Since EL Lambdas predate Java functional interfaces, custom EL resolvers must be used to as bridge between javax.el.LambdaExpression, and actual method with a accepting functional interface parameter.

Java EE 8 revision of EL should either

1. bring sufficient support for all Java SE methods accepting functional interfaces, or
2. offer implicit type conversion from LambdaExpression to appropriate types defined in java.util.function, or
3. offer implicit conversion to any Functional interface conforming to §9.8 of JLS.

Example implementation of the first approach for java.util.Optional can be seen here