Is there a destructor for Java

Question

Is there a destructor for Java  I don t seem to be able to find any documentation on this  If there isn t  how can I achieve the same effect   To make my question more specific  I am writing an application that deals with data and the specification say that there should be a  reset  button that brings the application back to its original just launched state  However  all data have to be  live  unless the application is closed or reset button is pressed   Being usually a C C   programmer  I thought this would be trivial to implement   And hence I planned to implement it last   I structured my program such that all the  reset-able  objects would be in the same class so that I can just destroy all  live  objects when a reset button is pressed   I was thinking if all I did was just to dereference the data and wait for the garbage collector to collect them  wouldn t there be a memory leak if my user repeatedly entered data and pressed the reset button  I was also thinking since Java is quite mature as a language  there should be a way to prevent this from happening or gracefully tackle this

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Just thinking about the original question... which, I think we can conclude from all the other learned answers, and also from Bloch's essential Effective Java, item 7, "Avoid finalizers", seeks the solution to a legitimate question in a manner which is inappropriate to the Java language...:

... wouldn't a pretty obvious solution to do what the OP actually wants be to keep all your objects which need to be reset in a sort of "playpen", to which all other non-resettable objects have references only through some sort of accessor object...

And then when you need to "reset" you disconnect the existing playpen and make a new one: all the web of objects in the playpen is cast adrift, never to return, and one day to be collected by the GC.

If any of these objects are Closeable (or not, but have a close method) you could put them in a Bag in the playpen as they are created (and possibly opened), and the last act of the accessor before cutting off the playpen would be to go through all the Closeables closing them... ?

The code would probably look something like this:

accessor.getPlaypen().closeCloseables();
accessor.setPlaypen( new Playpen() );

closeCloseables would probably be a blocking method, probably involving a latch (e.g. CountdownLatch), to deal with (and wait as appropriate for) any Runnables/Callables in any threads specific to the Playpen to be ended as appropriate, in particular in the JavaFX thread.

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I agree with most of the answers  You should not depend fully on either finalize or ShutdownHook finalize  The JVM does not guarantee when this finalize   method will be invoked   finalize   gets called only once by GC thread  If an object revives itself from finalizing method  then finalize will not be called again   In your application  you may have some live objects  on which garbage collection is never invoked   Any Exception that is thrown by the finalizing method is ignored by the GC thread  System runFinalization true  and Runtime getRuntime   runFinalization true  methods increase the probability of invoking finalize   method but now these two methods have been deprecated  These methods are very dangerous due to lack of thread safety and possible deadlock creation    shutdownHooks public void addShutdownHook Thread hook    Registers a new virtual-machine shutdown hook   The Java virtual machine shuts down in response to two kinds of events   The program exits normally  when the last non-daemon thread exits or when the exit  equivalently  System exit  method is invoked  or  The virtual machine is terminated in response to a user interrupt  such as typing  C  or a system-wide event  such as user logoff or system shutdown   A shutdown hook is simply an initialized but non-started thread  When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently  When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled   Finally  the virtual machine will halt  Note that daemon threads will continue to run during the shutdown sequence  as will non-daemon threads if the shutdown was initiated by invoking the exit method   Shutdown hooks should also finish their work quickly  When a program invokes exit the expectation is that the virtual machine will promptly shut down and exit  But even Oracle documentation quoted that    In rare circumstances the virtual machine may abort  that is  stop running without shutting down cleanly  This occurs when the virtual machine is terminated externally  for example with the SIGKILL signal on Unix or the TerminateProcess call on Microsoft Windows  The virtual machine may also abort if a native method goes awry by  for example  corrupting internal data structures or attempting to access nonexistent memory  If the virtual machine aborts then no guarantee can be made about whether or not any shutdown hooks will be run  Conclusion   use try   catch   finally   blocks appropriately and release critical resources in  finally   block  During release of resources in finally   block  catch Exception and Throwable

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No  java lang Object finalize is the closest you can get   However  when  and if  it is called  is not guaranteed  See  java lang Runtime runFinalizersOnExit boolean

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If it s just memory you are worried about  don t  Just trust the GC it does a decent job  I actually saw something about it being so efficient that it could be better for performance to create heaps of tiny objects than to utilize large arrays in some instances

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The closest equivalent to a destructor in Java is the finalize   method   The big difference to a traditional destructor is that you can t be sure when it ll be called  since that s the responsibility of the garbage collector   I d strongly recommend carefully reading up on this before using it  since your typical RAIA patterns for file handles and so on won t work reliably with finalize

