Generating a hard-coded build number/version name in your Java app

TLDR;

It’s now possible to easily generate a version number for Java apps during the build as a static final field without any runtime dependencies using the annotation processor of the cloudogu/versionName library.


While developing and operating apps with Java for more than a decade, I found one of the recurring problems when working with (or debugging) an app is the matching of the app to the corresponding source code revision/commit/version. So it’s usually a good idea to add a version name, commit id, or build number to your app. I wrote about this topic multiple times and authored a library (now cloudogu/versionName) to automate this task. I thought I’d be through with this topic. Thouroughly. Honestly.

However, while getting started with GraalVM native images, I struck me that the cloudogu/versionName library has shortcomings: It reads the version name from a file (MANIFEST.MF or properties file) within the jar. GraalVM native images are not packaged as jar, though. While we could of course copy the file manually, it made me wonder if a reading a read-only value from a text files isn’t the proper solution. Why don’t we hard-code this into a Java constant? After all, this feels more read-only than a text file and is much easier to read. Also, there’s no more risk of IOExceptions, different behaviours in JDKs, or environments (jar/war). But obviously the writing of the version name field (i.e. generating code) has to be automated.

I sat and thought how this could be realized. The official way of generating code with Java are annotation processors.  From a user perspective an implementation that uses byte code manipulation could be easier to use. OTOH this would require lots of vodoo. If you’re interested in the amount of voodoo, I recommend reading this classic: Project Lombok – Trick Explained.

So, annotations processors it is! Now, I’m happy to announce that the new version 2.1.0 of cloudogu/versionName contains an annotation processor, that generates a Version.NAME constant. This has another neat side effect: at runtime, there is no longer a need to include the library itself. All is done at compile time, zero dependencies at runtime!

All we have to do is add the dependency during compilation (e.g. using Maven’s provided scope) and set the version we want to create as a compiler argument. When using a build tool, it’s easy to generate the version name or build number as described in one the previous articles, and then pass them on to the compiler. For example with Maven:

<dependency>
    <groupId>com.cloudogu.versionName</groupId>
    <artifactId>processor</artifactId>
       <version>2.1.0</version>
   </dependency>
    <!-- This dependency is only needed during compile time and should not be packaged into final JAR -->
    <scope>provided</scope>
</dependency>
 
<!--- .. -->
 
<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.8.0</version>
    <configuration>
        <compilerArgs>
            <arg>-AversionName=${versionName}</arg>
        </compilerArgs>
    </configuration>
</plugin>

As it is an annotation processor, we finally have to add an annotation @VersionName to a package or class for the annotation processor to hook in the compilation process. This will cause the processor to create a class Version containing a static final field NAME, during compilation. In the application, this can be read like any other field without the risk of any failures at runtime. Why didn’t I think of this before!
Here’s an example for switiching from the regular versionName library to the annotation processor.

One last side note: The annotation processor’s features are verified using unit tests, which is more complex as it might seem with the compilation process and all. Fortunately, I could reuse the approach of the Shiro Static Permission library (by my colleage @cloudogu, Sebastian Sdorra) that uses Google’s compile-testing library. Unit tests for annotation processors, how awesome is that?!

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