Define Maven setup for all SmallRye projects.
Starting with version 13, the SmallRye Parent POM provides a framework for multi-release JAR build and test.
The multi-release JAR support works in two parts: compilation and testing.
Compilation works by providing extra executions of the compiler plugin in order to build the additional JAR layers. The
base layer is built by the standard default-compile execution. After that, Maven profiles are activated based on the
presence of extra layer source directories (e.g. src/main/java18, src/main/java19 etc.). These profiles contain
additional executions of the compiler plugin which compile the sources in the layer directory, while putting the output
of the previous step on the class path.
Each present layer is in turn compiled with the results of all the previous layers on the classpath in the correct
order. The additional layer class files are output under the target/classes directory in the appropriate location for
multi-release JAR layers.
In order to select the correct class files for the given Java version, the <release> property is used.
This prevents accidental usage of APIs which are only present in later versions than the one
being compiled.
Testing using maven-surefire-plugin is supported by running the project unit tests on every supported Java version.
In order to do so, it is expected that the following system property or properties are set as needed:
-
java11.home: this property must be set to the location of a Java 11 JDK installation -
java17.home: this property must be set to the location of a Java 17 JDK installation -
java18.home: this property must be set to the location of a Java 18 JDK installation -
java19.home: this property must be set to the location of a Java 19 JDK installation -
java20.home: this property must be set to the location of a Java 20 JDK installation -
java21.home: this property must be set to the location of a Java 21 JDK installation
In order to simplify development, it is recommended to project maintainers to set these
properties in your personal Maven settings.xml file.
Extra unit tests are run for a given platform whenever a newer version than that platform was used to build the project and the appropriate control file is found (see Build control files reference).
To configure a multi-release JAR, you need the following pieces of information:
-
The minimum (oldest) version of Java that will be supported by the project
-
The maximum (newest) version of Java for which your project has sources
Choose your base layer version. This can be Java 11 or anything later. Configure the version by configuring the
release property in the default-compile execution of maven-compiler-plugin:
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<executions>
<execution>
<id>default-compile</id>
<configuration>
<release>11</release>
</configuration>
</execution>
</executions>
</plugin>If the build-release-11, build-release-17, or build-release-21 file is present in the root of your project, then this step is automatically done for you, for the corresponding version. Only one such file should be present.
Configure the jdk.min.version property as described above to match either:
-
The maximum (newest) Java version for which sources exist in your project, or
-
Some Java version higher than that
This is the version of Java that will build all of your layers, so it necessarily must be able to compile every version of Java sources from oldest to newest.
The sources for your base layer continue to reside in src/main/java and src/test/java.
Additional layers are in directories whose names correspond to the version of Java that
is targeted by that directory. For example, sources which are specific to Java 18 and later
would be in src/main/java18, whereas sources which are specific to Java 19 and later would
be in src/main/java19.
If you have a class that needs an alternative version for a given Java version, you only need to provide the replacement source file in the directory corresponding to the oldest version that supports the alternative source. It is not necessary to copy identical classes into more than one layer; doing so will increase the size of the resultant artifact needlessly.
There are restrictions on these directories. You may only provide sources that correspond to sources that exist in the base layer - that is, it is a violation of the MR JAR specification to provide sources that introduce new APIs only in later Java versions. The JDK does enforce this at run time. In addition, providing additional public members in later versions is generally not recommended.
Using this functionality with GitHub Actions is relatively simple. It entails adding the additional JDK version(s) by way of a setup action, and then passing the location of each additional JDK to the build.
As an example, for a project that is built on Java 17 but must also be tested against JDK 11 your build.yml
might look something like this:
jobs:
build:
runs-on: ubuntu-latest
name: Build using Maven
steps:
- uses: actions/checkout@v2
name: Checkout
- uses: actions/setup-java@v3
name: Set up JDKs
with:
distribution: temurin
java-version: |
11
17
- name: Build
run: mvn -B verify --file pom.xml -Djava11.home=${{env.JAVA_HOME_11_X64}}See also the README for actions/setup-java.
Note that this configuration causes the default JAVA_HOME environment to be set to JDK 17.
These build control files are tested only for their presence. They do not need to have any content (i.e. they can be zero-sized).
| File name | Purpose | Reference |
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Use the |
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Use the |
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Use the |
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Run tests for Java 11 when |
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Run tests for Java 17 when |
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Run tests for Java 18 when |
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Run tests for Java 19 when |
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Run tests for Java 20 when |
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Run tests for Java 21 when |