By default, the run goal runs in the same process unless jvm arguments or an agent have been specified. You can enable or disable forking explicitly using the fork attribute.

If you need to fork the process and debug it you can add the necessary JVM arguments to enable remote debugging. The following configuration suspend the process until a debugger has joined on port 5005:

<project>
  ...
  <build>
    ...
    <plugins>
      ...
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
        <version>1.5.17.RELEASE</version>
        <configuration>
          <jvmArguments>
            -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=5005
          </jvmArguments>
        </configuration>
        ...
      </plugin>
      ...
    </plugins>
    ...
  </build>
  ...
</project>

Note that since you specified some JVM arguments, the process is forked automatically. These arguments can be specified on the command line as well, make sure to wrap that properly, that is:

mvn spring-boot:run -Drun.jvmArguments="-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=5005"