Apache Felix SCR Plugin Frequently Asked Questions

This page provides answers to frequently asked questions using the Maven SCR Plugin. See subprojects/apache-felix-maven-scr-plugin.adoc for documentation on that plugin.

Should I still use the Apache Felix SCR annotations over the official OSGi annotations?

Starting with the R6 release of the OSGi Declarative Services and Metatype specification, the official annotations support the same features as the Apache Felix SCR annotations in a more elegant manner and even provide additional functionality. Therefore the Apache Felix SCR annotations are now in maintenance mode and therefore you should rather use the official annotations. The Apache Felix maven-bundle-plugin, version 3.0.1 or higher supports those directly and there is no need for an additional plugin anymore.

Why are the (standard) annotations not processed?

In order to process any annotations, a processor for them needs to be added as a dependency to your project. There are currently two different processors, one for the annotations defined within the Apache Felix project and another one for the standard annotations from the Declarative Services specification.

Syntax Error when Enums are used?

During a SCR plugin run, a syntax error might occur if enums are used in the source code. This is a know problem of the used QDox version that reads the Java source files. Usually this happens when an enum is used inline and does not end with a ";". So adding this character should solve the problem:

public interface MyTest {

    /** more error codes may be added */
    enum Result {
        OK, UNKNOWN, ERROR, DENIED
    };     // <!-- HERE

    /** @return the result */
    public void getResult();
}

NoClassDefFoundError during build

This error might happen with older versions of the Maven SCR Plugins in combination with javadoc tags or newer versions in combination with the annotations. In both cases, the scanned classes have to be loaded and static fields have to be initialized. If you have have for example something like

 private static final org.slf4f.Logger LOGGER = org.slf4j.LoggerFactory.getLogger("name");

in your code, during the plugin run, a slf4j logger is tried to be instantiated. This requires an implementation of the logger. Ususally your module only depends on the slf4j API and therefore a

 java.lang.NoClassDefFoundError: org/slf4j/impl/StaticLoggerBinder

is thrown - this often happens with slf4j but might also happen with other libraries.

In these cases you should add a dependency to the missing class to the dependency list of the plugin, for example:

 <plugin>
     <groupId>org.apache.felix</groupId>
     <artifactId>maven-scr-plugin</artifactId>
     <version>...</version>
     <dependencies>
         <dependency>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-simple</artifactId>
             <version>1.5.2</version>
         </dependency>
     </dependencies>
     ...
 </plugin>

or in the special case of slf4j, using slf4j API 1.6 or higher solves the problem as well

No components or services available at runtime

The SCR plugin generates a descriptor for Declarative Services (see OSGi compendium specification). Therefore at runtime, you must have an implementation of DS running in your OSGi framework, like the SCR implementation from the Apache Felix project.

Otherwise your bundle might resolve fine, however no services are registered with the service registry and no components are activated.