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First  note that since Java is garbage-collected  it is rare to need to do anything about object destruction  Firstly because you don t usually have any managed resources to free  and secondly because you can t predict when or if it will happen  so it s inappropriate for things that you need to occur  as soon as nobody is using my object any more    You can be notified after an object has been destroyed using java lang ref PhantomReference  actually  saying it has been destroyed may be slightly inaccurate  but if a phantom reference to it is queued then it s no longer recoverable  which usually amounts to the same thing   A common use is    Separate out the resource s  in your class that need to be destructed into another helper object  note that if all you re doing is closing a connection  which is a common case  you don t need to write a new class  the connection to be closed would be the  helper object  in that case   When you create your main object  create also a PhantomReference to it  Either have this refer to the new helper object  or set up a map from PhantomReference objects to their corresponding helper objects  After the main object is collected  the PhantomReference is queued  or rather it may be queued - like finalizers there is no guarantee it ever will be  for example if the VM exits then it won t wait   Make sure you re processing its queue  either in a special thread or from time to time   Because of the hard reference to the helper object  the helper object has not yet been collected  So do whatever cleanup you like on the helper object  then discard the PhantomReference and the helper will eventually be collected too    There is also finalize    which looks like a destructor but doesn t behave like one  It s usually not a good option

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Many great answers here  but there is some additional information about why you should avoid using finalize     If the JVM exits due to System exit   or Runtime getRuntime   exit    finalizers will not be run by default  From Javadoc for Runtime exit        The virtual machine s shutdown sequence consists of two phases  In the first phase all registered shutdown hooks  if any  are started in some unspecified order and allowed to run concurrently until they finish  In the second phase all uninvoked finalizers are run if finalization-on-exit has been enabled  Once this is done the virtual machine halts    You can call System runFinalization   but it only makes  a best effort to complete all outstanding finalizations       not a guarantee   There is a System runFinalizersOnExit   method  but don t use it     it s unsafe  deprecated long ago

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Though there have been considerable advancements in Java s GC technology  you still need to be mindful of your references   Numerous cases of seemingly trivial reference patterns that are actually rats nests under the hood come to mind     From your post it doesn t sound like you re trying to implement a reset method for the purpose of object reuse  true     Are your objects holding any other type of resources that need to be cleaned up  i e   streams that must be closed  any pooled or borrowed objects that must be returned    If the only thing you re worried about is memory dealloc then I would reconsider my object structure and attempt to verify that my objects are self contained structures that will be cleaned up at GC time

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Use of finalize   methods should be avoided  They are not a reliable mechanism for resource clean up and it is possible to cause problems in the garbage collector by abusing them   If you require a deallocation call in your object  say to release resources  use an explicit method call  This convention can be seen in existing APIs  e g  Closeable  Graphics dispose    Widget dispose    and is usually called via try finally   Resource r   new Resource    try         work   finally       r dispose        Attempts to use a disposed object should throw a runtime exception  see IllegalStateException      EDIT      I was thinking  if all I did was just   to dereference the data and wait for   the garbage collector to collect them    wouldn t there be a memory leak if my   user repeatedly entered data and   pressed the reset button    Generally  all you need to do is dereference the objects - at least  this is the way it is supposed to work  If you are worried about garbage collection  check out Java SE 6 HotSpot tm  Virtual Machine Garbage Collection Tuning  or the equivalent document for your JVM version

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I fully agree to other answers  saying not to rely on the execution of finalize   In addition to try-catch-finally blocks  you may use Runtime addShutdownHook  introduced in Java 1 3  to perform final cleanups in your program    That isn t the same as destructors are  but one may implement a shutdown hook having listener objects registered on which cleanup methods  close persistent database connections  remove file locks  and so on  can be invoked - things that would normally be done in destructors  Again - this is not a replacement for destructors but in some cases  you can approach the wanted functionality with this   The advantage of this is having deconstruction behavior loosely coupled from the rest of your program

User · Answer

The closest equivalent to a destructor in Java is the finalize   method   The big difference to a traditional destructor is that you can t be sure when it ll be called  since that s the responsibility of the garbage collector   I d strongly recommend carefully reading up on this before using it  since your typical RAIA patterns for file handles and so on won t work reliably with finalize

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With Java 1 7 released  you now have the additional option of using the try-with-resources block   For example    public class Closeable implements AutoCloseable        Override     public void close             System out println  closing                  public static void main String   args            try  Closeable c   new Closeable                  System out println  trying                    throw new Exception  throwing                          catch  Exception e                System out println  catching                          finally               System out println  finalizing                             If you execute this class  c close   will be executed when the try block is left  and before the catch and finally blocks are executed   Unlike in the case of the finalize   method  close   is guaranteed to be executed   However  there is no need of executing it explicitly in the finally clause

