java.lang.ClassNotFoundException: org.springframework.boot.SpringApplication Maven

If you would like to stay in spring boot space just set the pom packaging to jar

<packaging>jar</packaging>

and add the spring-boot-maven-plugin to you build properties in the pom.xml file:

<build>
  <plugins>
    <plugin>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-maven-plugin</artifactId>
    </plugin>
  </plugins>
</build>

then a simple mvn package command will create a complete executable jar file.

See the very good spring reference doc for more details (doing it gradle style also) spring reference doc

Your jar does not contain the dependencies such as spring-boot which it is looking for during runtime. You can build a consolidated jar with all the dependencies as outlined in this question.

Mine was caused by a corrupt Maven repository.

I deleted everything under C:\Users\<me>\.m2\repository.

Then did an Eclipse Maven Update, and it worked first time.

So it was simply spring-boot.jar got corrupted.