Support externalized configuration for launching with bazel run

[plaird]

It is a useful feature for Spring Boot to load external configuration files (files not included in the exectuable jar) at runtime. This allows you to override some properties in production for example. You can do this via command line arguments, but the property file approach is common.

https://docs.spring.io/spring-boot/docs/current/reference/html/features.html#features.external-config.files

Spring Boot will automatically find and load application.properties and application.yaml files from the following locations when your application starts:

• The current directory
• The /config subdirectory in the current directory
• Immediate child directories of the /config subdirectory

This works if you launch the rules_spring built executable jar with java -jar:

java -jar example.jar

But we don't have a great way to do this when launching the application with bazel run. As a workaround the developer can invoke bazel run like this:

bazel run //examples/demoapp -- --spring.config.location=/tmp/demoapp/config/

but it feels like we should automatically honor package level files like:

• //examples/demoapp/application.properties
• //examples/demoapp/config/application.properties
• //examples/demoapp/config/secure/application.properties

When working on this, remember to support profile specific prop files: https://docs.spring.io/spring-boot/docs/current/reference/html/features.html#features.external-config.files.profile-specific

[plaird]

The right solution to this is to use bazelrun_data.

filegroup(
    name = "bazelrun_data_files",
    srcs = [
        "application.properties",
        "config/application.properties",
    ],
)

springboot(
   ...
   bazelrun_data = [":bazelrun_data_files"],
)

which adds the config files to the target's runfiles. Then, we should derive the directories from the data list that contain application*.properties files, and add those directories to a auto spring.config.location option passed when invoking the JVM.

That said, I am having problems finding a good way to resolve the paths to each data file prior to handing off to the write_bazelrun_env.sh shell script. This probably has to wait until we remove shell scripts from the impl.

[plaird]

It turns out this was doable with our current bash script implementation.

Docs:
https://github.com/salesforce/rules_spring/blob/main/springboot/bazelrun.md#external-configuration-with-applicationproperties