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There is no exactly destructor class in Java  class destroyed in java automatically by garbage collector   but you could do that using below one but it s not exact same thing    finalize    There was a question that spawned in-depth discussion of finalize   so that you should get more depth if required

User · Answer

The closest equivalent to a destructor in Java is the finalize   method   The big difference to a traditional destructor is that you can t be sure when it ll be called  since that s the responsibility of the garbage collector   I d strongly recommend carefully reading up on this before using it  since your typical RAIA patterns for file handles and so on won t work reliably with finalize

User · Answer

I fully agree to other answers  saying not to rely on the execution of finalize   In addition to try-catch-finally blocks  you may use Runtime addShutdownHook  introduced in Java 1 3  to perform final cleanups in your program    That isn t the same as destructors are  but one may implement a shutdown hook having listener objects registered on which cleanup methods  close persistent database connections  remove file locks  and so on  can be invoked - things that would normally be done in destructors  Again - this is not a replacement for destructors but in some cases  you can approach the wanted functionality with this   The advantage of this is having deconstruction behavior loosely coupled from the rest of your program

User · Answer

The finalize   function is the destructor   However  it should not be normally used because it is invoked after the GC and you can t tell when that will happen  if ever    Moreover  it takes more than one GC to deallocate objects that have finalize     You should try to clean up in the logical places in your code using the try      finally      statements

User · Answer

Though there have been considerable advancements in Java s GC technology  you still need to be mindful of your references   Numerous cases of seemingly trivial reference patterns that are actually rats nests under the hood come to mind     From your post it doesn t sound like you re trying to implement a reset method for the purpose of object reuse  true     Are your objects holding any other type of resources that need to be cleaned up  i e   streams that must be closed  any pooled or borrowed objects that must be returned    If the only thing you re worried about is memory dealloc then I would reconsider my object structure and attempt to verify that my objects are self contained structures that will be cleaned up at GC time

User · Answer

I fully agree to other answers  saying not to rely on the execution of finalize   In addition to try-catch-finally blocks  you may use Runtime addShutdownHook  introduced in Java 1 3  to perform final cleanups in your program    That isn t the same as destructors are  but one may implement a shutdown hook having listener objects registered on which cleanup methods  close persistent database connections  remove file locks  and so on  can be invoked - things that would normally be done in destructors  Again - this is not a replacement for destructors but in some cases  you can approach the wanted functionality with this   The advantage of this is having deconstruction behavior loosely coupled from the rest of your program

User · Answer

Perhaps you can use a try     finally block to finalize the object in the control flow at which you are using the object  Of course it doesn t happen automatically  but neither does destruction in C    You often see closing of resources in the finally block

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Because Java is a garbage collected language you cannot predict when  or even if  an object will be destroyed  Hence there is no direct equivalent of a destructor   There is an inherited method called finalize  but this is called entirely at the discretion of the garbage collector  So for classes that need to explicitly tidy up  the convention is to define a close method and use finalize only for sanity checking  i e  if close has not been called do it now and log an error    There was a question that spawned in-depth discussion of finalize recently  so that should provide more depth if required

User · Answer

Because Java is a garbage collected language you cannot predict when  or even if  an object will be destroyed  Hence there is no direct equivalent of a destructor   There is an inherited method called finalize  but this is called entirely at the discretion of the garbage collector  So for classes that need to explicitly tidy up  the convention is to define a close method and use finalize only for sanity checking  i e  if close has not been called do it now and log an error    There was a question that spawned in-depth discussion of finalize recently  so that should provide more depth if required

User · Answer

No Java doesn t have any destructors  The main reason behind it in Java is the Garbage Collectors that passively works in the background always and all the objects are made in the heap memory   that is the place where GC works  In c   there we have to explicitly call the delete function since there is no Garbage collector like thing

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Perhaps you can use a try     finally block to finalize the object in the control flow at which you are using the object  Of course it doesn t happen automatically  but neither does destruction in C    You often see closing of resources in the finally block

