Can you use both application.yml and application.properties in a Spring Boot project? If so, how are they prioritized?

Question

Accepted Answer

Spring Boot allows both application.yml and application.properties to exist in the same project, and it merges their settings, with application.properties taking precedence on any overlapping key.

Key Points:
• Both files are loaded from the same locations (classpath root, /config subdirectory, etc.).
• When a key is defined in both, the value from application.properties wins.
• YAML supports a hierarchical, nested structure, which is more readable for complex configuration.
• Properties files are flatter, using dot-separated keys, which some teams prefer for simplicity or tooling compatibility.
• Mixing both in a real project is uncommon and can be confusing; most teams pick one format for consistency.

Example:
If application.yml sets server.port: 8080 and application.properties sets server.port=9090, the application will actually start on port 9090 because properties files take priority.

Code Example:
```
# application.yml
server:
  port: 8080

# application.properties
server.port=9090
```

Interview Tip:
A concise interview answer is:

"Yes, both can coexist and Spring Boot merges them, but application.properties wins on any duplicate key. In practice I'd avoid mixing formats in one project -- pick YAML for hierarchical configuration or properties for simplicity, and stick with it to avoid confusing precedence surprises."