Java: is Exception class thread-safe?

Question

As I understand, Java's Exception class is certainly not immutable (methods like initCause and setStackTrace give some clues about that). So is it at least thread-safe? Suppose one of my classes has a field like this:

private final Exception myException;

Can I safely expose this field to multiple threads? I'm not willing to discuss concrete cases where and why this situation could occur. My question is more about the principle: can I tell that a class which exposes field of Exception type is thread-safe?

Another example:

class CustomException extends Exception
{
   ...
}

Is this class thread-safe?

Answer 1

Note that initCause() is synchronized and setStackTrace() copies its parameter and then does a single assignment.

So Exception actually does seem to be implemented with thread-safety in mind. Still, I'd be wary of any design where exceptions are routinely passed around between threads (i.e. for any reason other than handling a very serious error condition). It just feels wrong.

Answer 2

I believe safely publishing Throwables/Exceptions is a perfectly valid question. DJClayworth's comment that "if you are sharing an instance of Exception between threads then you are using it for a purpose for which it was not designed" does not account for task management code with Futures. It is common for a worker-thread to throw an exception, and for that exception to need to be handled by a different thread. In addition to all the comments above that mention the synchronized methods of Throwable, Future publishes Exceptions between threads, thus I believe it's safe to say that this is expected, safe, and supported functionality.