User · Answer

Use of finalize   methods should be avoided  They are not a reliable mechanism for resource clean up and it is possible to cause problems in the garbage collector by abusing them   If you require a deallocation call in your object  say to release resources  use an explicit method call  This convention can be seen in existing APIs  e g  Closeable  Graphics dispose    Widget dispose    and is usually called via try finally   Resource r   new Resource    try         work   finally       r dispose        Attempts to use a disposed object should throw a runtime exception  see IllegalStateException      EDIT      I was thinking  if all I did was just   to dereference the data and wait for   the garbage collector to collect them    wouldn t there be a memory leak if my   user repeatedly entered data and   pressed the reset button    Generally  all you need to do is dereference the objects - at least  this is the way it is supposed to work  If you are worried about garbage collection  check out Java SE 6 HotSpot tm  Virtual Machine Garbage Collection Tuning  or the equivalent document for your JVM version

User · Answer

Though there have been considerable advancements in Java s GC technology  you still need to be mindful of your references   Numerous cases of seemingly trivial reference patterns that are actually rats nests under the hood come to mind     From your post it doesn t sound like you re trying to implement a reset method for the purpose of object reuse  true     Are your objects holding any other type of resources that need to be cleaned up  i e   streams that must be closed  any pooled or borrowed objects that must be returned    If the only thing you re worried about is memory dealloc then I would reconsider my object structure and attempt to verify that my objects are self contained structures that will be cleaned up at GC time

User · Answer

Many great answers here  but there is some additional information about why you should avoid using finalize     If the JVM exits due to System exit   or Runtime getRuntime   exit    finalizers will not be run by default  From Javadoc for Runtime exit        The virtual machine s shutdown sequence consists of two phases  In the first phase all registered shutdown hooks  if any  are started in some unspecified order and allowed to run concurrently until they finish  In the second phase all uninvoked finalizers are run if finalization-on-exit has been enabled  Once this is done the virtual machine halts    You can call System runFinalization   but it only makes  a best effort to complete all outstanding finalizations       not a guarantee   There is a System runFinalizersOnExit   method  but don t use it     it s unsafe  deprecated long ago

User · Answer

With Java 1 7 released  you now have the additional option of using the try-with-resources block   For example    public class Closeable implements AutoCloseable        Override     public void close             System out println  closing                  public static void main String   args            try  Closeable c   new Closeable                  System out println  trying                    throw new Exception  throwing                          catch  Exception e                System out println  catching                          finally               System out println  finalizing                             If you execute this class  c close   will be executed when the try block is left  and before the catch and finally blocks are executed   Unlike in the case of the finalize   method  close   is guaranteed to be executed   However  there is no need of executing it explicitly in the finally clause

User · Answer

Use of finalize   methods should be avoided  They are not a reliable mechanism for resource clean up and it is possible to cause problems in the garbage collector by abusing them   If you require a deallocation call in your object  say to release resources  use an explicit method call  This convention can be seen in existing APIs  e g  Closeable  Graphics dispose    Widget dispose    and is usually called via try finally   Resource r   new Resource    try         work   finally       r dispose        Attempts to use a disposed object should throw a runtime exception  see IllegalStateException      EDIT      I was thinking  if all I did was just   to dereference the data and wait for   the garbage collector to collect them    wouldn t there be a memory leak if my   user repeatedly entered data and   pressed the reset button    Generally  all you need to do is dereference the objects - at least  this is the way it is supposed to work  If you are worried about garbage collection  check out Java SE 6 HotSpot tm  Virtual Machine Garbage Collection Tuning  or the equivalent document for your JVM version

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Have a look at the try-with-resources statement  For example   try  BufferedReader br   new BufferedReader new FileReader path        System out println br readLine       catch  Exception e            finally             Here the resource that is no longer needed is freed in the BufferedReader close   method  You can create your own class that implements AutoCloseable and use it in a similar fashion    This statement is more limited than finalize in terms of code structuring  but at the same time it makes the code simpler to understand and maintain  Also  there is no guarantee that a finalize method is called at all during the livetime of the application

User · Answer

Just thinking about the original question... which, I think we can conclude from all the other learned answers, and also from Bloch's essential Effective Java, item 7, "Avoid finalizers", seeks the solution to a legitimate question in a manner which is inappropriate to the Java language...:

... wouldn't a pretty obvious solution to do what the OP actually wants be to keep all your objects which need to be reset in a sort of "playpen", to which all other non-resettable objects have references only through some sort of accessor object...

And then when you need to "reset" you disconnect the existing playpen and make a new one: all the web of objects in the playpen is cast adrift, never to return, and one day to be collected by the GC.

If any of these objects are Closeable (or not, but have a close method) you could put them in a Bag in the playpen as they are created (and possibly opened), and the last act of the accessor before cutting off the playpen would be to go through all the Closeables closing them... ?

