Code from the java 2d tutorial at cokeandcode. Copied here with permission from author.
gradle jar
gradle run
OR, after "Build with gradle"
java -jar build/libs/*.jar
javac -d build/classes/main/ src/main/java/org/newdawn/spaceinvaders/*.java
java -cp build/classes/main/:src/main/resources org.newdawn.spaceinvaders.Game