This a simple skeleton project that I've found useful that just creates a simple spring-boot application that has a react front end as well.
To run the front end, cd into ./example-frontend and run
yarn start
By default, this will start the front end at http://localhost:3000. If you want to change the default port, modify package.json and change the start script section to
start: export PORT=3006 && react-scripts start
To run the back end, from the project root directory, run
mvn spring-boot:run
By default, this will start the backend server at http://localhost:8080.
To change the port, modify src/main/resources/application.properties to include server.port=xxxx where xxxx is your desired port number.
In the source directory, run
mvn install -DskipTests
In the example-frontend directory run
yarn install
You might have to install the appropriate versions of npm or mvn.
As of writing this tutorial, I am using npm 4.6.1 and I am using maven 3.3.9 with JDK version 1.8.0_92 for building ReactJS and Java code respectively.