The code would probably look something like this:

accessor.getPlaypen().closeCloseables();
accessor.setPlaypen( new Playpen() );

closeCloseables would probably be a blocking method, probably involving a latch (e.g. CountdownLatch), to deal with (and wait as appropriate for) any Runnables/Callables in any threads specific to the Playpen to be ended as appropriate, in particular in the JavaFX thread.

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I used to mainly deal with C   and that is what lead me to the search of a destructor as well  I am using JAVA a lot now  What I did  and it may not be the best case for everyone  but I implemented my own destructor by reseting all the values to either 0 or there default through a function   Example   public myDestructor      variableA   0    INT variableB   0 0    DOUBLE  amp  FLOAT variableC    NO NAME ENTERED     TEXT  amp  STRING variableD   false    BOOL      Ideally this won t work for all situations  but where there are global variables it will work as long as you don t have a ton of them   I know I am not the best Java programmer  but it seems to be working for me

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The finalize   function is the destructor   However  it should not be normally used because it is invoked after the GC and you can t tell when that will happen  if ever    Moreover  it takes more than one GC to deallocate objects that have finalize     You should try to clean up in the logical places in your code using the try      finally      statements

User · Answer

In Java  the garbage collector automatically deletes the unused objects to free up the memory  So it   s sensible Java has no destructors available

User · Answer

I fully agree to other answers  saying not to rely on the execution of finalize   In addition to try-catch-finally blocks  you may use Runtime addShutdownHook  introduced in Java 1 3  to perform final cleanups in your program    That isn t the same as destructors are  but one may implement a shutdown hook having listener objects registered on which cleanup methods  close persistent database connections  remove file locks  and so on  can be invoked - things that would normally be done in destructors  Again - this is not a replacement for destructors but in some cases  you can approach the wanted functionality with this   The advantage of this is having deconstruction behavior loosely coupled from the rest of your program

User · Answer

Nope  no destructors here  The reason is that all Java objects are heap allocated and garbage collected  Without explicit deallocation  i e  C   s delete operator  there is no sensible way to implement real destructors   Java does support finalizers  but they are meant to be used only as a safeguard for objects holding a handle to native resources like sockets  file handles  window handles  etc  When the garbage collector collects an object without a finalizer it simply marks the memory region as free and that s it  When the object has a finalizer  it s first copied into a temporary location  remember  we re garbage collecting here   then it s enqueued into a waiting-to-be-finalized queue and then a Finalizer thread polls the queue with very low priority and runs the finalizer   When the application exits  the JVM stops without waiting for the pending objects to be finalized  so there practically no guarantees that your finalizers will ever run

User · Answer

Use of finalize   methods should be avoided  They are not a reliable mechanism for resource clean up and it is possible to cause problems in the garbage collector by abusing them   If you require a deallocation call in your object  say to release resources  use an explicit method call  This convention can be seen in existing APIs  e g  Closeable  Graphics dispose    Widget dispose    and is usually called via try finally   Resource r   new Resource    try         work   finally       r dispose        Attempts to use a disposed object should throw a runtime exception  see IllegalStateException      EDIT      I was thinking  if all I did was just   to dereference the data and wait for   the garbage collector to collect them    wouldn t there be a memory leak if my   user repeatedly entered data and   pressed the reset button    Generally  all you need to do is dereference the objects - at least  this is the way it is supposed to work  If you are worried about garbage collection  check out Java SE 6 HotSpot tm  Virtual Machine Garbage Collection Tuning  or the equivalent document for your JVM version

User · Answer

Because Java is a garbage collected language you cannot predict when  or even if  an object will be destroyed  Hence there is no direct equivalent of a destructor   There is an inherited method called finalize  but this is called entirely at the discretion of the garbage collector  So for classes that need to explicitly tidy up  the convention is to define a close method and use finalize only for sanity checking  i e  if close has not been called do it now and log an error    There was a question that spawned in-depth discussion of finalize recently  so that should provide more depth if required

User · Answer

Because Java is a garbage collected language you cannot predict when  or even if  an object will be destroyed  Hence there is no direct equivalent of a destructor   There is an inherited method called finalize  but this is called entirely at the discretion of the garbage collector  So for classes that need to explicitly tidy up  the convention is to define a close method and use finalize only for sanity checking  i e  if close has not been called do it now and log an error    There was a question that spawned in-depth discussion of finalize recently  so that should provide more depth if required

