SCR Annotations Excerpt: Using Java 5 Annotations to describe the component or service.

The maven-scr-plugin uses the SCR annotations from the corresponding subproject at Apache Felix. All annotations are in the org.apache.felix.scr.annotations package. If you want to use the annotations in your project, you have to use a maven-scr-plugin version >= 1.24.0 and make sure that you add a dependency to the annotations to your POM:

 <dependency>
     <groupId>org.apache.felix</groupId>
     <artifactId>org.apache.felix.scr.annotations</artifactId>
     <version>1.12.0</version>
 </dependency>

The following annotations are supported:

The annotations itself do not support the new features from R6 or above. It is suggested to use the official OSGi annotations for Declarative Services instead.

@Component

The @Component annotation is the only required annotation. If this annotation is not declared for a Java class, the class is not declared as a component.

This annotation is used to declare the <component> element of the component declaration. See section 112.4.3, Component Element, in the OSGi Service Platform Service Compendium Specification for more information. The required <implementation> element is automatically generated with the fully qualified name of the class containing the @Component annotation.

Supported attributes:

Property Default SCR Descriptor Metatype Descriptor Description

ds

true

Whether Declarative Services descriptor is generated or not. If this parameter is not set or set to true the Declarative Services descriptor is generated in the service descriptor file for this component. Otherwise no Declarative Services descriptor is generated for this component.

specVersion

1.0

Defines what Declarative Services specification the component is written against. Though the Maven SCR Plugin is very good at detecting whether components are written against the original or a newer specification, there are some cases, where the plugin may fail. For these cases, the specVersion attribute may be set to the correct version. Currently supported values for this attribute are 1.0 and 1.1. Since version 1.4.1 of the Maven SCR Plugin and version 1.0.1 of the SCR Annotations.

metatype

false

Whether Metatype Service data is generated or not. If this parameter is set to true Metatype Service data is generated in the metatype.xml file for this component. Otherwise no Metatype Service data is generated for this component.

componentAbstract

This marks an abstract service description which is not added to the descriptor but intended for reuse through inheritance. This attribute defaults to true for abstract classes and false for concrete classes.

inherit

true

Whether any service, property and reference declarations from base classes should be inherited by this class.

createPid

true

service.pid

Generate the service.pid property if non is declared.

name

Fully qualified name of the Java class

component.name

OCD.id

Defines the Component name also used as the PID for the Configuration Admin Service

enabled

true

component.enabled

Whether the component is enabled when the bundle starts

factory

component.factory

Whether the component is a factory component

immediate

component.immediate

Whether the component is immediately activated

policy

OPTIONAL

component.policy

The configuration policy for this component: OPTIONAL, IGNORE, or REQUIRE. This attribute is supported since version 1.4.0 of the plugin and requires a Declarative Service implementation 1.1 or higher.

label

%<name>.name

OCD.name

This is generally used as a title for the object described by the meta type. This name may be localized by prepending a % sign to the name.

description

%<name>.name

OCD.description

This is generally used as a description for the object described by the meta type. This name may be localized by prepending a % sign to the name.

configurationFactory

false

Designate.factoryPid

Is this a configuration factory? (since 1.4.0)

Abstract Service Descriptions

If the @Component annotations contains the attribute componentAbstract with a value of true, the containing class is regarded as an abstract class. It is not added to the service descriptor and the tags are not validated. The information about this class is added to the bundle. Classes from other bundles (or the same) can extends this abstract class and do not need to specify the references of the abstract class if they set the inherit parameter on the scr.component tag to true.

This allows to create abstract classes which already provide some valuable functionality without having to deal with the details like reference definitions in each and every subclass.

@Activate, @Deactivate, and @Modified

The Declarative Service version 1.1 allows to specify the name for the activate, deactivate and modified method (see the spec for more information). The @Activate, @Deactivate, and @Modified annotation can be used to mark a method to be used for the specified purpose. However, as the DS specifies a method search algorithm, there are rare cases where the marked method is not used (if there is another method with the same name, but a different signature this might happen).

