When is the finalize method called in Java

Question

I need to know when the finalize   method is called in the JVM  I created a test class which writes into a file when the finalize   method is called by overriding it  It is not executed  Can anybody tell me the reason why it is not executing

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Check out Effective Java  2nd edition page 27  Item 7  Avoid finalizers     Finalizers are unpredictable  often dangerous  and generally unnecessary  never do anything time-critical in a finalizer  never   depend on a finalizer to update critical persistent state    To terminate a resource  use try-finally instead       try-finally block guarantees execution of termination method Foo foo   new Foo       try          Do what must be done with foo           finally       foo terminate       Explicit termination method

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finalize   is called just before garbage collection  It is not called when an object goes out of scope  This means that you cannot know when or even if finalize   will be executed   Example   If your program end before garbage collector occur  then finalize   will not execute  Therefore  it should be used as backup procedure to ensure the proper handling of other resources  or for special use applications  not as the means that your program uses in its normal operation

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The Java finalize   method is not a destructor and should not be used to handle logic that your application depends on  The Java spec states there is no guarantee that the finalize method is called at all during the livetime of the application  What you problably want is a combination of finally and a cleanup method  as in  MyClass myObj   try       myObj   new MyClass                  finally       if  null    myObj            myObj cleanup             This will correctly handle the situation when the MyClass   constructor throws an exception

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Having wrestled with finalizer methods lately  in order to dispose connection pools during testing   I have to say that finalizer lacks many things  Using VisualVM to observe as well as using weak references to track the actual interaction I found that following things are true in a Java 8 environment  Oracle JDK  Ubuntu 15     Finalize is not called immediately the Finalizer  GC part  individually owns the reference elusively The default Garbage Collector pools unreachable objects Finalize is called in bulk pointing to an implementation detail that there is a certain phase the garbage collector frees the resources  Calling System gc   often does not result in objects being finalized more often  it just results in the Finalizer getting aware of an unreachable object more rapidly Creating a thread dump almost always result in triggering the finalizer due to high heap overhead during performing the heap dump or some other internal mechanism Finalization seams to be bound by either memory requirements  free up more memory  or by the list of objects being marked for finalization growing of a certain internal limit  So if you have a lot of objects getting finalized the finalization phase will be triggered more often and earlier when compared with only a few There were circumstances a System gc   triggered a finalize directly but only if the reference was a local and short living  This might be generation related    Final Thought  Finalize method is unreliable but can be used for one thing only  You can ensure that an object was closed or disposed before it was garbage collected making it possible to implement a fail safe if objects with a more complex life-cylce involving a end-of-life action are handled correctly  That is the one reason I can think of that makes it worth in order to override it

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finalize method is not guaranteed This method is called when the object becomes eligible for GC  There are many situations where the objects may not be garbage collected

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When is the finalize   method called in Java    The finalize method will be called after the GC detects that the object is no longer reachable  and before it actually reclaims the memory used by the object    If an object never becomes unreachable  finalize   will never be called on it  If the GC doesn t run then finalize   may never be called    Normally  the GC only runs when the JVM decides that there is likely to enough garbage to make it worthwhile   It may take more than one GC cycle before the GC determines that a specific object is unreachable    Java GCs are typically  generational  collectors      Once the GC detects an object is unreachable and finalizable  it is places on a finalization queue   Finalization typically occurs asynchronously with the normal GC     The JVM spec actually allows a JVM to never run finalizers     provided that it doesn t reclaim the space used by the objects   A JVM that was implemented this way would be crippled   useless  but it this behavior is  allowed     The upshot is that it is unwise to rely on finalization to do things that have to be done in a definite time-frame   It is  best practice  not to use them at all   There should be a better  i e  more reliable  way to do whatever it is you are trying to do in the finalize   method     The only legitimate use for finalization is to clean up resources associated with objects that have been lost by application code   Even then  you should try to write the application code so that it doesn t lose the objects in the first place    For example  use Java 7  try-with-resources to ensure that close   is always called            I created a test class which writes into a file when the finalize   method is called by overriding it   It is not executed  Can anybody tell me the reason why it is not executing    It is hard to say  but there are a few possibilities    The object is not garbage collected because it is still reachable  The object is not garbage collected because the GC doesn t run before your test finishes  The object is found by the GC and placed in the finalization queue by the GC  but finalization isn t completed before your test finishes