User · Answer

Nope  no destructors here  The reason is that all Java objects are heap allocated and garbage collected  Without explicit deallocation  i e  C   s delete operator  there is no sensible way to implement real destructors   Java does support finalizers  but they are meant to be used only as a safeguard for objects holding a handle to native resources like sockets  file handles  window handles  etc  When the garbage collector collects an object without a finalizer it simply marks the memory region as free and that s it  When the object has a finalizer  it s first copied into a temporary location  remember  we re garbage collecting here   then it s enqueued into a waiting-to-be-finalized queue and then a Finalizer thread polls the queue with very low priority and runs the finalizer   When the application exits  the JVM stops without waiting for the pending objects to be finalized  so there practically no guarantees that your finalizers will ever run

User · Answer

If it s just memory you are worried about  don t  Just trust the GC it does a decent job  I actually saw something about it being so efficient that it could be better for performance to create heaps of tiny objects than to utilize large arrays in some instances

User · Answer

There is no exactly destructor class in Java  class destroyed in java automatically by garbage collector   but you could do that using below one but it s not exact same thing    finalize    There was a question that spawned in-depth discussion of finalize   so that you should get more depth if required

User · Answer

No  java lang Object finalize is the closest you can get   However  when  and if  it is called  is not guaranteed  See  java lang Runtime runFinalizersOnExit boolean

User · Answer

If it s just memory you are worried about  don t  Just trust the GC it does a decent job  I actually saw something about it being so efficient that it could be better for performance to create heaps of tiny objects than to utilize large arrays in some instances

User · Answer

There is a  Cleanup annotation in Lombok that mostly resembles C   destructors    Cleanup ResourceClass resource   new ResourceClass      When processing it  at compilation time   Lombok inserts appropriate try-finally block so that resource close   is invoked  when execution leaves the scope of the variable  You can also specify explicitly another method for releasing the resource  e g  resource dispose       Cleanup  dispose   ResourceClass resource   new ResourceClass

User · Answer

In Java  the garbage collector automatically deletes the unused objects to free up the memory  So it   s sensible Java has no destructors available

User · Answer

I agree with most of the answers  You should not depend fully on either finalize or ShutdownHook finalize  The JVM does not guarantee when this finalize   method will be invoked   finalize   gets called only once by GC thread  If an object revives itself from finalizing method  then finalize will not be called again   In your application  you may have some live objects  on which garbage collection is never invoked   Any Exception that is thrown by the finalizing method is ignored by the GC thread  System runFinalization true  and Runtime getRuntime   runFinalization true  methods increase the probability of invoking finalize   method but now these two methods have been deprecated  These methods are very dangerous due to lack of thread safety and possible deadlock creation    shutdownHooks public void addShutdownHook Thread hook    Registers a new virtual-machine shutdown hook   The Java virtual machine shuts down in response to two kinds of events   The program exits normally  when the last non-daemon thread exits or when the exit  equivalently  System exit  method is invoked  or  The virtual machine is terminated in response to a user interrupt  such as typing  C  or a system-wide event  such as user logoff or system shutdown   A shutdown hook is simply an initialized but non-started thread  When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently  When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled   Finally  the virtual machine will halt  Note that daemon threads will continue to run during the shutdown sequence  as will non-daemon threads if the shutdown was initiated by invoking the exit method   Shutdown hooks should also finish their work quickly  When a program invokes exit the expectation is that the virtual machine will promptly shut down and exit  But even Oracle documentation quoted that    In rare circumstances the virtual machine may abort  that is  stop running without shutting down cleanly  This occurs when the virtual machine is terminated externally  for example with the SIGKILL signal on Unix or the TerminateProcess call on Microsoft Windows  The virtual machine may also abort if a native method goes awry by  for example  corrupting internal data structures or attempting to access nonexistent memory  If the virtual machine aborts then no guarantee can be made about whether or not any shutdown hooks will be run  Conclusion   use try   catch   finally   blocks appropriately and release critical resources in  finally   block  During release of resources in finally   block  catch Exception and Throwable

User · Answer

Perhaps you can use a try     finally block to finalize the object in the control flow at which you are using the object  Of course it doesn t happen automatically  but neither does destruction in C    You often see closing of resources in the finally block

User · Answer

Nope  no destructors here  The reason is that all Java objects are heap allocated and garbage collected  Without explicit deallocation  i e  C   s delete operator  there is no sensible way to implement real destructors   Java does support finalizers  but they are meant to be used only as a safeguard for objects holding a handle to native resources like sockets  file handles  window handles  etc  When the garbage collector collects an object without a finalizer it simply marks the memory region as free and that s it  When the object has a finalizer  it s first copied into a temporary location  remember  we re garbage collecting here   then it s enqueued into a waiting-to-be-finalized queue and then a Finalizer thread polls the queue with very low priority and runs the finalizer   When the application exits  the JVM stops without waiting for the pending objects to be finalized  so there practically no guarantees that your finalizers will ever run

