When reading the document I'm curious why the @Memoized have these 2nd parameter. What is the use case? When is the relation between the garbage collection and cache replace behavior?
int protectedCacheSize: Number of cached return values to protect from garbage collection.
This documentation mentions:
By default, the size of the cache is unlimited and no cache result is protected from garbage collection.
Setting a protectedCacheSize>0 would create an unlimited cache with some results protected.
Setting maxCacheSize>0 would create a limited cache but without any protection from garbage protection.
Setting both would create a limited, protected cache.
So a use case would be if you e.g. have a memory intensive application that might run the risk of the cache being garbage collected too often or deeply, then using the protectedCacheSize can forcibly protect a certain number of the most recently used cache entries from garbage collection.
This will make even more sense if the cached values are expensive to compute and the protectedCacheSize is not too high as to prevent the GC from regaining needed memory.
Digging a bit deeper the AST for the @Memoized annotation is handled by the groovy.transform.MemoizedASTTransformation.java which converts the method call to a Closure.
Depending on the max and protected args being 0 or not different memoize methods are then called on the Closure, see the implementation for more detail (scroll down for the three other memoize methods).