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Since there is an uncertainity in calling of finalize   method by JVM  not sure whether finalize   which is overridden would be executed or not   for study purposes the better way to observe what happens when finalize   is called  is to force the JVM to call garbage collection by command System gc     Specifically  finalize   is called when an object is no longer in use  But when we try to call it by creating new objects there is no certainty of its call  So for certainty we create a null object c which obviously  has no future use  hence we see the object c s finalize call   Example  class Car        int maxspeed       Car             maxspeed   70             protected void finalize             Originally finalize method does nothing  but here we override finalize   saying it to print some stmt        Calling of finalize is uncertain  Difficult to observe so we force JVM to call it by System gc    GarbageCollection          System out println  Called finalize method in class Car                class Bike        int maxspeed       Bike             maxspeed   50             protected void finalize             System out println  Called finalize method in class Bike                class Example        public static void main String args              Car c   new Car            c   null        if c weren t null JVM wouldn t be certain it s cleared or not  null means has no future use or no longer in use hence clears it         Bike b   new Bike            System gc          should clear c  but not b         for  b maxspeed   1  b maxspeed  lt   70  b maxspeed                  System out print   t    b maxspeed               if  b maxspeed  gt  50                    System out println  Over Speed  Pls slow down                                       Output      Called finalize method in class Car                1       2       3       4       5       6       7       8       9     10      11      12      13      14      15      16      17      18      19     20      21      22      23      24      25      26      27      28      29     30      31      32      33      34      35      36      37      38      39     40      41      42      43      44      45      46      47      48      49     50      51Over Speed  Pls slow down              52Over Speed  Pls slow down              53Over Speed  Pls slow down              54Over Speed  Pls slow down              55Over Speed  Pls slow down              56Over Speed  Pls slow down              57Over Speed  Pls slow down              58Over Speed  Pls slow down               59Over Speed  Pls slow down              60Over Speed  Pls slow down              61Over Speed  Pls slow down              62Over Speed  Pls slow down              63Over Speed  Pls slow down              64Over Speed  Pls slow down              65Over Speed  Pls slow down              66Over Speed  Pls slow down              67Over Speed  Pls slow down              68Over Speed  Pls slow down              69Over Speed  Pls slow down              70Over Speed  Pls slow down    Note - Even after printing upto 70 and after which object b is not being used in the program  there is  uncertainty that b is cleared or not by JVM since  Called finalize method in class Bike     is not printed

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Try runiing this Program for better understanding  public class FinalizeTest               static           System out println Runtime getRuntime   freeMemory                public void run             System out println  run            System out println Runtime getRuntime   freeMemory                 protected void finalize   throws Throwable             System out println  finalize             while true               break                         public static void main String   args                for  int i   0   i  lt  500000   i                           new FinalizeTest   run

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The finalize method is called when an object is about to get garbage collected  That can be at any time after it has become eligible for garbage collection   Note that it s entirely possible that an object never gets garbage collected  and thus finalize is never called   This can happen when the object never becomes eligible for gc  because it s reachable through the entire lifetime of the JVM  or when no garbage collection actually runs between the time the object become eligible and the time the JVM stops running  this often occurs with simple test programs    There are ways to tell the JVM to run finalize on objects that it wasn t called on yet  but using them isn t a good idea either  the guarantees of that method aren t very strong either    If you rely on finalize for the correct operation of your application  then you re doing something wrong  finalize should only be used for cleanup of  usually non-Java  resources  And that s exactly because the JVM doesn t guarantee that finalize is ever called on any object

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Sometimes when it is destroyed  an object must make an action  For example  if an object has a non-java resource such as a file handle or a font  you can verify that these resources are released before destroying an object  To manage such situations  java offers a mechanism called  finalizing   By finalizing it  you can define specific actions that occur when an object is about to be removed from the garbage collector  To add a finalizer to a class simply define the finalize   method  Java execution time calls this method whenever it is about to delete an object of that class  Within the finalize method   you specify actions to be performed before destroying an object  The garbage collector is periodically searched for objects that no longer refer to any running state or indirectly any other object with reference  Before an asset is released  the Java runtime calls the finalize   method on the object  The finalize   method has the following general form   protected void finalize           This is where the finalization code is entered     With the protected keyword  access to finalize   by code outside its class is prevented  It is important to understand that finalize   is called just just before the garbage collection  It is not called when an object leaves the scope  for example  It means you can not know when  or if  finalize   will be executed  As a result  the program must provide other means to free system resources or other resources used by the object  You should not rely on finalize   for normal running of the program

