Support any interface as an ExtensionPoint

[decebals]

For more information about this feature, please read #289.

[decebals]

1. How it works

This feature is not enabled by default. To activate this feature you must use pf4j.ignoreExtensionPoint flag (option in ExtensionAnnotationProcessor).
If you use Maven, set this flag via compiler arguments

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.8.1</version>
    <configuration>
        <compilerArgs>
            <arg>-Apf4j.ignoreExtensionPoint</arg>
        </compilerArgs>
    </configuration>
</plugin>

2. Show me the code

The Greeting interface from demo was modified

public interface Greeting /*extends ExtensionPoint*/ {

    String getGreeting();

}

NOTE Don't forget to activate this feature via pf4j.ignoreExtensionPoint option in ExtensionAnnotationProcessor.

3. Constraints

If the class annotated with @Extension doesn't implement any interface

@Extension
public static class WelcomeGreeting /*implements Greeting*/ {

    /*@Override*/
    public String getGreeting() {
        return "Welcome";
    }

}

then ExtensionAnnotationProcessor will throw an error when the application is compiled

Error:(36, 19) java: com.mycompany.welcome.WelcomePlugin.WelcomeGreeting is not an extension (it doesn't implement any interface)

If the class annotated with @Extension implements multiple interface

@Extension
public static class WelcomeGreeting implements Greeting, AnInterface {

    @Override
    public String getGreeting() {
        return "Welcome";
    }

}

then ExtensionAnnotationProcessor will throw an error when the application is compiled

Error:(36, 19) java: com.mycompany.welcome.WelcomePlugin.WelcomeGreeting is not an extension (it implements multiple interfaces)