Merge pull request #42520 from holly-cummins/extensions-faq

FAQ (frequently asked questions) documentation for extensions

docs/src/main/asciidoc/extension-faq.adoc

@@ -0,0 +1,97 @@
+////
+This document is maintained in the main Quarkus repository
+and pull requests should be submitted there:
+https://github.com/quarkusio/quarkus/tree/main/docs/src/main/asciidoc
+////
+[id="extensions-faq"]
+= Frequently asked questions about writing extensions
+include::_attributes.adoc[]
+:diataxis-type: howto
+:categories: extensions
+////
+:extension-status: preview
+TODO: uncomment the above for experimental or tech-preview content.
+The document header ends at the first blank line. Do not remove the blank line between the header and the abstract summary.
+////
+
+## Should you write an extension?
+
+### Why would I want to write an extension?
+
+See the xref:writing-extensions#extension-philosophy[extension philosophy].
+
+One useful thing extensions can do is bundle other extensions.
+Have a look at the link:https://quarkus.io/extensions/io.quarkiverse.microprofile/quarkus-microprofile/[Quarkus MicroProfile extension] for an example of aggregator extensions.
+
+### Are there cases an extension isn't necessary?
+
+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 link:https://www.loicmathieu.fr/wordpress/en/informatique/quarkus-tip-comment-ne-pas-creer-une-extension-quarkus/[blog shows how]).
+
+
+## Bytecode transformation
+
+### How can I change the code of things on the classpath?
+
+A `BytecodeTransformerBuildItem` can be used to manipulate bytecode.
+For example, see this link:https://quarkus.io/blog/solving-problems-with-extensions/[blog about removed problematic bridge methods from a dependency].
+
+## CDI
+
+### I'm working with CDI, and I don't know how to ...
+
+The xref:cdi-integration.adoc[CDI integration guide] presents solutions to a number of CDI-related use cases for extension authors.
+
+### I have transformed a user class to add an injected field, but CDI isn't working
+
+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 link:https://github.com/quarkusio/quarkus/blob/main/extensions/arc/deployment/src/main/java/io/quarkus/arc/deployment/AnnotationsTransformerBuildItem.java[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.
+
+### Something in my classpath has @Inject annotations, which are confusing CDI. How can I fix that?
+
+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 link:https://quarkus.io/blog/solving-problems-with-extensions-2/[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.
+
+## Cross-cutting concerns
+
+### How can I redirect application logging to an external service?
+
+A `LogHandlerBuildItem` is a convenient way to redirect application logs. See this link:https://quarkus.io/blog/quarkus-aws-cloudwatch_extension/[worked example of an extension which directs output to AWS CloudWatch].
+
+## Build and hosting infrastructure for extensions
+
+### Can I use Gradle to build my extension?
+
+Yes, but it's not the most typical pattern.
+See the xref:building-my-first-extension.adoc#gradle-setup[Building Your First Extension Guide] for instructions on setting up a Gradle extension. Have a look at the link:https://quarkus.io/extensions/org.jobrunr/quarkus-jobrunr/[JobRunr extension] for an example implementation.
+
+### If I want my extension to be in code.quarkus.io, does it have to be in the Quarkiverse GitHub org?
+
+Registering an extension in the catalog is independent from where the source code is.
+The link:https://hub.quarkiverse.io[quarkiverse repository] has some shortcuts to make releasing and testing extensions easier, but any extension can link:https://hub.quarkiverse.io/checklistfornewprojects/#make-your-extension-available-in-the-tooling[register into the catalog].
+
+### My extension isn't showing up on extensions.quarkus.io
+
+Every extension in the link:https://github.com/quarkusio/quarkus-extension-catalog/tree/main/extensions[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:
+
+- Check your extension is present in link:https://central.sonatype.com/[Maven Central]
+- Check the extension is included the link:https://github.com/quarkusio/quarkus-extension-catalog/tree/main/extensions[extensions catalog list] (it only needs to be included once, and future versions will be automatically detected)
+- Check if the extension is listed in the http://https://registry.quarkus.io/q/swagger-ui/#/Client/get_client_extensions_all[Quarkus registry] list of all known extensions
+- Check if there has been a green link:https://github.com/quarkusio/extensions/actions/workflows/build_and_publish.yml[build of the extensions site] since updating the catalog
+
+## Other topics
+
+
+### What's the difference between a quickstart and a codestart?
+
+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.

docs/src/main/asciidoc/writing-extensions.adoc

@@ -17,7 +17,8 @@ Quarkus extensions add a new developer focused behavior to the core offering, an
 This means that metadata is only processed once at build time, which both saves on startup time, and also on memory
 usage as the classes etc that are used for processing are not loaded (or even present) in the runtime JVM.
 
-NOTE: This is an in-depth documentation, see the xref:building-my-first-extension.adoc[building my first extension] if you need an introduction.
+NOTE: This is an in-depth documentation, see the xref:building-my-first-extension.adoc[building my first extension] if you need an introduction,
+or the xref:extension-faq.adoc[frequently asked questions].
 
 == Extension philosophy