Why does @FunctionalInterface have a RUNTIME retention?

Question

Said in Javadoc:

If a type is annotated with this annotation type, compilers are required to generate an error message unless ...

Answer 1

@FunctionalInterface is for runtime reflection, compile check, and java runtime process probably.

javap is used to de-compile and compare two interfaces, one with @FunctionalInterface, the other none.

Just extra two lines byte code in @FunctionalInterface tagged interface:

Constant pool:
   #7 = ...   RuntimeVisibleAnnotations
   #8 = ...   Ljava/lang/FunctionalInterface;

And both implementation/lambda express are same at byte code level.

Except for interface reflection:

X.class.getAnnotation(FunctionalInterface.class) == null?;