Apache

Sling Models

Many Sling projects want to be able to create model objects - POJOs which are automatically mapped from Sling objects, typically resources, but also request objects. Sometimes these POJOs need OSGi services as well.

Design Goals

  • Entirely annotation driven. "Pure" POJOs.
  • Use standard annotations where possible.
  • Pluggable
  • OOTB, support resource properties (via ValueMap), SlingBindings, OSGi services, request attributes
  • Adapt multiple objects - minimal required Resource and SlingHttpServletRequest
  • Client doesn't know/care that these objects are different than any other adapter factory
  • Support both classes and interfaces.
  • Work with existing Sling infrastructure (i.e. not require changes to other bundles).

Basic Usage

In the simplest case, the class is annotated with @Model and the adaptable class. Fields which need to be injected are annotated with @Inject:

@Model(adaptables=Resource.class)
public class MyModel {

    @Inject
    private String propertyName;
}

In this case, a property named "propertyName" will be looked up from the Resource (after first adapting it to a ValueMap) and it is injected.

For an interface, it is similar:

@Model(adaptables=Resource.class)
public interface MyModel {

    @Inject
    String getPropertyName();
}

Constructor injection is also supported (as of Sling Models 1.1.0):

@Model(adaptables=Resource.class)
public class MyModel {    
    @Inject
    public MyModel(@Named("propertyName") String propertyName) {
      // constructor code
    }
}

Because the name of a constructor argument parameter cannot be detected via the Java Reflection API a @Named annotation is mandatory for injectors that require a name for resolving the injection.

In order for these classes to be picked up, there is a header which must be added to the bundle's manifest:

<Sling-Model-Packages>
  org.apache.sling.models.it.models
</Sling-Model-Packages>

This header must contain all packages which contain model classes or interfaces. However, subpackages need not be listed individually, e.g. the header above will also pick up model classes in org.apache.sling.models.it.models.sub. Multiple packages can be listed in a comma-separated list (any whitespace will be removed):

<Sling-Model-Packages>
  org.apache.sling.models.it.models,
  org.apache.sling.other.models
</Sling-Model-Packages>

Alternatively it is possible to list all classes individually that are Sling Models classes via the Sling-Model-Classes header.

If you use the Sling Models bnd plugin all required bundle headers are generated automatically at build time (see chapter 'Registration of Sling Models classes via bnd plugin' below).

Client Code

adaptTo()

Client code doesn't need to be aware that Sling Models is being used. It just uses the Sling Adapter framework:

MyModel model = resource.adaptTo(MyModel.class)

Or

<sling:adaptTo adaptable="${resource}" adaptTo="org.apache.sling.models.it.models.MyModel" var="model"/>

Or

${sling:adaptTo(resource, 'org.apache.sling.models.it.models.MyModel')}

As with other AdapterFactories, if the adaptation can't be made for any reason, adaptTo() returns null.

ModelFactory (since 1.2.0)

See also SLING-3709

Since Sling Models 1.2.0 there is another way of instantiating models. The OSGi service ModelFactory provides a method for instantiating a model that throws exceptions. This is not allowed by the Javadoc contract of the adaptTo method. That way null checks are not necessary and it is easier to see why instantiation of the model failed.

try {
    MyModel model = modelFactory.createModel(object, MyModel.class);
} catch (Exception e) {
    // give out error message that the model could not be instantiated. 
    // The exception contains further information. 
    // See the javadoc of the ModelFactory for which Exception can be expected here
}

In addition ModelFactory provides methods for checking whether a given class is a model at all (having the model annotation) or whether a class can be adapted from a given adaptable.