User · Answer

No Java doesn t have any destructors  The main reason behind it in Java is the Garbage Collectors that passively works in the background always and all the objects are made in the heap memory   that is the place where GC works  In c   there we have to explicitly call the delete function since there is no Garbage collector like thing

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First  note that since Java is garbage-collected  it is rare to need to do anything about object destruction  Firstly because you don t usually have any managed resources to free  and secondly because you can t predict when or if it will happen  so it s inappropriate for things that you need to occur  as soon as nobody is using my object any more    You can be notified after an object has been destroyed using java lang ref PhantomReference  actually  saying it has been destroyed may be slightly inaccurate  but if a phantom reference to it is queued then it s no longer recoverable  which usually amounts to the same thing   A common use is    Separate out the resource s  in your class that need to be destructed into another helper object  note that if all you re doing is closing a connection  which is a common case  you don t need to write a new class  the connection to be closed would be the  helper object  in that case   When you create your main object  create also a PhantomReference to it  Either have this refer to the new helper object  or set up a map from PhantomReference objects to their corresponding helper objects  After the main object is collected  the PhantomReference is queued  or rather it may be queued - like finalizers there is no guarantee it ever will be  for example if the VM exits then it won t wait   Make sure you re processing its queue  either in a special thread or from time to time   Because of the hard reference to the helper object  the helper object has not yet been collected  So do whatever cleanup you like on the helper object  then discard the PhantomReference and the helper will eventually be collected too    There is also finalize    which looks like a destructor but doesn t behave like one  It s usually not a good option

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If you re writing a Java Applet  you can override the Applet  destroy    method  It is         Called by the browser or applet viewer to inform    this applet that it is being reclaimed and that it should destroy    any resources that it has allocated  The stop   method    will always be called before destroy       Obviously not what you want  but might be what other people are looking for

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If you re writing a Java Applet  you can override the Applet  destroy    method  It is         Called by the browser or applet viewer to inform    this applet that it is being reclaimed and that it should destroy    any resources that it has allocated  The stop   method    will always be called before destroy       Obviously not what you want  but might be what other people are looking for

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The closest equivalent to a destructor in Java is the finalize   method   The big difference to a traditional destructor is that you can t be sure when it ll be called  since that s the responsibility of the garbage collector   I d strongly recommend carefully reading up on this before using it  since your typical RAIA patterns for file handles and so on won t work reliably with finalize

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Have a look at the try-with-resources statement  For example   try  BufferedReader br   new BufferedReader new FileReader path        System out println br readLine       catch  Exception e            finally             Here the resource that is no longer needed is freed in the BufferedReader close   method  You can create your own class that implements AutoCloseable and use it in a similar fashion    This statement is more limited than finalize in terms of code structuring  but at the same time it makes the code simpler to understand and maintain  Also  there is no guarantee that a finalize method is called at all during the livetime of the application

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First  note that since Java is garbage-collected  it is rare to need to do anything about object destruction  Firstly because you don t usually have any managed resources to free  and secondly because you can t predict when or if it will happen  so it s inappropriate for things that you need to occur  as soon as nobody is using my object any more    You can be notified after an object has been destroyed using java lang ref PhantomReference  actually  saying it has been destroyed may be slightly inaccurate  but if a phantom reference to it is queued then it s no longer recoverable  which usually amounts to the same thing   A common use is    Separate out the resource s  in your class that need to be destructed into another helper object  note that if all you re doing is closing a connection  which is a common case  you don t need to write a new class  the connection to be closed would be the  helper object  in that case   When you create your main object  create also a PhantomReference to it  Either have this refer to the new helper object  or set up a map from PhantomReference objects to their corresponding helper objects  After the main object is collected  the PhantomReference is queued  or rather it may be queued - like finalizers there is no guarantee it ever will be  for example if the VM exits then it won t wait   Make sure you re processing its queue  either in a special thread or from time to time   Because of the hard reference to the helper object  the helper object has not yet been collected  So do whatever cleanup you like on the helper object  then discard the PhantomReference and the helper will eventually be collected too    There is also finalize    which looks like a destructor but doesn t behave like one  It s usually not a good option

