Why is exception printStackTrace considered bad practice

Question

There is a lot of material out there which suggests that printing the stack trace of an exception is bad practice  E g  from the RegexpSingleline check in Checkstyle   This check can be used       to find common bad practice such as calling ex printStacktrace    However  I m struggling to find anywhere which gives a valid reason why since surely the stack trace is very useful in tracking down what caused the exception  Things that I am aware of   A stack trace should never be visible to end users  for user experience and security purposes   Generating a stack trace is a relatively expensive process  though unlikely to be an issue in most  exceptional  circumstances   Many logging frameworks will print the stack trace for you  ours does not and no  we can t change it easily   Printing the stack trace does not constitute error handling   It should be combined with other information logging and exception handling    What other reasons are there for avoiding printing a stack trace in your code

User · Answer

Printing the exception s stack trace in itself doesn t constitute bad practice  but only printing the stace trace when an exception occurs is probably the issue here -- often times  just printing a stack trace is not enough   Also  there s a tendency to suspect that proper exception handling is not being performed if all that is being performed in a catch block is a e printStackTrace  Improper handling could mean at best an problem is being ignored  and at worst a program that continues executing in an undefined or unexpected state   Example  Let s consider the following example   try     initializeState       catch  TheSkyIsFallingEndOfTheWorldException e      e printStackTrace       continueProcessingAssumingThatTheStateIsCorrect      Here  we want to do some initialization processing before we continue on to some processing that requires that the initialization had taken place   In the above code  the exception should have been caught and properly handled to prevent the program from proceeding to the continueProcessingAssumingThatTheStateIsCorrect method which we could assume would cause problems   In many instances  e printStackTrace   is an indication that some exception is being swallowed and processing is allowed to proceed as if no problem every occurred   Why has this become a problem   Probably one of the biggest reason that poor exception handling has become more prevalent is due to how IDEs such as Eclipse will auto-generate code that will perform a e printStackTrace for the exception handling   try     Thread sleep 1000     catch  InterruptedException e         TODO Auto-generated catch block   e printStackTrace         The above is an actual try-catch auto-generated by Eclipse to handle an InterruptedException thrown by Thread sleep    For most applications  just printing the stack trace to standard error is probably not going to be sufficient  Improper exception handling could in many instances lead to an application running in a state that is unexpected and could be leading to unexpected and undefined behavior

User · Answer

Throwable printStackTrace   writes the stack trace to System err PrintStream  The System err stream and the underlying standard  error  output stream of the JVM process can be redirected by   invoking System setErr   which changes the destination pointed to by System err  or by redirecting the process  error output stream  The error output stream may be redirected to a file device   whose contents may be ignored by personnel  the file device may not be capable of log rotation  inferring that a process restart is required to close the open file device handle  before archiving the existing contents of the file device  or the file device actually discards all data written to it  as is the case of  dev null     Inferring from the above  invoking Throwable printStackTrace   constitutes valid  not good great  exception handling behavior  only   if you do not have System err being reassigned throughout the duration of the application s lifetime  and if you do not require log rotation while the application is running  and if accepted designed logging practice of the application is to write to System err  and the JVM s standard error output stream     In most cases  the above conditions are not satisfied  One may not be aware of other code running in the JVM  and one cannot predict the size of the log file or the runtime duration of the process  and a well designed logging practice would revolve around writing  machine-parseable  log files  a preferable but optional feature in a logger  in a known destination  to aid in support   Finally  one ought to remember that the output of Throwable printStackTrace   would definitely get interleaved with other content written to System err  and possibly even System out if both are redirected to the same file device   This is an annoyance  for single-threaded apps  that one must deal with  for the data around exceptions is not easily parseable in such an event  Worse  it is highly likely that a multi-threaded application will produce very confusing logs as Throwable printStackTrace   is not thread-safe   There is no synchronization mechanism to synchronize the writing of the stack trace to System err when multiple threads invoke Throwable printStackTrace   at the same time  Resolving this actually requires your code to synchronize on the monitor associated with System err  and also System out  if the destination file device is the same   and that is rather heavy price to pay for log file sanity  To take an example  the ConsoleHandler and StreamHandler classes are responsible for appending log records to console  in the logging facility provided by java util logging  the actual operation of publishing log records is synchronized - every thread that attempts to publish a log record must also acquire the lock on the monitor associated with the StreamHandler instance  If you wish to have the same guarantee of having non-interleaved log records using System out System err  you must ensure the same - the messages are published to these streams in a serializable manner   Considering all of the above  and the very restricted scenarios in which Throwable printStackTrace   is actually useful  it often turns out that invoking it is a bad practice     Extending the argument in the one of the previous paragraphs  it is also a poor choice to use Throwable printStackTrace in conjunction with a logger that writes to the console  This is in part  due to the reason that the logger would synchronize on a different monitor  while your application would  possibly  if you don t want interleaved log records  synchronize on a different monitor  The argument also holds good when you use two different loggers that write to the same destination  in your application

