# org.junit.jupiter.api.extension.ExtensionConfigurationException: Extension [com.example.MyExtension] is not registered

- **ID:** `java/junit-extension-not-registered`
- **Domain:** java
- **Category:** config_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Custom JUnit extension is used via @ExtendWith but not properly registered or class not found.

## Version Compatibility

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

## Workarounds

1. **Register extension with @RegisterExtension** (90% success)
   ```
   @RegisterExtension
static MyExtension extension = new MyExtension();
   ```
2. **Ensure extension class is public and has no-arg constructor** (85% success)
   ```
   public class MyExtension implements BeforeEachCallback {
    public MyExtension() { }
}
   ```

## Dead Ends

- **Removing @ExtendWith annotation** — Extension functionality is lost; tests may fail. (60% fail)
- **Adding extension as a dependency in build file** — Extension is already available; registration issue is different. (40% fail)