Other Options

Names

If the field or method name doesn't exactly match the property name, @Named can be used:

@Model(adaptables=Resource.class)
public class MyModel {

    @Inject @Named("secondPropertyName")
    private String otherName;
} 

Optional and Required

@Injected fields/methods are assumed to be required. To mark them as optional, use @Optional:

@Model(adaptables=Resource.class)
public class MyModel {

    @Inject @Optional
    private String otherName;
}

If a majority of @Injected fields/methods are optional, it is possible (since Sling Models API 1.0.2/Impl 1.0.6) to change the default injection strategy by using adding defaultInjectionStrategy = DefaultInjectionStrategy.OPTIONAL to the @Model annotation:

@Model(adaptables=Resource.class, defaultInjectionStrategy=DefaultInjectionStrategy.OPTIONAL)
public class MyModel {

    @Inject
    private String otherName;
}

To still mark some fields/methods as being mandatory while relying on defaultInjectionStrategy = DefaultInjectionStrategy.OPTIONAL for all other fields, the annotation @Required can be used.

@Optional annotations are only evaluated when using the defaultInjectionStrategy = DefaultInjectionStrategy.REQUIRED (which is the default), @Required annotations only if using defaultInjectionStrategy = DefaultInjectionStrategy.OPTIONAL.

Defaults

A default value can be provided (for Strings & primitives):

@Model(adaptables=Resource.class)
public class MyModel {

    @Inject @Default(values="defaultValue")
    private String name;
}

Defaults can also be arrays:

@Model(adaptables=Resource.class)
public class MyModel {

    @Inject @Default(intValues={1,2,3,4})
    private int[] integers;
}

OSGi services can be injected:

@Model(adaptables=Resource.class)
public class MyModel {

    @Inject
    private ResourceResolverFactory resourceResolverFactory;
} 

In this case, the name is not used -- only the class name.

Collections

Lists and arrays are supported by some injectors. For the details look at the table given in Available Injectors:

@Model(adaptables=Resource.class)
public class MyModel {

    @Inject
    private List<Servlet> servlets;
}

List injection for child resources works by injecting grand child resources (since Sling Models Impl 1.0.6). For example, the class

@Model(adaptables=Resource.class)
public class MyModel {

    @Inject
    private List<Resource> addresses;
}

Is suitable for a resource structure such as:

+- resource (being adapted)
 |
 +- addresses
    |
    +- address1
    |
    +- address2

In this case, the addresses List will contain address1 and address2.

OSGi Service Filters

OSGi injection can be filtered:

@Model(adaptables=SlingHttpServletRequest.class)
public class MyModel {

    @Inject
    private PrintWriter out;

    @Inject
    @Named("log")
    private Logger logger;

    @Inject
    @Filter("(paths=/bin/something)")
    private List<Servlet> servlets;
}

PostConstruct Methods

The @PostConstruct annotation can be used to add methods which are invoked upon completion of all injections:

@Model(adaptables=SlingHttpServletRequest.class)
public class MyModel {

    @Inject
    private PrintWriter out;

    @Inject
    @Named("log")
    private Logger logger;

    @PostConstruct
    protected void sayHello() {
         logger.info("hello");
    }
}

@PostConstruct methods in a super class will be invoked first. If a @PostConstruct method exists in a subclass with the same name as in the parent class, only the subclass method will be invoked. This is the case regardless of the scope of either method.

Since Sling Models Implementation 1.4.6, @PostConstruct methods may return a false boolean value in which case the model creation will fail without logging any exception (a message will be logged at the DEBUG level).

Via

In some cases, a different object should be used as the adaptable instead of the original adaptable. This can be done using the @Via annotation. By default, this can be done using a JavaBean property of the adaptable:

@Model(adaptables=SlingHttpServletRequest.class)
public interface MyModel {

    // will return request.getResource().getValueMap().get("propertyName", String.class)
    @Inject @Via("resource")
    String getPropertyName();
} 

A different strategy can be used to define the adaptable by specifying a type attribute:

@Model(adaptables=Resource.class)
public interface MyModel {

    // will return resource.getChild("jcr:content").getValueMap().get("propertyName", String.class)
    @Inject @Via(value = "jcr:content", type = ChildResource.class)
    String getPropertyName();

}

See the Via Types section below for details on the included types for @Via.

