Upgrade Spring Boot Admin to 4.0.0-M2 for Spring Boot 4 compatibility

[yousfiSaad]

Summary

This PR fixes DNS resolution issues in Docker environments for Spring Boot Admin
when running with Spring Boot 4.

Problem

Spring Boot Admin fails to resolve container hostnames in Docker environments
due to Netty's native epoll resolver, which has issues resolving container
hostnames in Docker.

Solution

• Upgrade Spring Boot Admin from 3.4.1 to 4.0.0-M2 for Spring Boot 4 compatibility
• Add WebClientConfig with DefaultAddressResolverGroup to use JVM's built-in DNS
  resolver instead of Netty's native resolver

Changes

• spring-petclinic-admin-server/pom.xml: Spring Boot Admin 3.4.1 → 4.0.0-M2
• spring-petclinic-admin-server/src/main/java/.../admin/config/WebClientConfig.java: NEW

Related Issues

Fixes #487

[yousfiSaad]

Hi @arey,

I understand your hesitation about the milestone version (4.0.0-M2). I'm not entirely convinced we should merge this as-is either - I mainly wanted to share the findings and the potential workaround.

Some context on why I created this PR:

The WebClientConfig with DefaultAddressResolverGroup is the key fix here - it resolves the Docker DNS resolution issue by using the JVM's built-in DNS resolver instead of Netty's native epoll resolver. This is a documented workaround for container hostname resolution issues.

Regarding the milestone version:

• The WebClientConfig approach could potentially work with Spring Boot Admin 3.4.x as well
• Upgrading to 4.0.0-M2 was primarily for Spring Boot 4 compatibility, but milestone versions do carry risk

Options moving forward:

1. Close this PR and wait for Spring Boot Admin 4.x stable release
2. Test the WebClientConfig fix with Spring Boot Admin 3.4.x and revert just the dependency upgrade
3. Keep this open as a WIP/draft until Spring Boot Admin 4.0.0 is officially released

I'm open to your thoughts on the best path forward.

[arey]

Hi @yousfiSaad, could you describe the scenario that leads to the DNS resolution issue you mentioned?
Did you find some documentation about into the Netty or Spring Boot Admin documentation?

[yousfiSaad]

Update: Investigation Results

After further investigation, the original diagnosis of a "Netty DNS resolution issue" was incorrect.

What was actually wrong

The real issue was Spring Boot 4 incompatibility with Spring Boot Admin 3.4.1, which caused:

• ClassNotFoundException: WebClientAutoConfiguration

The fix

Upgrading Spring Boot Admin from 3.4.1 to 4.0.0-M2 resolved the compatibility issue.

Why no WebClientConfig?

The originally proposed WebClientConfig class (to work around DNS resolution) was not needed because:

• No actual DNS resolution errors (UnknownHostException) were observed
• The default Netty behavior works correctly in Docker environments

Lesson learned

Always verify the root cause before implementing workarounds. The original symptoms were likely caused by insufficient VM memory, not DNS issues.

---

Note: The branch name (fix/admin-server-dns-resolution) and PR title reference DNS, but the actual fix was the SBA version upgrade for Spring Boot 4 compatibility. I'm happy to rename the branch/PR title if preferred.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[arey]

Duplicated with #513