# Failed to filter resource 'src/main/resources/application.properties' with value '${db.url}' for project com.example:my-app: Could not resolve placeholder 'db.url' in value '${db.url}'

- **ID:** `java/maven-resource-filtering-error`
- **Domain:** java
- **Category:** build_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Maven resource filtering is enabled, but the placeholder ${db.url} is not defined in any properties file or POM.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 8+ | active | — | — |

## Workarounds

1. **Define the property in pom.xml** (95% success)
   ```
   <properties><db.url>jdbc:mysql://localhost:3306/mydb</db.url></properties>
   ```
2. **Use a properties file with filtering enabled** (90% success)
   ```
   Create src/main/filters/db.properties with db.url=jdbc:mysql://localhost:3306/mydb and add <filters><filter>src/main/filters/db.properties</filter></filters>
   ```

## Dead Ends

- **Disabling resource filtering entirely** — Other placeholders may need filtering, and disabling it breaks them. (70% fail)
- **Adding a random property with a different name** — The placeholder must match exactly; a different name won't be resolved. (90% fail)