Frequently asked questions about writing extensions

See the extension philosophy.

One useful thing extensions can do is bundle other extensions. Have a look at the Quarkus MicroProfile extension for an example of aggregator extensions.

Not every problem needs an extension! If you’re just bundling up external libraries (that aren’t already extensions) and making minor adjustments, you might not need an extension. For example, plain libraries can create new configuration elements and register classes with Jandex (this blog shows how).

A BytecodeTransformerBuildItem can be used to manipulate bytecode. For example, see this blog about removed problematic bridge methods from a dependency.

The CDI integration guide presents solutions to a number of CDI-related use cases for extension authors.

What happens if an extension transforms a user class using BytecodeTransformerBuildItem, and replaces @jakarta.annotation.Resource with @jakarta.inject.Inject? The field will not be injected by Arc. Debugging will show the transformed class being loaded in the app, but it looks like Arc doesn’t see the new code.

Arc-related transformations should generally be done with AnnotationsTransformerBuildItem. The reason is that all Quarkus’s bytecode transformations are done after Jandex indexing. This means changes are never reflected back in Jandex.

Most extensions use Jandex as a source of truth to find out what to do. Those extensions won’t see new/modified endpoints in the bytecode itself. The solution to this limitation is annotation transformers. You should also be aware that while Arc and Quarkus REST honour annotation transformers, not all extensions do.

You will need to implement an AnnotationsTransformer and strip out out the problematic injection sites. (Remember, if the use case involves CDI, it needs to be an AnnotationsTransformer, not a BytecodeTransformer`.) See this blog about on using an AnnotationsTransformer extension to clean non @Inject annotations from the Airline library so that it can be used in CDI-enabled runtimes.

A LogHandlerBuildItem is a convenient way to redirect application logs. See this worked example of an extension which directs output to AWS CloudWatch.

Yes, but it’s not the most typical pattern. See the Building Your First Extension Guide for instructions on setting up a Gradle extension. Have a look at the JobRunr extension for an example implementation.

Registering an extension in the catalog is independent from where the source code is. The quarkiverse repository has some shortcuts to make releasing and testing extensions easier, but any extension can register into the catalog.

Every extension in the extension catalog should appear in http://code.quarkus.io, http://extensions.quarkus.io, and the command line tools. The web pages at http://extensions.quarkus.io are refreshed a few times a delay, so there may be a delay in new extensions showing up there. To debug a missing extension, first:

Both codestarts and quickstarts are designed to help users get coding quickly. A codestarts is a generated application and a quickstart is browsable source code. Codestarts allow the creation of customised apps, which makes them quite powerful.