User · Answer

As some guys already mentioned here the problem is with the exception swallowing in case you just call e printStackTrace   in the catch block  It won t stop the thread execution and will continue after the try block as in normal condition    Instead of that you need either try to recover from the exception  in case it is recoverable   or to throw RuntimeException  or to bubble the exception to the caller in order to avoid silent crashes  for example  due to improper logger configuration

User · Answer

printStackTrace   prints to a console  In production settings  nobody is ever watching at that  Suraj is correct  should pass this information to a logger

User · Answer

I think your list of reasons is a pretty comprehensive one   One particularly bad example that I ve encountered more than once goes like this       try            do stuff       catch  Exception e            e printStackTrace       and swallow the exception         The problem with the above code is that the handling consists entirely of the printStackTrace call  the exception isn t really handled properly nor is it allowed to escape   On the other hand  as a rule I always log the stack trace whenever there s an unexpected exception in my code  Over the years this policy has saved me a lot of debugging time   Finally  on a lighter note  God s Perfect Exception

User · Answer

First thing printStackTrace   is not expensive as you state  because the stack trace is filled when the exception is created itself   The idea is to pass anything that goes to logs through a logger framework  so that the logging can be controlled  Hence instead of using printStackTrace  just use something like Logger log msg  exception

User · Answer

It is not bad practice because something is  wrong  about PrintStackTrace    but because it s  code smell    Most of the time the PrintStackTrace   call is there because somebody failed to properly handle the exception  Once you deal with the exception in a proper way you generally don t care about the StackTrace any more    Additionally  displaying the stacktrace on stderr is generally only useful when debugging  not in production because very often stderr goes nowhere  Logging it makes more sense  But just replacing PrintStackTrace   with logging the exception still leaves you with an application which failed but keeps running like nothing happened

User · Answer

You are touching multiple issues here      1  A stack trace should never be visibile to end users  for user experience and security purposes    Yes  it should be accessible to diagnose problems of end-users  but end-user should not see them for two reasons    They are very obscure and unreadable  the application will look very user-unfriendly  Showing a stack trace to end-user might introduce a potential security risk  Correct me if I m wrong  PHP actually prints function parameters in stack trace - brilliant  but very dangerous - if you would you get exception while connecting to the database  what are you likely to in the stacktrace       2  Generating a stack trace is a relatively expensive process  though unlikely to be an issue in most  exception al circumstances    Generating a stack trace happens when the exception is being created thrown  that s why throwing an exception comes with a price   printing is not that expensive  In fact you can override Throwable fillInStackTrace   in your custom exception effectively making throwing an exception almost as cheap as a simple GOTO statement      3  Many logging frameworks will print the stack trace for you  ours does not and no  we can t change it easily    Very good point  The main issue here is  if the framework logs the exception for you  do nothing  but make sure it does   If you want to log the exception yourself  use logging framework like Logback or Log4J  to not put them on the raw console because it is very hard to control it   With logging framework you can easily redirect stack traces to file  console or even send them to a specified e-mail address  With hardcoded printStackTrace   you have to live with the sysout      4  Printing the stack trace does not constitute error handling  It should be combined with other information logging and exception handling    Again  log SQLException correctly  with the full stack trace  using logging framework  and show nice   Sorry  we are currently not able to process your request  message  Do you really think the user is interested in the reasons  Have you seen StackOverflow error screen  It s very humorous  but does not reveal any details  However it ensures the user that the problem will be investigated   But he will call you immediately and you need to be able to diagnose the problem  So you need both  proper exception logging and user-friendly messages     To wrap things up  always log exceptions  preferably using logging framework   but do not expose them to the end-user  Think carefully and about error-messages in your GUI  show stack traces only in development mode

User · Answer

In server applications the stacktrace blows up your stdout stderr file  It may become larger and larger and is filled with useless data because usually you have no context and no timestamp and so on   e g  catalina out when using tomcat as container

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