Your environment is made of: mvn, and java.

A template is a blueprint for a dynamic page. 

-bash-4.2$ pwd
/u/dgerman/apache/htdocs/spring-boot/fall2019/step02
-bash-4.2$ tree
.
+-- pom.xml
+-- src
    +-- main
        +-- java
        |   +-- com
        |       +-- example
        |           +-- demo
        |               +-- HomeController.java
        |               +-- SpringBoot02Application.java
        +-- resources
            +-- templates
                +-- hometemplate.html

8 directories, 4 files
-bash-4.2$
Then:
  1. mvn package
  2. java -jar target/demo-0.0.1-SNAPSHOT.jar --server.port=31415
Then access online like before.