Log4j is not logging some stacktraces [duplicate]

Question

I've had instances of our Java code catch a NullPointerException, but when I try to log the StackTrace (which basically ends up calling Throwable.printStackTrace() ), all I get is:

java.lang.NullPointerException

Has anyone else come across this? I tried googling for "java null pointer empty stack trace" but didn't come across anything like this.

Answer 1

You are probably using the HotSpot JVM (originally by Sun Microsystems, later bought by Oracle, part of the OpenJDK), which performs a lot of optimization. To get the stack traces back, you need to pass the following option to the JVM:

-XX:-OmitStackTraceInFastThrow

The optimization is that when an exception (typically a NullPointerException) occurs for the first time, the full stack trace is printed and the JVM remembers the stack trace (or maybe just the location of the code). When that exception occurs often enough, the stack trace is not printed anymore, both to achieve better performance and not to flood the log with identical stack traces.

To see how this is implemented in the HotSpot JVM, grab a copy of it and search for the global variable OmitStackTraceInFastThrow. Last time I looked at the code (in 2019), it was in the file graphKit.cpp.

Answer 2

As you mentioned in a comment, you're using log4j. I discovered (inadvertently) a place where I had written

LOG.error(exc);

instead of the typical

LOG.error("Some informative message", e);

through laziness or perhaps just not thinking about it. The unfortunate part of this is that it doesn't behave as you expect. The logger API actually takes Object as the first argument, not a string - and then it calls toString() on the argument. So instead of getting the nice pretty stack trace, it just prints out the toString - which in the case of NPE is pretty useless.

Perhaps this is what you're experiencing?