What is a stack trace and how can I use it to debug my application errors

Question

Sometimes when I run my application it gives me an error that looks like   Exception in thread  main  java lang NullPointerException         at com example myproject Book getTitle Book java 16          at com example myproject Author getBookTitles Author java 25          at com example myproject Bootstrap main Bootstrap java 14    People have referred to this as a  stack trace   What is a stack trace  What can it tell me about the error that s happening in my program     About this question - quite often I see a question come through where a novice programmer is  getting an error   and they simply paste their stack trace and some random block of code without understanding what the stack trace is or how they can use it  This question is intended as a reference for novice programmers who might need help understanding the value of a stack trace

User · Answer

The other posts describe what a stack trace is, but it can still be hard to work with.

If you get a stack trace and want to trace the cause of the exception, a good start point in understanding it is to use the Java Stack Trace Console in Eclipse. If you use another IDE there may be a similar feature, but this answer is about Eclipse.

First, ensure that you have all of your Java sources accessible in an Eclipse project.

Then in the Java perspective, click on the Console tab (usually at the bottom). If the Console view is not visible, go to the menu option Window -> Show View and select Console.

Then in the console window, click on the following button (on the right)

Consoles button

and then select Java Stack Trace Console from the drop-down list.

Paste your stack trace into the console. It will then provide a list of links into your source code and any other source code available.

This is what you might see (image from the Eclipse documentation):

Diagram from Eclipse documentation

The most recent method call made will be the top of the stack, which is the top line (excluding the message text). Going down the stack goes back in time. The second line is the method that calls the first line, etc.

If you are using open-source software, you might need to download and attach to your project the sources if you want to examine. Download the source jars, in your project, open the Referenced Libraries folder to find your jar for your open-source module (the one with the class files) then right click, select Properties and attach the source jar.

User · Answer

To understand the name  A stack trace is a a list of Exceptions  or you can say a list of  Cause by    from the most surface Exception e g  Service Layer Exception  to the deepest one  e g  Database Exception   Just like the reason we call it  stack  is because stack is First in Last out  FILO   the deepest exception was happened in the very beginning  then a chain of exception was generated a series of consequences  the surface Exception was the last one happened in time  but we see it in the first place   Key 1 A tricky and important thing here need to be understand is   the deepest cause may not be the  root cause   because if you write some  bad code   it may cause some exception underneath which is deeper than its layer  For example  a bad sql query may cause SQLServerException connection reset in the bottem instead of syndax error  which may just in the middle of the stack   -  Locate the root cause in the middle is your job    Key 2 Another tricky but important thing is inside each  Cause by  block  the first line was the deepest layer and happen first place for this block  For instance   Exception in thread  main  java lang NullPointerException         at com example myproject Book getTitle Book java 16             at com example myproject Author getBookTitles Author java 25                 at com example myproject Bootstrap main Bootstrap java 14    Book java 16 was called by Auther java 25 which was called by Bootstrap java 14  Book java 16 was the root cause  Here attach a diagram sort the trace stack in chronological order

