When should we call System exit in Java

Question

In Java  What is the difference with or without System exit 0  in following code    public class TestExit             public static void main String   args                 System out println  hello world             System exit 0       is it necessary  And when it must be called                   The document says   This method never returns normally   What does it mean

User · Accepted Answer

System exit   can be used to run shutdown hooks before the program quits  This is a convenient way to handle shutdown in bigger programs  where all parts of the program can t  and shouldn t  be aware of each other  Then  if someone wants to quit  he can simply call System exit    and the shutdown hooks  if properly set up  take care of doing all necessary shutdown ceremonies such as closing files  releasing resources etc    This method never returns normally   means just that the method won t return  once a thread goes there  it won t come back   Another  maybe more common  way to quit a program is to simply to reach the end of the main method  But if there are any non-daemon threads running  they will not be shut down and thus the JVM will not exit  Thus  if you have any such non-daemon threads  you need some other means  than the shutdown hooks  to shut down all non-daemon threads and release other resources  If there are no other non-daemon threads  returning from main will shut down the JVM and will call the shutdown hooks   For some reason shutdown hooks seem to be an undervalued and misunderstood mechanism  and people are reinventing the wheel with all kind of proprietary custom hacks to quit their programs  I would encourage using shutdown hooks  it s all there in the standard Runtime that you ll be using anyway

User · Answer

In applications that may have complex shutdown hooks  this method should not be called from an unknown thread   System exit never exits normally because the call will block until the JVM is terminated   It s as if whatever code is running that has the power plug pulled on it before it can finish   Calling System exit will initiate the program s shutdown hooks and whatever thread that calls System exit will block until program termination   This has the implication that if the shutdown hook in turn submits a task to the thread from which System exit was called  the program will deadlock   I m handling this in my code with the following   public static void exit final int status        new Thread  App-exit              Override         public void run                 System exit status                   start

User · Answer

System exit 0  terminates the JVM  In simple examples like this it is difficult to percieve the difference  The parameter is passed back to the OS and is normally used to indicate abnormal termination  eg some kind of fatal error   so if you called java from a batch file or shell script you d be able to get this value and get an idea if the application was successful   It would make a quite an impact if you called System exit 0  on an application deployed to an application server  think about it before you try it

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The method never returns because it s the end of the world and none of your code is going to be executed next   Your application  in your example  would exit anyway at the same spot in the code  but  if you use System exit  you have the option of returning a custom code to the enviroment  like  say  System exit 42     Who is going to make use of your exit code  A script that called the application  Works in Windows  Unix and all other scriptable environments   Why return a code  To say things like  I did not succeed    The database did not answer    To see how to get the value od an exit code and use it in a unix shell script or windows cmd script  you might check this answer on this site

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Java Language Specification says that  Program Exit A program terminates all its activity and exits when one of two things happens  All the threads that are not daemon threads terminate  Some thread invokes the exit method of class Runtime or class System  and the exit operation is not forbidden by the security manager   It means that You should use it when You have big program  well  at lest bigger than this one  and want to finish its execution

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One should NEVER call System exit 0  for these reasons    It is a hidden  goto  and  gotos  break the control flow  Relying on hooks in this context is a mental mapping every developer in the team has to be aware of  Quitting the program  normally  provides the same exit code to the operating system as System exit 0  so it is redundant   If your program cannot quit  normally  you have lost control of your development  design   You should have always full control of the system state  Programming problems such as running threads that are not stopped normally become hidden  You may encounter an inconsistent application state interrupting threads abnormally   Refer to  3    By the way  Returning other return codes than 0 does make sense if you want to indicate abnormal program termination

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Though answer was really helpful but some how it missed some more extra detail  I hope below will help understand the shutdown process in java  in addition to the answer above    In an orderly  shutdown  the JVM first starts all registered shutdown hooks  Shutdown hooks are unstarted threads that are registered with Runtime addShutdownHook  JVM makes no guarantees on the order in which shutdown hooks are started  If any application threads  daemon or nondaemon  are still running at shutdown time  they continue to run concurrently with the shutdown process  When all shutdown hooks have completed  the JVM may choose to run finalizers if runFinalizersOnExit is true  and then halts  JVM makes no attempt to stop or interrupt any application threads that are still running at shutdown time  they are abruptly terminated when the JVM eventually halts   If the shutdown hooks or finalizers don   t complete  then the orderly shutdown process    hangs    and the JVM must be shut down abruptly   In an abrupt shutdown  the JVM is not required to do anything other than halt the JVM  shutdown hooks will not run    PS  The JVM can shut down in either an orderly or abrupt manner     An orderly shutdown is initiated when the last    normal     nondaemon  thread terminates  someone calls System exit  or by other platform-specific means  such as sending a SIGINT or hitting Ctrl-C    While above is the standard and preferred way for the JVM to shut down  it can also be shut down abruptly by calling Runtime halt or by killing the JVM process through the operating system  such as sending a SIGKILL

User · Answer

In that case  it s not needed  No extra threads will have been started up  you re not changing the exit code  which defaults to 0  - basically it s pointless   When the docs say the method never returns normally  it means the subsequent line of code is effectively unreachable  even though the compiler doesn t know that   System exit 0   System out println  This line will never be reached      Either an exception will be thrown  or the VM will terminate before returning  It will never  just return    It s very rare to be worth calling System exit   IME  It can make sense if you re writing a command line tool  and you want to indicate an error via the exit code rather than just throwing an exception    but I can t remember the last time I used it in normal production code

User · Answer

System exit is needed    when you want to return a non-0 error code when you want to exit your program from somewhere that isn t main     In your case  it does the exact same thing as the simple return-from-main

User · Answer

If you have another program running in the JVM and you use System exit  that second program will be closed  too  Imagine for example that you run a java job on a cluster node and that the java program that manages the cluster node runs in the same JVM  If the job would use System exit it would not only quit the job but also  shut down the complete node   You would not be able to send another job to that cluster node since the management program has been closed accidentally    Therefore  do not use System exit if you want to be able to control your program from another java program within the same JVM    Use System exit if you want to close the complete JVM on purpose and if you want to take advantage of the possibilities that have been described in the other answers  e g  shut down hooks  Java shutdown hook  non-zero return value for command line calls  How to get the exit status of a Java program in Windows batch file    Also have a look at Runtime Exceptions  System exit num  or throw a RuntimeException from main

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