Source

If there is ambiguity where a given injection could be handled by more than one injector, the @Source annotation can be used to define which injector is responsible:

@Model(adaptables=SlingHttpServletRequest.class)
public interface MyModel {

    // Ensure that "resource" is retrived from the bindings, not a request attribute 
    @Inject @Source("script-bindings")
    Resource getResource();
} 

Adaptations

If the injected object does not match the desired type and the object implements the Adaptable interface, Sling Models will try to adapt it. This provides the ability to create rich object graphs. For example:

@Model(adaptables=Resource.class)
public interface MyModel {

    @Inject
    ImageModel getImage();
}

@Model(adaptables=Resource.class)
public interface ImageModel {

    @Inject
    String getPath();
}

When a resource is adapted to MyModel, a child resource named image is automatically adapted to an instance of ImageModel.

Constructor injection is supported for the adaptable itself. For example:

@Model(adaptables=Resource.class)
public class MyModel {

    public MyModel(Resource resource) {
        this.resource = resource;
    }

    private final Resource resource;

    @Inject
    private String propertyName;
}

Sling Validation (since 1.2.0)

See also SLING-4161

You can use the attribute validation on the Model annotation to call a validation service on the resource being used by the Sling model. That attribute supports three different values:

Value Description Invalid validation model No validation model found Resource invalid according to model
DISABLED (default) don't validate the resource bound to the Model Model instantiated Model instantiated Model instantiated
REQUIRED enforce validation of the resource bound to the Model Model not instantiated Model not instantiated Model not instantiated
OPTIONAL validate the resource bound to the Model (if a validation model is found) Model not instantiated Model instantiated Model not instantiated

In case the model is not instantiated an appropriate error message is logged (if adaptTo() is used) or an appropriate exception is thrown (if ModelFactory.createModel() is used).

The only implementation for this Sling Models validation service is leveraging Sling Validation and is located in the bundle org.apache.sling.models.validation-impl. Validation is only working on models which are adapted from either Resource or SlingHttpServletRequest and if the Sling Validation Bundle is deployed.

Custom Injectors

To create a custom injector, simply implement the org.apache.sling.models.spi.Injector interface and register your implementation with the OSGi service registry. Please refer to the standard injectors in Git for examples.

Injectors are invoked in order of their service ranking, from lowest to highest. See the table below for the rankings of the standard injectors.

Annotation Reference

@Model : declares a model class or interface

@Inject : marks a field or method as injectable

@Named : declare a name for the injection (otherwise, defaults based on field or method name).

@Optional : marks a field or method injection as optional

@Source : explictly tie an injected field or method to a particular injector (by name). Can also be on other annotations.

@Filter : an OSGi service filter

@PostConstruct : methods to call upon model option creation (only for model classes)

@Via : change the adaptable as the source of the injection

@Default : set default values for a field or method

@Path : only used together with the resource-path injector to specify the path of a resource

@Exporters/@Exporter/@ExporterOptions/@ExporterOption : See Exporter Framework section below

In addition all injector-specific annotations.