User · Answer

In simple terms  a stack trace is a list of the method calls that the application was in the middle of when an Exception was thrown   Simple Example  With the example given in the question  we can determine exactly where the exception was thrown in the application  Let s have a look at the stack trace   Exception in thread  main  java lang NullPointerException         at com example myproject Book getTitle Book java 16          at com example myproject Author getBookTitles Author java 25          at com example myproject Bootstrap main Bootstrap java 14    This is a very simple stack trace  If we start at the beginning of the list of  at       we can tell where our error happened  What we re looking for is the topmost method call that is part of our application  In this case  it s   at com example myproject Book getTitle Book java 16    To debug this  we can open up Book java and look at line 16  which is   15   public String getTitle     16      System out println title toString     17      return title  18       This would indicate that something  probably title  is null in the above code   Example with a chain of exceptions  Sometimes applications will catch an Exception and re-throw it as the cause of another Exception   This typically looks like   34   public void getBookIds int id    35      try   36         book getId id         this method it throws a NullPointerException on line 22 37        catch  NullPointerException e    38         throw new IllegalStateException  A book has a null property   e  39        40       This might give you a stack trace that looks like   Exception in thread  main  java lang IllegalStateException  A book has a null property         at com example myproject Author getBookIds Author java 38          at com example myproject Bootstrap main Bootstrap java 14  Caused by  java lang NullPointerException         at com example myproject Book getId Book java 22          at com example myproject Author getBookIds Author java 36              1 more   What s different about this one is the  Caused by   Sometimes exceptions will have multiple  Caused by  sections  For these  you typically want to find the  root cause   which will be one of the lowest  Caused by  sections in the stack trace  In our case  it s   Caused by  java lang NullPointerException  lt -- root cause         at com example myproject Book getId Book java 22   lt -- important line   Again  with this exception we d want to look at line 22 of Book java to see what might cause the NullPointerException here   More daunting example with library code  Usually stack traces are much more complex than the two examples above  Here s an example  it s a long one  but demonstrates several levels of chained exceptions    javax servlet ServletException  Something bad happened     at com example myproject OpenSessionInViewFilter doFilter OpenSessionInViewFilter java 60      at org mortbay jetty servlet ServletHandler CachedChain doFilter ServletHandler java 1157      at com example myproject ExceptionHandlerFilter doFilter ExceptionHandlerFilter java 28      at org mortbay jetty servlet ServletHandler CachedChain doFilter ServletHandler java 1157      at com example myproject OutputBufferFilter doFilter OutputBufferFilter java 33      at org mortbay jetty servlet ServletHandler CachedChain doFilter ServletHandler java 1157      at org mortbay jetty servlet ServletHandler handle ServletHandler java 388      at org mortbay jetty security SecurityHandler handle SecurityHandler java 216      at org mortbay jetty servlet SessionHandler handle SessionHandler java 182      at org mortbay jetty handler ContextHandler handle ContextHandler java 765      at org mortbay jetty webapp WebAppContext handle WebAppContext java 418      at org mortbay jetty handler HandlerWrapper handle HandlerWrapper java 152      at org mortbay jetty Server handle Server java 326      at org mortbay jetty HttpConnection handleRequest HttpConnection java 542      at org mortbay jetty HttpConnection RequestHandler content HttpConnection java 943      at org mortbay jetty HttpParser parseNext HttpParser java 756      at org mortbay jetty HttpParser parseAvailable HttpParser java 218      at org mortbay jetty HttpConnection handle HttpConnection java 404      at org mortbay jetty bio SocketConnector Connection run SocketConnector java 228      at org mortbay thread QueuedThreadPool PoolThread run QueuedThreadPool java 582  Caused by  com example myproject MyProjectServletException     at com example myproject MyServlet doPost MyServlet java 169      at javax servlet http HttpServlet service HttpServlet java 727      at javax servlet http HttpServlet service HttpServlet java 820      at org mortbay jetty servlet ServletHolder handle ServletHolder java 511      at org mortbay jetty servlet ServletHandler CachedChain doFilter ServletHandler java 1166      at com example myproject OpenSessionInViewFilter doFilter OpenSessionInViewFilter java 30          27 more Caused by  org hibernate exception ConstraintViolationException  could not insert   com example myproject MyEntity      at org hibernate exception SQLStateConverter convert SQLStateConverter java 96      at org hibernate exception JDBCExceptionHelper convert JDBCExceptionHelper java 66      at org hibernate id insert AbstractSelectingDelegate performInsert AbstractSelectingDelegate java 64      at org hibernate persister entity AbstractEntityPersister insert AbstractEntityPersister java 2329      at org hibernate persister entity AbstractEntityPersister insert AbstractEntityPersister java 2822      at org hibernate action EntityIdentityInsertAction execute EntityIdentityInsertAction java 71      at org hibernate engine ActionQueue execute ActionQueue java 268      at org hibernate event def AbstractSaveEventListener performSaveOrReplicate AbstractSaveEventListener java 321      at org hibernate event def AbstractSaveEventListener performSave AbstractSaveEventListener java 204      at org hibernate event def AbstractSaveEventListener saveWithGeneratedId AbstractSaveEventListener java 130      at org hibernate event def DefaultSaveOrUpdateEventListener saveWithGeneratedOrRequestedId DefaultSaveOrUpdateEventListener java 210      at org hibernate event def DefaultSaveEventListener saveWithGeneratedOrRequestedId DefaultSaveEventListener java 56      at org hibernate event def DefaultSaveOrUpdateEventListener entityIsTransient DefaultSaveOrUpdateEventListener java 195      at org hibernate event def DefaultSaveEventListener performSaveOrUpdate DefaultSaveEventListener java 50      at org hibernate event def DefaultSaveOrUpdateEventListener onSaveOrUpdate DefaultSaveOrUpdateEventListener java 93      at org hibernate impl SessionImpl fireSave SessionImpl java 705      at org hibernate impl SessionImpl save SessionImpl java 693      at org hibernate impl SessionImpl save SessionImpl java 689      at sun reflect GeneratedMethodAccessor5 invoke Unknown Source      at sun reflect DelegatingMethodAccessorImpl invoke DelegatingMethodAccessorImpl java 25      at java lang reflect Method invoke Method java 597      at org hibernate context ThreadLocalSessionContext TransactionProtectionWrapper invoke ThreadLocalSessionContext java 344      at  Proxy19 save Unknown Source      at com example myproject MyEntityService save MyEntityService java 59   lt -- relevant call  see notes below      at com example myproject MyServlet doPost MyServlet java 164          32 more Caused by  java sql SQLException  Violation of unique constraint MY ENTITY UK 1  duplicate value s  for column s  MY COLUMN in statement           at org hsqldb jdbc Util throwError Unknown Source      at org hsqldb jdbc jdbcPreparedStatement executeUpdate Unknown Source      at com mchange v2 c3p0 impl NewProxyPreparedStatement executeUpdate NewProxyPreparedStatement java 105      at org hibernate id insert AbstractSelectingDelegate performInsert AbstractSelectingDelegate java 57          54 more   In this example  there s a lot more  What we re mostly concerned about is looking for methods that are from our code  which would be anything in the com example myproject package  From the second example  above   we d first want to look down for the root cause  which is   Caused by  java sql SQLException   However  all the method calls under that are library code  So we ll move up to the  Caused by  above it  and look for the first method call originating from our code  which is   at com example myproject MyEntityService save MyEntityService java 59    Like in previous examples  we should look at MyEntityService java on line 59  because that s where this error originated  this one s a bit obvious what went wrong  since the SQLException states the error  but the debugging procedure is what we re after