These annotations have no attributes.

@Service

The @Service annotation defines whether and which service interfaces are provided by the component. This is a class annotation.

This annotation is used to declare <service> and <provide> elements of the component declaration. See section 112.4.6, Service Elements, in the OSGi Service Platform Service Compendium Specification for more information.

Supported attributes:

Property Default SCR Descriptor Description

value

All implemented interfaces

provide.interface

The name of the service interface provided by the component. This can either be the fully qualified name or just the interface class name if the interface is either in the same package or is imported. If this property is not set provide elements will be generated for all interfaces generated by the class

serviceFactory

false

service.servicefactory

Whether the component is registered as a ServiceFactory or not

Omitting the Service annotation will just define (and activate if required) the component but not register it as a service. Multiple Service annotations may be declared each with its own value. These annotations need to be wrapped into a Services annotation. The component is registered as a ServiceFactory if at least on Service annotations declares the serviceFactory attribute as true.

@Property

The @Property annotation defines properties which are made available to the component through the ComponentContext.getProperties() method. These tags are not strictly required but may be used by components to defined initial configuration. Additionally properties may be set here to identify the component if it is registered as a service, for example the service.description and service.vendor properties.

This annotation can be applied on the component class level or on a field defining a constant with the name of the property.

This annotation is used to declare <property> elements of the component declaration. See section 112.4.5, Properties and Property Elements, in the OSGi Service Platform Service Compendium Specification for more information.

Supported attributes:

Property Default SCR Descriptor Metatype Descriptor Description

name

The name of constant

property.name

AD.id

The name of the property. If this tag is defined on a field with an initialization expression, the value of that expression is used as the name if the field is of type String.

value

property.value

AD.default

The string value of the property. This can either be a single value or an array.

longValue

property.value

AD.default

The long value of the property. This can either be a single value or an array.

doubleValue

property.value

AD.default

The double value of the property. This can either be a single value or an array.

floatValue

property.value

AD.default

The float value of the property. This can either be a single value or an array.

intValue

property.value

AD.default

The int value of the property. This can either be a single value or an array.

byteValue

property.value

AD.default

The byte value of the property. This can either be a single value or an array.

charValue

property.value

AD.default

The char value of the property. This can either be a single value or an array.

boolValue

property.value

AD.default

The boolean value of the property. This can either be a single value or an array.

shortValue

property.value

AD.default

The short value of the property. This can either be a single value or an array.

label

%<name>.name

AD.name

The label to display in a form to configure this property. This name may be localized by prepending a % sign to the name.

description

%<name>.description

AD.description

A descriptive text to provide the client in a form to configure this property. This name may be localized by prepending a % sign to the name.

propertyPrivate

Depending on the name

See description Boolean flag defining whether a metatype descriptor entry should be generated for this property or not. By default a metatype descriptor entry, i.e. an AD element, is generated except for the properties service.pid, service.description, service.id, service.ranking, service.vendor, service.bundlelocation and service.factoryPid. If a property should not be available for display in a configuration user interface, this parameter should be set to true.

cardinality

Depends on property value(s)

AD.cardinality

Defines the cardinality of the property and its collection type. If the cardinality is negative, the property is expected to be stored in a java.util.Vector (primitive types such as boolean are boxed in the Wrapper class), if the cardinality is positive, the property is stored in an array (primitve types are unboxed, that is Boolean type values are stored in boolean). The actual value defines the maximum number of elements in the vector or array, where Integer.MIN*INT describes an unbounded Vector and Integer.MAX*INT describes an unbounded array. If the cardinality is zero, the property is a scalar value. If the defined value of the property is set in the value attribute, the cardinality defaults to 0 (zero for scalar value). If the property is defined in one or more properties starting with values, the cardinality defaults to Integer.MAX_INT, that is an unbounded array.

options

See below for a description of the options attribute.

Generating <properties> elements referring to bundle entries is not currently supported.

