Java Reflection: Why is it so slow?

Question

I\'ve always avoided Java reflection soley based on its reputation for slowness. I reached a point in the design of my current project where being able to use it would make

Answer 1

The JITted code for instantiating B is incredibly lightweight. Basically it needs to allocate enough memory (which is just incrementing a pointer unless a GC is required) and that's about it - there's no constructor code to call really; I don't know whether the JIT skips it or not but either way there's not a lot to do.

Compare that with everything that reflection has to do:

• Check that there's a parameterless constructor
• Check the accessibility of the parameterless constructor
• Check that the caller has access to use reflection at all
• Work out (at execution time) how much space needs to be allocated
• Call into the constructor code (because it won't know beforehand that the constructor is empty)

... and probably other things I haven't even thought of.

Typically reflection isn't used in a performance-critical context; if you need dynamic behaviour like that, you could use something like BCEL instead.