Available Injectors

Title Name (for @Source) Service Ranking Available Since (Implementation Version) Description Applicable To (including using @Via) Accepts Null Name? Array Support Parameterized Type Support
Script Bindings script-bindings 1000 1.0.0 Lookup objects in the script bindings object by name. A ServletRequest object which has the Sling Bindings attribute defined no no conversion is done If a parameterized type is passed, the bindings value must be of a compatible type of the parameterized type.
Value Map valuemap 2000 1.0.0 Gets a property from a ValueMap by name. Any object which is or can be adapted to a ValueMap no Primitive arrays wrapped/unwrapped as necessary. Wrapper object arrays are unwrapped/wrapped as necessary. Parameterized List and Collection injection points are injected by getting an array of the component type and creating an unmodifiable List from the array.
Child Resources child-resources 3000 1.0.0 Gets a child resource by name. Resource objects no none if a parameterized type List or Collection is passed, a List<Resource> is returned (the contents of which may be adapted to the target type) filled with all child resources of the resource looked up by the given name.
Request Attributes request-attributes 4000 1.0.0 Get a request attribute by name. ServletRequest objects no no conversion is done If a parameterized type is passed, the request attribute must be of a compatible type of the parameterized type.
OSGi Services osgi-services 5000 1.0.0 Lookup services based on class name. Since Sling Models Impl 1.2.8 (SLING-5664) the service with the highest service ranking is returned. In case multiple services are returned, they are ordered descending by their service ranking (i.e. the one with the highest ranking first). Any object yes yes Parameterized List and Collection injection points are injected by getting an array of the services and creating an unmodifiable List from the array.
Resource Path resource-path 2500 1.1.0 Injects one or multiple resources. The resource paths are either given by @Path annotations, the element path or paths of the annotation @ResourcePath or by paths given through a resource property being referenced by either @Named or element name of the annotation @ResourcePath. Resource or SlingHttpServletRequest objects yes yes none
Self self Integer.MAX_VALUE 1.1.0 Injects the adaptable object itself (if the class of the field matches or is a supertype). If the @Self annotation is present it is tried to adapt the adaptable to the field type. Any object yes none none
Sling Object sling-object Integer.MAX_VALUE 1.1.0 Injects commonly used sling objects if the field matches with the class: request, response, resource resolver, current resource, SlingScriptHelper. This works only if the adaptable can get the according information, i.e. all objects are available via SlingHttpServletRequest while ResourceResolver can only resolve the ResourceResolver object and nothing else. A discussion around this limitation can be found at SLING-4083. Also Resources can only be injected if the according injector-specific annotation is used (@SlingObject). Resource, ResourceResolver or SlingHttpServletRequest objects (not all objects can be resolved by all adaptables). yes none none

Injector-specific Annotations

Introduced with SLING-3499 in Sling Models Impl 1.0.6

Sometimes it is desirable to use customized annotations which aggregate the standard annotations described above. This will generally have the following advantages over using the standard annotations:

  • Less code to write (only one annotation is necessary in most of the cases)
  • More robust (in case of name collisions among the different injectors, you make sure that the right injector is used)
  • Better IDE support (because the annotations provide elements for each configuration which is available for that specific injector, i.e. filter only for OSGi services)

The follow annotations are provided which are tied to specific injectors:

Annotation Supported Optional Elements Injector Description
@ScriptVariable injectionStrategy and name script-bindings Injects the script variable defined via Sling Bindings. If name is not set the name is derived from the method/field name.
@ValueMapValue injectionStrategy, name and via valuemap Injects a ValueMap value. If via is not set, it will automatically take resource if the adaptable is the SlingHttpServletRequest. If name is not set the name is derived from the method/field name.
@ChildResource injectionStrategy, name and via child-resources Injects a child resource by name. If via is not set, it will automatically take resource if the adaptable is the SlingHttpServletRequest. If name is not set the name is derived from the method/field name.
@RequestAttribute injectionStrategy, name and via request-attributes Injects a request attribute by name. If name is not set the name is derived from the method/field name.
@ResourcePath injectionStrategy, path, and name resource-path Injects a resource either by path or by reading a property with the given name.
@OSGiService injectionStrategy, filter osgi-services Injects an OSGi service by type. The filter can be used give an OSGi service filter.
@Self injectionStrategy self Injects the adaptable itself. If the field type does not match with the adaptable it is tried to adapt the adaptable to the requested type.
@SlingObject injectionStrategy sling-object Injects commonly used sling objects if the field matches with the class: request, response, resource resolver, current resource, SlingScriptHelper

Hints

Those annotations replace @Via, @Filter, @Named, @Optional, @Required, @Source and @Inject. Instead of using the deprecated annotation element optional you should rather use injectionStrategy with the values DEFAULT, OPTIONAL or REQUIRED (see also SLING-4155). @Default may still be used in addition to the injector-specific annotation to set default values. All elements given above are optional.

Custom Annotations

To create a custom annotation, implement the org.apache.sling.models.spi.injectorspecific.StaticInjectAnnotationProcessorFactory interface. This interface may be implemented by the same class as implements an injector, but this is not strictly necessary. Please refer to the injectors in Git for examples.

