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 >= 1.2.0 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>1.2.0</version>
</dependency>

The following annotations are supported:

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:

Name

Default Value

Required

SCR

Metatype

Description

name

Fully qualified name of the Java class

no

component.name

OCD.id

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

ds

true

no

–

–

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

no

–

–

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.

componentAbstract

see description

no

–

–

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.

enabled

true

no

component.enabled

–

Whether the component is enabled when the bundle starts

factory

–

no

component.factory

–

Whether the component is a factory component

immediate

–

no

component.immediate

–

Whether the component is immediately activated

inherit

true

no

–

–

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

metatype

false

no

–

–

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.

label

%<name>.name

no

–

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

no

–

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.

createPid

true

no

service.pid

–

Generate the service.pid property if non is declared.

configurationFactory

false

no

–

Designate.factoryPid

Is this a configuration factory? (since 1.4.0)

The follwing attributes are supported since version 1.4.0 of the plugin and required a Declarative Service implementation 1.1:

policy

OPTIONAL

no

component.policy

–

The configuration policy for this component: OPTIONAL, IGNORE, or REQUIRE

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).

Service

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

This tag 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:

Name

Default Value

Required

Descriptor

Description

value

All implemented interfaces

no

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

no

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 anotation. 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 tag may be defined in the Java Class comment of the component or in a coment to a field defining a constant with the name of the property.

This tag 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 parameters:

Name

Default Value

Required

SCR

Metatype

Description

name

The name of constant

yes

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

–

no

property.value

AD.default

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

longValue

–

no

property.value

AD.default

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

doubleValue

–

no

property.value

AD.default

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

floatValue

–

no

property.value

AD.default

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

intValue

–

no

property.value

AD.default

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

byteValue

–

no

property.value

AD.default

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

charValue

–

no

property.value

AD.default

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

boolValue

–

no

property.value

AD.default

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

shortValue

–

no

property.value

AD.default

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

label

%<name>.name

no

–

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

no

–

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

no

–

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)

no

–

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

–

no

–

See below

See below for a description of the options attribute.

  • Generating <properties> elements referring to bundle entries is not currently supported.
  • Multiple property annotations can be embedded in the Properties annoation.

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 point to another constant.

The options Parameter

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. All parameters in the form of name-value pairs occurring after the options attribute are used to build the list of available value options. The parameter 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.

Please note, that all parameters of the Property annotation occurring after the options parameter are used to build the options list. Hence no non-option value parameters should actually follow the options parameter.

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 an 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 explicilty 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:

Name

Default Value

Required

Descriptor

Description

name

Name of the field

yes

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

yes

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

no

reference.cardinality

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

policy

static

no

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

–

no

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

no

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

no

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

no

reference.strategy

The strategy used for this reference, one of event or lookup

Notes:

  • If you define a reference on a field with a strategy of event and there is no bind or unbind method, the plugin will create the necessary methods.
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