Create Your Own Incubating Java Modules

Java Modules have some secret features, that are mostly used by the JDK itself. One of these features is the incubating modules.

In the JEP 11: Incubator Modules it is described how it should work

If we use the jdk.incubator.concurrent module as an example - from JEP 428: Structured Concurrency (Incubator) - we can observe that:

• This module is listed in the java --list-modules
• It is not resolved by default
• It warns in the console of the use of the incubating module

One way of resolving an incubating module is by adding a requires some.incubator.module in a module-info.java or by including the --add-modules=some.incubator.module to the JVM command-line flag.

However, JEP 11 only defines how it should work with JMod files. While JMods are great for use with JLink, it is not the current deploy format for Maven Central.

Looking through the JDK source-code, though, I have found out that not only JMod tool supports the creation of incubating modules, but also the Jar tool.

WARNING These command-line flags and the class file attribute are not in the specification. They may be changed or removed without any notice!

There are two main flags:

• –do-not-resolve-by-default
• –warn-if-resolved=value (where value can be one of):
  • incubating
  • deprecated-for-removal
  • deprecated

Using the Javap tool, it is possible to see that the module-info.class is changed to include a (unspecified) attribute: ModuleResolution that has a few valid values:

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ModuleResolution:
  1     //  DO_NOT_RESOLVE_BY_DEFAULT
ModuleResolution:
  2     //  WARN_DEPRECATED
ModuleResolution:
  4     //  WARN_DEPRECATED_FOR_REMOVAL
ModuleResolution:
  8     //  WARN_INCUBATING

Only the first option (DO_NOT_RESOLVE_BY_DEFAULT) can be combined with any of ther other options.

Note Due a bug, it is needed to specify the --warn-if-resolved flag before --do-not-resolve-by-default

In Maven, you can achive this by updating the jar after it has been already created, like this:

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<plugins>
    <plugin>
        <groupId>io.github.wiverson</groupId>
        <artifactId>jtoolprovider-plugin</artifactId>
        <version>1.0.34</version>
        <executions>
            <execution>
                <phase>package</phase>
                <goals>
                    <goal>java-tool</goal>
                </goals>
                <configuration>
                    <toolName>jar</toolName>
                    <writeOutputToLog>true</writeOutputToLog>
                    <writeErrorsToLog>true</writeErrorsToLog>
                    <failOnError>true</failOnError>
                    <args>
                        <arg>--update</arg>
                        <arg>--warn-if-resolved=incubating</arg>                        
                        <arg>--do-not-resolve-by-default</arg>
                        <arg>--file</arg>
                        <arg>${project.build.directory}/${project.build.finalName}.jar</arg>
                        <!-- It seems to be needed to include twice -->
                        <arg>${project.build.directory}/${project.build.finalName}.jar</arg>
                    </args>
                </configuration>
            </execution>
        </executions>
    </plugin>
</plugins>

You can checkout a full example here: https://github.com/Thihup/incubator-module-example