Specifying an Alternate Adapter Class (since 1.1.0)

By default, each model class is registered using its own implementation class as adapter. If the class has additional interfaces this is not relevant.

The @Model annotations provides an optional adapters attribute which allows specifying under which type(s) the model implementation should be registered in the Models Adapter Factory. Prior to Sling Models Impl 1.3.10 only the given class names are used as adapter classes, since 1.3.10 the implementation class is always being registered implicitly as adapter as well (see SLING-6658). With this attribute it is possible to register the model to one (or multiple) interfaces, or a superclass. This allows separating the model interface from the implementation, which makes it easier to provide mock implementations for unit tests as well.

Example:

@Model(adaptables = Resource.class, adapters = MyService.class)
public class MyModel implements MyService {
    // injects fields and implements the MyService methods
}

In this example a Resource can be adapted to a MyService interface, and the Sling Models implementation instantiates a MyModel class for this.

It is possible to have multiple models implementing the same interface. By default Sling Models will just take the first one ordered alphabetically by the class name. Applications can provide an OSGi service implementing the ImplementationPicker SPI interface which could use context to determine which implementation can be chosen, e.g. depending an a tenant or content path context. If multiple implementations of the ImplementationPicker interface are present, they are queried one after another in order of their service ranking property, the first one that picks an implementation wins.

Associating a Model Class with a Resource Type (since 1.3.0)

The @Model annotation provides an optional resourceType attribute which allows for model classes to be associated with one or more resource types. This is used in three different ways.

In the case of multiple model classes implementing the same interface, the class with the "closest" resource type will be used when adapting to the interface.

The ModelFactory service interface has methods Object getModelFromResource(Resource) and Object getModelFromRequest(SlingHttpServletRequest) which will dynamically determine the adapter class based on the Resource using its type. In the case of the SlingHttpServletRequest method, it uses the request's Resource object (i.e. by calling request.getResource())

The resource type is also used as part of the Exporter framework (see next section).

Exporter Framework (since 1.3.0)

Sling Models objects can be exported to arbitrary Java objects through the Sling Models Exporter framework. Model objects can be programatically exported by calling the ModelFactory method exportModel(). This method takes as its arguments:

  • the model object
  • an exporter name
  • a target class
  • a map of options

The exact semantics of the exporting will be determined by an implementation of the ModelExporter service interface. Sling Models currently includes a single exporter, using the Jackson framework, which is capable of serializing models as JSON or transforming them to java.util.Map objects.

In addition, model objects can have servlets automatically registered for their resource type (if it is set) using the @Exporter annotation. For example, a model class with the annotation

@Model(adaptable = Resource.class, resourceType = "myco/components/foo")
@Exporter(name = "jackson", extensions = "json")

results in the registration of a servlet with the resource type and extension specified and a selector of 'model' (overridable through the @Exporter annotation's selector attribute). When this servlet is invoked, the Resource will be adapted to the model, exported as a java.lang.String (via the named Exporter) and then returned to the client.

Registration of Sling Models classes via bnd plugin

With the Sling Models bnd plugin it is possible to automatically generated the necessary bundle header to register the Sling Models classes contained in the Maven bundle project - either with maven-bundle-plugin or with bnd-maven-plugin. By default the plugin generates a Sling-Model-Classes header (only compatible with Sling Models Impl since version 1.3.4, see SLING-6308).

Example configuration:

<plugin>
    <groupId>org.apache.felix</groupId>
    <artifactId>maven-bundle-plugin</artifactId>
    <extensions>true</extensions>
    <configuration>
        <instructions>
            <_plugin>org.apache.sling.bnd.models.ModelsScannerPlugin</_plugin>
        </instructions>
    </configuration>
    <dependencies>
        <dependency>
            <groupId>org.apache.sling</groupId>
            <artifactId>org.apache.sling.bnd.models</artifactId>
            <version>1.0.0</version>
        </dependency>
    </dependencies>
</plugin>

If a Sling-Model-Packages or Sling-Model-Classes was already manually defined for the bundle the bnd plugin does nothing. So if you want to migrate an existing project to use this plugin remove the existing header definitions.

