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 well):

Scanner scanner = new Scanner(System.in); Day day = Day.valueOf(scanner.next()); switch (day) {     case MONDAY, TUESDAY -> System.out.println("Back to work.") ;     case WEDNESDAY -> System.out.println("Wait for the end of week...") ;     case THURSDAY,FRIDAY -> System.out.println("Plan for the weekend?");     case SATURDAY, SUNDAY -> System.out.println("Enjoy the holiday!"); } 

where Day being an enum as

public enum Day {     MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY } 

The Preview Language and VM Features JEP-12 already elaborate how a feature can be enabled during compile and runtime using javac and java.

How can one try out this feature using Maven?

Answer 1

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>