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Java allows objects to implement a method called finalize     that might get called       finalize   method gets called if the garbage collector tries to   collect the object       If the garbage collector doesn t run  the method doesn t get called       If the garbage collector fails to collect the object and tries to run   it again  the method doesn t get called in the second time       In practice  you are highly unlikely to use it in real projects       Just keep in mind that it might not get called and that it definitely   won t be called twice  The finalize   method could run zero or one   time       In the following code  finalize   method produces no output when we   run it since the program exits before there is any need to run the   garbage collector    Source

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As pointed out in https   wiki sei cmu edu confluence display java MET12-J  Do not use finalizers      There is no fixed time at which finalizers must be executed because time of execution depends on the Java Virtual Machine  JVM   The only guarantee is that any finalizer method that executes will do so sometime after the associated object has become unreachable  detected during the first cycle of garbage collection  and sometime before the garbage collector reclaims the associated object s storage  during the garbage collector s second cycle   Execution of an object s finalizer may be delayed for an arbitrarily long time after the object becomes unreachable  Consequently  invoking time-critical functionality such as closing file handles in an object s finalize   method is problematic

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Class where we override finalize method  public class TestClass           public TestClass             System out println  constructor               public void display             System out println  display               Override     public void finalize             System out println  destructor              The chances of finalize method being called  public class TestGarbageCollection       public static void main String   args            while  true                TestClass s   new TestClass                s display                System gc                        when the memory is overloaded with dump objects the gc will call finalize method  run and see the console  where you dont find the finalize method being called frequently  when the memory is getting overloaded then the finalize method will be called

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finalize will print out the count for class creation    protected void finalize   throws Throwable       System out println  Run F         if   checkedOut          System out println  Error  Checked out            System out println  Class Create Count      classCreate       main  while   true        Book novel new Book true         System out println novel checkedOut         Runtime getRuntime   runFinalization        novel checkIn        new Book true         System runFinalization        System gc      As you can see  The following out put show the gc got executed first time when the class count is 36   C  javaCode firstClass gt java TerminationCondition Run F Error  Checked out Class Create Count  36 Run F Error  Checked out Class Create Count  48 Run F

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An Object becomes eligible for Garbage collection or GC if its not reachable from any live threads or any static refrences in other words you can say that an object becomes eligible for garbage collection if its all references are null  Cyclic dependencies are not counted as reference so if Object A has reference of object B and object B has reference of Object A and they don t have any other live reference then both Objects A and B will be eligible for Garbage collection   Generally an object becomes eligible for garbage collection in Java on following cases    All references of that object explicitly set to null e g  object   null Object is created inside a block and reference goes out scope once control exit that block  Parent object set to null  if an object holds reference of another object and when you set container object s reference null  child or contained object automatically becomes eligible for garbage collection  If an object has only live references via WeakHashMap it will be eligible for garbage collection

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In general it s best not to rely on finalize   to do any cleaning up etc   According to the Javadoc  which it would be worth reading   it is      Called by the garbage collector on an object when garbage collection determines that there are no more references to the object    As Joachim pointed out  this may never happen in the life of a program if the object is always accessible   Also  the garbage collector is not guaranteed to run at any specific time  In general  what I m trying to say is finalize   is probably not the best method to use in general unless there s something specific you need it for

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protected void finalize   throws Throwable             every class inherits the finalize   method from   java lang Object   the method is called by the garbage collector when it determines   no more references to the object   exist   the Object finalize method performs no actions but it may be overridden by   any class   normally it should be overridden to clean-up non-Java resources ie closing   a file   if overridding finalize   it is good programming practice to use a   try-catch-finally statement and to   always call super finalize    This   is a safety measure to ensure you do   not inadvertently miss closing a   resource used by the objects calling   class   protected void finalize   throws Throwable        try            close              close open files        finally            super finalize                 any exception thrown by finalize   during garbage collection halts the   finalization but is otherwise ignored   finalize   is never run more than once on any object      quoted from  http   www janeg ca scjp gc finalize html  You could also check this article    Object finalization and cleanup

[java] When is the finalize() method called in Java?

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