Since the beginning of AEM development we used annotations to declare OSGI services; @Component, @Service, @Property and @Reference should be known to everyone how has ever developed backend stuff for AEM. The implementation behind these annotations came from the Apache Felix project, and they were called the SCR annotations (SCR = Service Component Runtime). But unlike the Service Component Runtime, which is part of the OSGI standard for quite some, these annotations were not standardized. This changed with OSGI Release 6.
With this release annotations were also standardized, but they are 100% compatible to the SCR annotations. And there are a lot of resources out there, which can help to explain the differences:
- Carsten Ziegeler wrote some blog posts about: Overview, part 1, part 2, part 3
- Feike wrote about them in the context of AEM with a focus on OSGI configuration
- Also Nate Yolles wrote about it
- (And there are definitly much more resources out there).
I recently worked on migrating a lot of the code from ACS AEM Commons from SCR annotations to OSGI annotations, and I want to share some learning I gained on the way. Because in some subtle areas the conversion isn’t that easy.
Mixed use of SCR annotations and OSGI annotations
You can mix SCR annotations and OSGI annotations in a project, you don’t need to migrate them all at once. But you can to be consistent on a class level, you cannot mix SCR and OSGI annotations in a single class. This is achieved by an extension to the maven-bundle-plugin (see below).
SCR property annotations give you a lot of freedom. You can annotate them on top of the class (using the @Properties annotation as container with nested @Property annotations), you can annotate individual constant values to be properties. You can make them visible in the OSGI webconsole (technically you are creating a metatype for them), or you can mark them as private (no metatype is created).
With OSGI annotations this is different.
- Metatype properties are handled in the dedicated configuration class marked with @ObjectClassDefinition. They cannot be private.
- Properties which are considered to be private are attached to the @Component annotation. They cannot be changed anymore.
A limitation from a backward compatibility point of view: With SCR annotations you are not limited in the naming of properties, next to characters often the “.” (dot) and the “-” (dash, minus) was used. With OSGI r6 annotations you can easily create a property with a “.” in it
String before_after() default "something";
will result in the property with the name “before.after”; but with OSGI r6 annotations you cannot create properties with a “-” in it. Only OSGI r7 (which is supported in AEM 6.4 onwards) supports it with a construct like this:
String before$_$after() default "something";
If you want to keep compatibility with AEM 6.3, expect the breakage of property names or you need to investigate in workarounds (see #1631 of ACS AEM Commons). But my recommendation is to avoid the use of the “-” in property names alltogether and harmonize this in your project.
Update: I posted an additional blog post specifically on migrating SCR properties, mostly in the context of OSGI DS and OSGI Metatypes.
Labels & description
All the metatype stuff (that means, how OSGI configurations appear in the /system/console/configMgr view) is handled on the level of the @ObjectClassDefinition annotation and the method annotated with it. With the SCR annotations this was all mixed up between the @Component annotation and the @Property fields.
Update the tooling to make it work
If you want to work with OSGI annotations, you should update some elements in your POM as well:
- Update the maven-bundle-plugin to 4.1.0
- Remove the dependency to the maven-scr-plugin
- Add a dependency to org.osgi:org.osgi.annotations:6.0.0 to your POM.
- Then you need to add an additional execution to your maven-bundle-plugin (it’s called “generate-scr-metadata-for-unittests“) and update its configuration (see it on ACS AEM Commons POM).
The interesting part is here is the plugin to the maven-bundle-plugin, which can also handle SCR annotations; this statement allows you to mix both types of annotations.
This blog post should have given you some hints how you migrate the SCR annotations of an existing codebase to OSGI annotations. It’s definitly not a hard task, but some details can be tricky. Therefor it’s cool if you have the chance to mix both types of annotations, so you don’t need a big-bang migration for this.