Spring Boot Client Extension fails with [invalid_token_response] An error occurred while attempting to retrieve the OAuth 2.0

Issue

We have a Spring Boot extension client that interacts with Liferay APIs and makes this OAuth2 challenge:

OAuth2AccessToken oAuth2AccessToken = liferayOAuth2AccessTokenManager.getOAuth2AccessToken(clientRegistrationId);

When we launch the Spring Boot application via bootRun, the OAuth2 Challenge completes without issues. However, when launching it via java -jar spring-boot-client-extension.jar, we receive this error:

DEBUG 80178 --- [main]            : HTTP POST http://localhost:8080/o/oauth2/token
DEBUG 80178 --- [main]            : Accept=[application/json, application/*+json]
DEBUG 80178 --- [main]            : Writing [{grant_type=[client_credentials], client_id=[id-da997878-ba98-79d5-d494-9fa68e7daf0], client_secret=[<secret>]}] as "application/x-www-form-urlencoded;charset=UTF-8"
DEBUG 80178 --- [main]            : Response 401 UNAUTHORIZED
ERROR 80178 --- [main]     : Error during the import : [invalid_token_response] An error occurred while attempting to retrieve the OAuth 2.0 Access Token Response: 401 : [no body]

Environment

Liferay DXP with Client Extensions

Resolution

Running the task via bootRun sets two environment variables on the process: LIFERAY_ROUTES_DXP and LIFERAY_ROUTES_CLIENT_EXTENSION. These point to the routes directories, which hold the client extension's required OAuth2 and Liferay connection details.
When running the jar directly, required steps depend on your deployment model:
- Liferay SaaS: These environment variables are set automatically (no additonal actions are required).
- Liferay PaaS: Set the two environment variables manually before starting the Client Extension process. See Deploying Microservice Client Extensions to Liferay PaaS for more information.
- Liferay Self-Hosted: Set the two environment variables before starting the java process, for example in a shell script.
  - By default, LIFERAY_ROUTES_DXP points to [Liferay Home]/routes/default/dxp and LIFERAY_ROUTES_CLIENT_EXTENSION points to [Liferay Home]/routes/default/[client extension project name].

Additional Information

If the client extension microservice runs on a different host than Liferay, you must also share the $[Liferay Home]/routes directory across systems (using something like NFS, sshfs, etc.)
Working with Client Extensions