Question

We are making an Excel-like system. When we open a document and found unsupported functions we threw exception. We just support small subset of excel functions, this could happen frequently. The problem is when there are a lot of cells that contains unsupported functions, lots and lots of exception instances are created. And creating those many exception instances consumes unignorable amount of time.

We don’t have any special properties within the exception class. What we need to know is the fact that the exception is thrown. We just found that the error has occurred and mark the cell as error.

So we decided to share one exception instance and throw it whenever needed. The exception instance can be thrown by multiple threads. I suspect that the stack trace could be corrupted, however, we don’t see it. We just catch the exception, and mark the corresponding cell as error.

My question is:
In this situation, is it safe to share exception instance?
Well, I read the following article:
Java: is Exception class thread-safe?
But the context seems to be different.

Thank you for reading this long question and response in advance.

Answer 1

[…] is it safe to share exception instance?

Yes, if you’re careful.

If you’re not careful, the getStackTrace for instance may get messed up. Either make sure each thread has it’s own exception object, or override getStackTrace and return an empty array.

(The JVM actually reuses exception instances in some cases. If it runs out of memory, it will reuse a preallocated OutOfMemoryError instead of trying to create a new one for instance. In this case the getStackTrace returns an empty array.)

Related question:

• How can a preallocated OutOfMemoryError truthfully implement Throwable.getStackTrace if thrown twice?