If you want to generate a bundle header compliant with Sling Models < 1.3.4 (i.e. Sling-Model-Packages) you need to specify the attribute generatePackagesHeader=true. An example configuration looks like this

<configuration>
    <instructions>
        <_plugin>org.apache.sling.bnd.models.ModelsScannerPlugin;generatePackagesHeader=true</_plugin>
    </instructions>
</configuration>

Caching

By default, Sling Models do not do any caching of the adaptation result and every request for a model class will result in a new instance of the model class. However, there are two notable cases when the adaptation result can be cached. The first case is when the adaptable extends the SlingAdaptable base class. Most significantly, this is the case for many Resource adaptables as AbstractResource extends SlingAdaptable. SlingAdaptable implements a caching mechanism such that multiple invocations of adaptTo() will return the same object. For example:

// assume that resource is an instance of some subclass of AbstractResource
ModelClass object1 = resource.adaptTo(ModelClass.class); // creates new instance of ModelClass
ModelClass object2 = resource.adaptTo(ModelClass.class); // SlingAdaptable returns the cached instance
assert object1 == object2;

While this is true for AbstractResource subclasses, it is notably not the case for SlingHttpServletRequest as this class does not extend SlingAdaptable. So:

// assume that request is some SlingHttpServletRequest object
ModelClass object1 = request.adaptTo(ModelClass.class); // creates new instance of ModelClass
ModelClass object2 = request.adaptTo(ModelClass.class); // creates another new instance of ModelClass
assert object1 != object2;

Since API version 1.3.4, Sling Models can cache an adaptation result, regardless of the adaptable by specifying cache = true on the @Model annotation.

@Model(adaptable = SlingHttpServletRequest.class, cache = true)
public class ModelClass {}

...

// assume that request is some SlingHttpServletRequest object
ModelClass object1 = request.adaptTo(ModelClass.class); // creates new instance of ModelClass
ModelClass object2 = request.adaptTo(ModelClass.class); // Sling Models returns the cached instance
assert object1 == object2;

When cache = true is specified, the adaptation result is cached regardless of how the adaptation is done:

@Model(adaptable = SlingHttpServletRequest.class, cache = true)
public class ModelClass {}

...

// assume that request is some SlingHttpServletRequest object
ModelClass object1 = request.adaptTo(ModelClass.class); // creates new instance of ModelClass
ModelClass object2 = modelFactory.createModel(request, ModelClass.class); // Sling Models returns the cached instance
assert object1 == object2;

Via Types (Since API 1.3.4/Implementation 1.4.0)

As discussed in the Via section above, it is possible to select a different adaptable than the original value using the @Via annotation. The following standard types are provided (all types are in the package org.apache.sling.models.annotations.via)

@Via type value Description
BeanProperty (default) Uses a JavaBean property from the adaptable.
ChildResource Uses a child resource from the adaptable, assuming the adaptable is a Resource. In case the adaptable is a SlingHttpServletRequest uses a wrapper overwriting the getResource() to point to the given child resource (SLING-7321).
ForcedResourceType Creates a wrapped resource with the provided resource type. If the adaptable is a SlingHttpServletRequest, a wrapped request is created as well to contain the wrapped resource.
ResourceSuperType Creates a wrapped resource with the resource type set to the adaptable's resource super type. If the adaptable is a SlingHttpServletRequest, a wrapped request is created as well to contain the wrapped resource.

Custom Via Type

Defining your own type for the @Via annotation is a two step process. The first step is to create a marker class implementing the @ViaProviderType annotation. This class can be entirely empty, e.g.

public class MyCustomProviderType implements ViaProviderType {}

The second step is to create an OSGi service implementing the ViaProvider interface. This interface defines two methods:

  • getType() should return the marker class.
  • getAdaptable() should return the new adaptable or ViaProvider.ORIGINAL to indicate that the original adaptable should be used.