How to create Java mixins: @Slf4j

Question

I'm trying to create a Java mixin and use the @Slf4j annotation. However intellij shows an error @Slf4j is only legal for classes and enums.

import lombok.extern.slf4j.Slf4j;

@Slf4j
public interface Foo {
    default void foo() {
        log.info("Hello world");
    }
}

Answer 1

If you want to use logging for the interface you could use a constant:

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public interface Foo {
    Logger log = LoggerFactory.getLogger(Foo.class);

    default void foo() {
        log.info("Hello from Foo");
    }
}

If will have some implementation for this interface and you could overwrite a logger:

public class Bar implements Foo {
    Logger log = LoggerFactory.getLogger(Bar.class);

    public void bar() {
        log.info("Hello from Bar");
    }
}

With Lombok it could be done even simpler:

@Slf4j
class Bar implements Foo {
    public void bar() {
        log.info("Hello from Bar");
    }
}

Demo code:

public class LoggerDemo {
    public static void main(String[] args) {
        Bar bar = new Bar();
        bar.foo();
        bar.bar();
    }
}

Output:

20:40:48.010 [main] INFO demo.Foo - Hello from Foo 
20:40:48.014 [main] INFO demo.Bar - Hello from Bar