NullPointerException in Java with no StackTrace

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

User · Answer

toString   only returns the exception name and the optional message   I would suggest calling   exception printStackTrace     to dump the message  or if you need the gory details    StackTraceElement   trace   exception getStackTrace

User · Answer

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

User · Answer

This will output the Exception  use only to debug you should handle you exceptions better   import java io PrintWriter  import java io StringWriter      public static String getStackTrace Throwable t                StringWriter sw   new StringWriter            PrintWriter pw   new PrintWriter sw  true           t printStackTrace pw           pw flush            sw flush            return sw toString

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When you are using AspectJ in your project  it may happen that some aspect hides its portion of the stack trace  For example  today I had   java lang NullPointerException    at com company product MyTest test MyTest java 37    This stack trace was printed when running the test via Maven s surefire   On the other hand  when running the test in IntelliJ  a different stack trace was printed   java lang NullPointerException   at com company product library ArgumentChecker nonNull ArgumentChecker java 67    at       at com company product aspects CheckArgumentsAspect wrap CheckArgumentsAspect java 82    at       at com company product MyTest test MyTest java 37

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Here is an explanation   Hotspot caused exceptions to lose their stack traces in production     and the fix  I ve tested it on Mac OS X   java version  1 6 0 26  Java TM  SE Runtime Environment  build 1 6 0 26-b03-383-11A511   Java HotSpot TM  64-Bit Server VM  build 20 1-b02-383  mixed mode   Object string    abcd   int i   0  while  i  lt  12289        i        try           Integer a    Integer  string        catch  Exception e            e printStackTrace               For this specific fragment of code  12288 iterations   frequency   seems to be the limit where JVM has decided to use preallocated exception

User · Answer

Alternate suggestion - if you re using Eclipse  you could set a breakpoint on NullPointerException itself  in the Debug perspective  go to the  Breakpoints  tab and click on the little icon that has a   in it   Check both the  caught  and  uncaught  options - now when you trigger the NPE  you ll immediately breakpoint and you can then step through and see how exactly it is handled and why you re not getting a stack trace

User · Answer

We have seen this same behavior in the past   It turned out that  for some crazy reason  if a NullPointerException occurred at the same place in the code multiple times  after a while  using Log error String  Throwable  would stop including full stack traces   Try looking further back in your log   You may find the culprit   EDIT  this bug sounds relevant  but it was fixed so long ago it s probably not the cause

User · Answer

Your question is still unclear on whether your code is calling printStackTrace   or this is being done by a logging handler    Here are some possible explanations about what might be happening    The logger   handler being used has been configured to only output the exception s message string  not a full stack trace  Your application  or some third-party library  is logging the exception using LOG error ex   rather than the 2-argument form of  for example  the log4j Logger method  The message is coming from somewhere different to where you think it is  e g  it is actually coming some third-party library method  or some random stuff left over from earlier attempts to debug  The exception that is being logged has overloaded some methods to obscure the stacktrace   If that is the case  the exception won t be a genuine NullPointerException  but will be some custom subtype of NPE or even some unconnected exception      I think that the last possible explanation is pretty unlikely  but people do at least contemplate doing this kind of thing to  prevent  reverse engineering   Of course it only really succeeds in making life difficult for honest developers

User · Answer

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 option -XX -OmitStackTraceInFastThrow to the JVM   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

User · Answer

exception toString does not give you the StackTrace  it only returns     a short description of this throwable    The result is the concatenation of     the name of the class of this object         a colon and a space    the result of invoking this object s getLocalizedMessage   method      Use exception printStackTrace instead to output the StackTrace

[java] NullPointerException in Java with no StackTrace

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