Java Reflection: Why is it so slow?

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Reflection is slow for a few obvious reasons:

  1. The compiler can do no optimization whatsoever as it can have no real idea about what you are doing. This probably goes for the JIT as well
  2. Everything being invoked/created has to be discovered (i.e. classes looked up by name, methods looked at for matches etc)
  3. Arguments need to be dressed up via boxing/unboxing, packing into arrays, Exceptions wrapped in InvocationTargetExceptions and re-thrown etc.
  4. All the processing that Jon Skeet mentions here.

Just because something is 100x slower does not mean it is too slow for you assuming that reflection is the “right way” for you to design your program. For example, I imagine that IDEs make heavy use of reflection and my IDE is mostly OK from a performance perspective.

After all, the overhead of reflection is likely to pale into insignificance when compared with, say, parsing XML or accessing a database!

Another point to remember is that micro-benchmarks are a notoriously flawed mechanism for determining how fast something is in practice. As well as Tim Bender’s remarks, the JVM takes time to “warm up”, the JIT can re-optimize code hotspots on-the-fly etc.

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