I managed to load an application.properties file in external path while using -jar option.
The key was PropertiesLauncher.
To use PropertiesLauncher, pom.xml file must be changed like this:
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
<configuration> <!-- added -->
<layout>ZIP</layout> <!-- to use PropertiesLaunchar -->
</configuration>
</plugin>
</plugins>
</build>
For this, I referenced the following StackOverflow question: spring boot properties launcher unable to use . BTW, In Spring Boot Maven Plugin document(http://docs.spring.io/spring-boot/docs/1.1.7.RELEASE/maven-plugin/repackage-mojo.html), there is no mention that specifying ZIP triggers that PropertiesLauncher is used. (Perhaps in another document?)
After the jar file had been built, I could see that the PropertiesLauncher is used by inspecting Main-Class property in META-INF/MENIFEST.MF in the jar.
Now, I can run the jar as follows(in Windows):
java -Dloader.path=file:///C:/My/External/Dir,MyApp-0.0.1-SNAPSHOT.jar -jar MyApp-0.0.1-SNAPSHOT.jar
Note that the application jar file is included in loader.path.
Now an application.properties file in C:\My\External\Dir\config is loaded.
As a bonus, any file (for example, static html file) in that directory can also be accessed by the jar since it's in the loader path.
As for the non-jar (expanded) version mentioned in UPDATE 2, maybe there was a classpath order problem.