This marketplace solution provides Java 12 support for Eclipse 2019-03 (4.11). To install the feature, please ensure you have the 4.11 version of Eclipse IDE installed:

https://download.eclipse.org/eclipse/downloads/drops4/R-4.11-201903070500/

The feature can also be installed via the update site:

https://download.eclipse.org/eclipse/updates/4.11-P-builds

For an informal list of examples of Java 12 features, please have a look at the following page:

https://wiki.eclipse.org/Java12/Examples