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Though there have been considerable advancements in Java s GC technology  you still need to be mindful of your references   Numerous cases of seemingly trivial reference patterns that are actually rats nests under the hood come to mind     From your post it doesn t sound like you re trying to implement a reset method for the purpose of object reuse  true     Are your objects holding any other type of resources that need to be cleaned up  i e   streams that must be closed  any pooled or borrowed objects that must be returned    If the only thing you re worried about is memory dealloc then I would reconsider my object structure and attempt to verify that my objects are self contained structures that will be cleaned up at GC time

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If it s just memory you are worried about  don t  Just trust the GC it does a decent job  I actually saw something about it being so efficient that it could be better for performance to create heaps of tiny objects than to utilize large arrays in some instances

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Nope  no destructors here  The reason is that all Java objects are heap allocated and garbage collected  Without explicit deallocation  i e  C   s delete operator  there is no sensible way to implement real destructors   Java does support finalizers  but they are meant to be used only as a safeguard for objects holding a handle to native resources like sockets  file handles  window handles  etc  When the garbage collector collects an object without a finalizer it simply marks the memory region as free and that s it  When the object has a finalizer  it s first copied into a temporary location  remember  we re garbage collecting here   then it s enqueued into a waiting-to-be-finalized queue and then a Finalizer thread polls the queue with very low priority and runs the finalizer   When the application exits  the JVM stops without waiting for the pending objects to be finalized  so there practically no guarantees that your finalizers will ever run

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First  note that since Java is garbage-collected  it is rare to need to do anything about object destruction  Firstly because you don t usually have any managed resources to free  and secondly because you can t predict when or if it will happen  so it s inappropriate for things that you need to occur  as soon as nobody is using my object any more    You can be notified after an object has been destroyed using java lang ref PhantomReference  actually  saying it has been destroyed may be slightly inaccurate  but if a phantom reference to it is queued then it s no longer recoverable  which usually amounts to the same thing   A common use is    Separate out the resource s  in your class that need to be destructed into another helper object  note that if all you re doing is closing a connection  which is a common case  you don t need to write a new class  the connection to be closed would be the  helper object  in that case   When you create your main object  create also a PhantomReference to it  Either have this refer to the new helper object  or set up a map from PhantomReference objects to their corresponding helper objects  After the main object is collected  the PhantomReference is queued  or rather it may be queued - like finalizers there is no guarantee it ever will be  for example if the VM exits then it won t wait   Make sure you re processing its queue  either in a special thread or from time to time   Because of the hard reference to the helper object  the helper object has not yet been collected  So do whatever cleanup you like on the helper object  then discard the PhantomReference and the helper will eventually be collected too    There is also finalize    which looks like a destructor but doesn t behave like one  It s usually not a good option

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I used to mainly deal with C   and that is what lead me to the search of a destructor as well  I am using JAVA a lot now  What I did  and it may not be the best case for everyone  but I implemented my own destructor by reseting all the values to either 0 or there default through a function   Example   public myDestructor      variableA   0    INT variableB   0 0    DOUBLE  amp  FLOAT variableC    NO NAME ENTERED     TEXT  amp  STRING variableD   false    BOOL      Ideally this won t work for all situations  but where there are global variables it will work as long as you don t have a ton of them   I know I am not the best Java programmer  but it seems to be working for me

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No  java lang Object finalize is the closest you can get   However  when  and if  it is called  is not guaranteed  See  java lang Runtime runFinalizersOnExit boolean

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There is a  Cleanup annotation in Lombok that mostly resembles C   destructors    Cleanup ResourceClass resource   new ResourceClass      When processing it  at compilation time   Lombok inserts appropriate try-finally block so that resource close   is invoked  when execution leaves the scope of the variable  You can also specify explicitly another method for releasing the resource  e g  resource dispose       Cleanup  dispose   ResourceClass resource   new ResourceClass

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Perhaps you can use a try     finally block to finalize the object in the control flow at which you are using the object  Of course it doesn t happen automatically  but neither does destruction in C    You often see closing of resources in the finally block

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No  java lang Object finalize is the closest you can get   However  when  and if  it is called  is not guaranteed  See  java lang Runtime runFinalizersOnExit boolean

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The finalize   function is the destructor   However  it should not be normally used because it is invoked after the GC and you can t tell when that will happen  if ever    Moreover  it takes more than one GC to deallocate objects that have finalize     You should try to clean up in the logical places in your code using the try      finally      statements

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The finalize   function is the destructor   However  it should not be normally used because it is invoked after the GC and you can t tell when that will happen  if ever    Moreover  it takes more than one GC to deallocate objects that have finalize     You should try to clean up in the logical places in your code using the try      finally      statements

[java] Is there a destructor for Java?

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