Question

I’m implementing an object cache:

void cache(String request_type, String request_id, ProcessingStrategy request_strategy) 
{ 
    if (no_state(request_strategy) == true)
       map.put(request_type, request_strategy);
    else
       map.put(request_id, request_strategy); //no chance of keyspace collision
}

ProcessingStrategy lookup(String request_type, String request_id)
{
    ProcessingStrategy request_strategy = map.get(request_type);
    if (request_strategy == null) return map.get(request_id)
    else return request_strategy;
}

This scheme of sharing objects implementing interface ProcessingStrategy across all requests of a particular type will only work if there is no state being stored in the concrete class.

How do I write the no_state method?

I am thinking checking if all member fields are final using Reflection will suffice:

for(Field f : request_strategy.getClass().getDeclaredFields())
    if(!Modifier.isFinal(f.getModifiers()))
        return false;
//even if all fields are final;
//how to check that they are not being initialized with differently in the ProcessingStrategy constructor?

Answer 1

What you’re attempting to do (detect mutability of an arbitrary object), I don’t think can be done. Even if it could, through some reflective acrobatics, it still seems a bad idea.

If you control the ProcessingStrategy interface or its implementors, perhaps you can use a method, isStateless(), or a subinterface, StatelessProcessingStrategy which are, by convention, shareable across requests.