node resource.
Rendition If this is a dam:Asset rendition (nt:file under the rendition folder of a dam:Assert)
Tag If this is a cq:Tag node resource.
UserManager Based on the JCR session if this is a JCR-based resource and the user has permissions to access the UserManager.
Authorizable The Authorizable is the common base interface for User and Group.
User User is a special Authorizable that can be authenticated and impersonated.
SimpleSearch Searches below the resource (or use setSearchIn()) if this is a JCR-based resource.
WorkflowStatus Workflow status for the given page/workflow payload node.
ReplicationStatus Replication status for the given resource or its jcr:content subnode (checked first).
ConnectorResource Returns an adapted connector resource for certain types, if this is a JCR-node-based resource.
Config If this is a cq:ContentSyncConfig node resource.
ConfigEntry If this is below a cq:ContentSyncConfig node resource.

ResourceResolver adapts to:

Session The request's JCR session, if this is a JCR-based resource resolver (default).
PageManager  
ComponentManager  
Designer  
AssetManager Based on the JCR session, if this is a JCR-based resource resolver.
TagManager Based on the JCR session, if this is a JCR-based resource resolver.
UserManager The UserManager provides access to and means to maintain authorizable objects i.e. users and groups. The UserManager is bound to a particular Session.
Authorizable The current user.
User
 The current user.
QueryBuilder
Externalizer For externalizing absolute URLs, even with out the request object.

SlingHttpServletRequest adapts to:

No targets yet, but implements Adaptable and could be used as source in a custom AdapterFactory.

SlingHttpServletResponse adapts to:

ContentHandler
(XML)		 If this is a sling rewriter response.

WCM

Page adapts to:

Resource
 Resource of the page.
LabeledResource Labeled resource (== this).
Node Node of the page.
... Everything that the page's resource can be adapted to.

Component adapts to:

Resource Resource of the component.
LabeledResource Labeled resource (== this).
Node Node of the component.
… Everything that the component’s resource can be adapted to.

Template adapts to:

Resource
 Resource of the template.
LabeledResource Labeled resource (== this).
Node Node of this template.
... Everything that the template's resource can be adapted to.

Security

Authorizable, User and Group adapt to:

Node Returns the user/group home node.
ReplicationStatus Returns the replication status for the user/group home node.

DAM

Asset adapts to:

Resource Resource of the asset.
Node Node of the asset.
… Everything that the asset’s resource can be adapted to.

Tagging

Tag adapts to:

Resource Resource of the tag.
Node Node of the tag.
… Everything that the tag’s resource can be adapted to.

Other

Furthermore Sling / JCR / OCM also provides an [AdapterFactory](https://sling.apache.org/site/adapters.html#Adapters-AdapterFactory) for custom OCM (Object Content Mapping) objects.

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