Compile and execute a JDK preview feature with Maven

Question

With JDK/12 EarlyAccess Build 10, the JEP-325 Switch Expressions has been integrated as a preview feature in the JDK. A sample code for the expressions (as in the JEP as wel

Answer 1

To enable preview feature you must define --enable-preview in pom.xml under compilerArgs

in below I mention how to enable preview feature with java 13.

<build>
  <plugins>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-compiler-plugin</artifactId>
      <version>3.8.0</version>
      <configuration>
        <release>13</release>
        <compilerArgs>
          --enable-preview
        </compilerArgs>
      </configuration>
    </plugin>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-surefire-plugin</artifactId>
      <version>3.0.0-M3</version>
      <configuration>
        <argLine>--enable-preview</argLine>
      </configuration>
    </plugin>
  </plugins>
 </build>

Answer 2

Step 1

One can make use of the following maven configurations to compile the code using the --enable-preview along with --release 12+ (e.g. 13, 14, 15) argument.

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.8.0</version>
            <configuration>
                <release>12</release> <!-- <release>13/14/15</release> -->
                <compilerArgs>--enable-preview</compilerArgs>
            </configuration>
        </plugin>
        <!-- This is just to make sure the class is set as main class to execute from the jar-->
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-shade-plugin</artifactId>
            <version>3.1.0</version>
            <executions>
                <execution>
                    <phase>package</phase>
                    <goals>
                        <goal>shade</goal>
                    </goals>
                    <configuration>
                        <transformers>
                            <transformer
                                    implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer"/>
                            <transformer
                                    implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                                <mainClass>edu.forty.bits.expression.SwitchExpressions</mainClass>
                            </transformer>
                        </transformers>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

Note:- I had to also ensure on my MacOS that my ~/.mavenrc file was configured to mark java 13 as the default java configured for maven.

Step 2

Execute the maven command to build the jar from the module classes

mvn clean verify 

Step 3

Use the command line to execute the main class of the jar created in the previous step as :

java --enable-preview -jar target/forty-bits-of-java-1.0.0-SNAPSHOT.jar

the last argument is the path to the jar built by maven.

This produces the output as expected as:

(screenshot is from a previous execution.)

Source on GitHub

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Edit: A learning from an unwanted debugging session, use the arguments in the format as follows:

<compilerArgs>
    <arg>--enable-preview</arg>
</compilerArgs>

Reason being, if you specify two different arguments it doesn't fail during the configuration validation and the one found later overrules the effective config:

<compilerArgs>--enable-preview</compilerArgs>
<compilerArgs>-Xlint:all</compilerArgs>