How to know about OutOfMemory or StackOverflow errors ahead of time

Question

In Java, is there a way to know that a StackOverflow error or OutOfMemory exception may happen soon?

The OutOfMemory exception m

Answer 1

Usually memory useage is hard to predict.

Stack Overflows from infinite regression generally show up in the write/debug cycle.

Harder to catch are memory issues on things like large held collections, caches, etc.etc. As has been pointed out you can check runtime free and max memory, but be careful, as their meanings aren't obvious - "free" here, means "free now", e.g. meaning if you ran a full gc you might get more. Unfortunately there's no way to get the "total possible free, including garbage-collectible" without running System.gc(), which is not a nice thing to do on a production application (where you're liable to have large enough data sets to cause the problem in the first place) because the entire JVM will come to a screeching halt for a few seconds (or more, in a large app). Note that even System.gc() is not guaranteed to run "now", but my experience has been that it has whenever I've played with it.

You can print gc activity from the running jvm by starting java with -verbose:gc, -XX:+PrintGCTimeStamps, and -XX:+PrintGCDetails (more detail here), and in general if the collector starts to run more frequently, it's probably a sign that you're running out of memory.