User · Answer

To add on to what Rob has mentioned   Setting break points in your application allows for the step-by-step processing of the stack   This enables the developer to use the debugger to see at what exact point the method is doing something that was unanticipated   Since Rob has used the NullPointerException  NPE  to illustrate something common  we can help to remove this issue in the following manner     if we have a method that takes parameters such as   void  String firstName     In our code we would want to evaluate that firstName contains a value  we would do this like so  if firstName    null    firstName equals       return   The above prevents us from using firstName as an unsafe parameter   Therefore  by doing null checks before processing we can help to ensure that our code will run properly   To expand on an example that utilizes an object with methods we can look here     if dog    null    dog firstName    null   return   The above is the proper order to check for nulls  we start with the base object  dog in this case  and then begin walking down the tree of possibilities to make sure everything is valid before processing   If the order were reversed a NPE could potentially be thrown and our program would crash

User · Answer

Just to add to the other examples  there are inner nested  classes that appear with the   sign  For example   public class Test        private static void privateMethod             throw new RuntimeException               public static void main String   args  throws Exception           Runnable runnable   new Runnable                  Override public void run                     privateMethod                                     runnable run              Will result in this stack trace   Exception in thread  main  java lang RuntimeException         at Test privateMethod Test java 4          at Test access 000 Test java 1          at Test 1 run Test java 10          at Test main Test java 13

