[slf4j-dev] Lambda support in SLF4J

[Ceki Gülcü]

Hello all,

Given that JDK 7 earlier are used by over 50% of Java projects, I do not 
think we can force users to upgrade to Java 8. Thus, SLF4J is unlikely 
to bump its JDK requirement to version 8 for yet some time.

On the bright side, there is a way for org.slf4j.Logger to support Java8 
lambdas, or just lambdas, without requiring Java 8. As stated in [1], 
the compiler will treat any interface meeting the definition of a 
functional interface as a functional interface regardless of whether or 
not a @FunctionalInterface annotation is present on the interface 
declaration.

This means that we can create a Supplier interface such as

public interface Supplier<T> {
     T get();
}

and then add the following method to org.slf4j.Logger interface.

package org.slf4j;

public interface Logger {
   ....
   public void debug(Supplier<T> supplier);
   ....
}

This will add lambda support without requiring Java 8 support at build time.

The downside of this approach is that implementations of the 
org.slf4j.Logger interface as found in logback and modules such as 
slf4j-simple, slf4j-log412, slf4j-jdk14 will need to be updated. Given 
that the number of such implementations is limited, I think the 
requirement is quite acceptable.

Your comments are welcome,

--
Ceki Gülcü


[1] 
https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html