How do I tell Spring Boot which main class to use for the executable jar?

If you do NOT use the spring-boot-starter-parent pom, then from the Spring documentation:

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <version>1.1.3.RELEASE</version>
    <configuration>
        <mainClass>my.package.MyStartClass</mainClass>
        <layout>ZIP</layout>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>repackage</goal>
            </goals>
        </execution>
    </executions>
</plugin>

For those using Gradle (instead of Maven) :

springBoot {
    mainClass = "com.example.Main"
}

Add your start class in your pom:

<properties>
    <!-- The main class to start by executing java -jar -->
    <start-class>com.mycorp.starter.HelloWorldApplication</start-class>
</properties>

or

<build>
<plugins>
    <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>             
        <configuration>    
            <mainClass>com.mycorp.starter.HelloWorldApplication</mainClass>
        </configuration>
    </plugin>
</plugins>
</build>