Multiple property annotations on the class level can be embedded in the @Properties annotation. For example:

 @Properties({
     @Property(name = "prop1", value = "value1"),
     @Property(name = "prop2", value = "value2")
 })

Naming the Property

It is important to carefully define the name of properties. By using a constant of the form

 @Property(value="default value")
 static final String CONSTANT_NAME = "property.name";

and defining the @Property annotation on this constant, the name of the property is taken from the constant value. Thus it may easily be ensured, that both the property in the descriptor files and the property used by the implementation are actually the same. In addition the value attribute can refer to another constant.

The options Attribute

Some properties may only be set to a set of possible values. To support user interfaces which provide a selection list of values or a list of checkboxes the option values and labels may be defined as parameters to the @Property annotation.

The value of the options attribute is a list of @PropertyOptions annotations:

 @Property(name = "sample",
     options = {
         @PropertyOption(name = "option1", value = "&option.label.1"),
         @PropertyOption(name = "option2", value = "&option.label.2")
     }
 )

The @PropertyOption's name is used as the value while the parameter value is used as the label in the user interface. This label may be prepended with a % sign to localize the string.

The options are written to the metatype.xml file as Option elements inside the AD element defining the property. The name of the parameter will be used for the Option.value attribute while the value of the parameter defines the Option.label attribute.

Multivalue Properties

Generally the value of a property is scalar, that is a property has a single value such as true, 5 or "This is a String". Such scalar values are defined with the different value attributes of the Property annotation. In the case of a scalar property value, the cardinality parameter value is assumed to be 0 (zero) unless of course set otherwise.

There may be properties, which have a list of values, such as a list of possible URL mappings for a URL Mapper. Such multiple values are defined just by comma separate as the value of the annotation parameter.

If the cardinality of the property is not explicitly set with the cardinality property, it defaults to Integer.MAX_INT, i.e. unbound array, if multiple values are defined. Otherwise the cardinality parameter may be set for example to a negative value to store the values in a java.util.Vector instead.

@Reference

The @Reference annotation defines references to other services made available to the component by the Service Component Runtime.

This annotation may be declared on a Class level or any Java field to which it might apply. Depending on where the annotation is declared, the parameters may have different default values.

This annotation is used to declare <reference> elements of the component declaration. See section 112.4.7, Reference Element, in the OSGi Service Platform Service Compendium Specification for more information.

Supported parameters:

Property Default SCR Descriptor Description

name

Name of the field

reference.name

The local name of the reference. If the Reference annotation is declared in the class comment, this parameter is required. If the annotation is declared on a field, the default value for the name parameter is the name of the field

interfaceReference

Type of the field

reference.interface

The name of the service interface. This name is used by the Service Component Runtime to access the service on behalf of the component. If the Reference annotation is declared on a class level, this parameter is required. If the annoation is declared on a field, the default value for the interfaceReference parameter is the type of the field

cardinality

1..1

reference.cardinality

The cardinality of the service reference. This must be one of value from the enumeration ReferenceCardinality

policy

static

reference.policy

The dynamicity policy of the reference. If dynamic the service will be made available to the component as it comes and goes. If static the component will be deactivated and re-activated if the service comes and/or goes away. This must be one of static and dynamic

target

reference.target

A service target filter to select specific services to be made available. In order to be able to overwrite the value of this value by a configuration property, this parameter must be declared. If the parameter is not declared, the respective declaration attribute will not be generated

bind

See description

reference.bind

The name of the method to be called when the service is to be bound to the component. The default value is the name created by appending the reference name to the string bind. The method must be declared public or protected and take single argument which is declared with the service interface type

unbind

See description

reference.unbind

The name of the method to be called when the service is to be unbound from the component. The default value is the name created by appending the reference name to the string unbind. The method must be declared public or protected and take single argument which is declared with the service interface type

strategy

event

reference.strategy

The strategy used for this reference, one of event or lookup. If the reference is defined on a field with a strategy of event and there is no bind or unbind method, the plugin will create the necessary methods.