Using JDK preview features

Since the release of Java 9, the release cycles have become significantly shorter. This makes it possible to release certain features of the language as a preview. This blog post shortly describes how to use these preview features.

The example code

As an example, I wrote a very primitive application that makes use of PatternMatching and records. These features are fully available starting from JDK 16, but have been present as preview features since JDK 14.
The sample application can be found here

Building with maven

For this example, I used the maven-compiler-plugin as well as the maven-shade-plugin. When building the sample application the following error occurs:

...
 records are a preview feature and are disabled by default.
...

The error message is self explanatory and the error itself can be fixed with a minor change in the maven plugin configuration

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.8.1</version>
  <configuration>
    <release>14</release>
    <compilerArgs>
      <arg>--enable-preview</arg>
    </compilerArgs>
  </configuration>
</plugin>

Running the application

Now the build should be successful. When running the application with the jar flag, the following error occurs:

Error: LinkageError occurred while loading main class se.mbaeumer.javalab.java14.Main
	java.lang.UnsupportedClassVersionError: JVMCFRE163 Class file is a preview version but has the wrong major version or preview is not enabled.

The solution is simply to add a flag to enable preview features:

java --enable-preview -jar <path-to-jar>