Is it expensive to use try-catch blocks even if an exception is never thrown?

java performance try-catch
jsedano picture jsedano · May 9, 2013 · Viewed 44.9k times · Source

We know that it is expensive to catch exceptions. But, is it also expensive to use a try-catch block in Java even if an exception is never thrown?

I found the Stack Overflow question/answer Why are try blocks expensive?, but it is for .NET.

Answer

Patashu picture Patashu · May 9, 2013

try has almost no expense at all. Instead of doing the work of setting up the try at runtime, the code's metadata is structured at compile time such that when an exception is thrown, it now does a relatively expensive operation of walking up the stack and seeing if any try blocks exist that would catch this exception. From a layman's perspective, try may as well be free. It's actually throwing the exception that costs you - but unless you're throwing hundreds or thousands of exceptions, you still won't notice the cost.

try has some minor costs associated with it. Java cannot do some optimizations on code in a try block that it would otherwise do. For example, Java will often re-arrange instructions in a method to make it run faster - but Java also needs to guarantee that if an exception is thrown, the method's execution is observed as though its statements, as written in the source code, executed in order up to some line.

Because in a try block an exception can be thrown (at any line in the try block! Some exceptions are thrown asynchronously, such as by calling stop on a Thread (which is deprecated), and even besides that OutOfMemoryError can happen almost anywhere) and yet it can be caught and code continue to execute afterwards in the same method, it is more difficult to reason about optimizations that can be made, so they are less likely to happen. (Someone would have to program the compiler to do them, reason about and guarantee correctness, etc. It'd be a big pain for something meant to be 'exceptional') But again, in practice you won't notice things like this.

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