User · Answer

There is one more stacktrace feature offered by Throwable family - the possibility to manipulate stack trace information   Standard behavior   package test stack trace   public class SomeClass        public void methodA             methodB               public void methodB             methodC               public void methodC             throw new RuntimeException               public static void main String   args            new SomeClass   methodA              Stack trace   Exception in thread  main  java lang RuntimeException     at test stack trace SomeClass methodC SomeClass java 18      at test stack trace SomeClass methodB SomeClass java 13      at test stack trace SomeClass methodA SomeClass java 9      at test stack trace SomeClass main SomeClass java 27    Manipulated stack trace   package test stack trace   public class SomeClass                 public void methodC             RuntimeException e   new RuntimeException            e setStackTrace new StackTraceElement                    new StackTraceElement  OtherClass    methodX    String java   99                   new StackTraceElement  OtherClass    methodY    String java   55                      throw e             public static void main String   args            new SomeClass   methodA              Stack trace   Exception in thread  main  java lang RuntimeException     at OtherClass methodX String java 99      at OtherClass methodY String java 55

User · Answer

I am posting this answer so the topmost answer  when sorted by activity  is not one that is just plain wrong   What is a Stacktrace   A stacktrace is a very helpful debugging tool  It shows the call stack  meaning  the stack of functions that were called up to that point  at the time an uncaught exception was thrown  or the time the stacktrace was generated manually   This is very useful because it doesn t only show you where the error happened  but also how the program ended up in that place of the code  This leads over to the next question   What is an Exception   An Exception is what the runtime environment uses to tell you that an error occurred  Popular examples are NullPointerException  IndexOutOfBoundsException or ArithmeticException  Each of these are caused when you try to do something that is not possible  For example  a NullPointerException will be thrown when you try to dereference a Null-object   Object a   null  a toString                      this line throws a NullPointerException  Object   b   new Object 5   System out println b 10         this line throws an IndexOutOfBoundsException                                  because b is only 5 elements long int ia   5  int ib   0  ia   ia ib                      this line throws an  ArithmeticException with the                                  message    by 0   because you are trying to                                 divide by 0  which is not possible    How should I deal with Stacktraces Exceptions   At first  find out what is causing the Exception  Try googleing the name of the exception to find out  what is the cause of that exception  Most of the time it will be caused by incorrect code  In the given examples above  all of the exceptions are caused by incorrect code  So for the NullPointerException example you could make sure that a is never null at that time  You could  for example  initialise a or include a check like this one   if  a  null        a toString        This way  the offending line is not executed if a  null  Same goes for the other examples   Sometimes you can t make sure that you don t get an exception  For example  if you are using a network connection in your program  you cannot stop the computer from loosing it s internet connection  e g  you can t stop the user from disconnecting the computer s network connection   In this case the network library will probably throw an exception  Now you should catch the exception and handle it  This means  in the example with the network connection  you should try to reopen the connection or notify the user or something like that  Also  whenever you use catch  always catch only the exception you want to catch  do not use broad catch statements like catch  Exception e  that would catch all exceptions  This is very important  because otherwise you might accidentally catch the wrong exception and react in the wrong way   try       Socket x   new Socket  1 1 1 1   6789       x getInputStream   read     catch  IOException e        System err println  Connection could not be established  please try again later        Why should I not use catch  Exception e    Let s use a small example to show why you should not just catch all exceptions   int mult Integer a Integer b        try           int result   a b         return result        catch  Exception e            System err println  Error  Division by zero             return 0            What this code is trying to do is to catch the ArithmeticException caused by a possible division by 0  But it also catches a possible NullPointerException that is thrown if a or b are null  This means  you might get a NullPointerException but you ll treat it as an ArithmeticException and probably do the wrong thing  In the best case you still miss that there was a NullPointerException  Stuff like that makes debugging much harder  so don t do that   TLDR   Figure out what is the cause of the exception and fix it  so that it doesn t throw the exception at all  If 1  is not possible  catch the specific exception and handle it    Never just add a try catch and then just ignore the exception  Don t do that  Never use catch  Exception e   always catch specific Exceptions  That will save you a lot of headaches

[java] What is a stack trace, and how can I use it to debug my application errors?

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