How to force garbage collection in Java

Question

Is it possible to force garbage collection in Java  even if it is tricky to do  I know about System gc    and Runtime gc    but they only suggest to do GC  How can I force GC

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Really  I don t get you  But to be   clear about  Infinite Object Creation    I meant that there is some piece of   code at my big system do creation of   objects whom handles and alive in   memory  I could not get this piece of   code actually  just gesture     This is correct  only gesture  You have pretty much the standard answers already given by several posters  Let s take this one by one    I could not get this piece of code actually   Correct  there is no actual jvm - such is only a specification  a bunch of computer science describing a desired behaviour     I recently dug into initializing Java objects from native code  To get what you want  the only way is to do what is called aggressive nulling  The mistakes if done wrong are so bad doing that we have to limit ourselves to the original scope of the question     some piece of code at my big system do creation of objects   Most of the posters here will assume you are saying you are working to an interface  if such we would have to see if you are being handed the entire object or one item at a time   If you no longer need an object  you can assign null to the object but if you get it wrong there is a null pointer exception generated  I bet you can achieve better work if you use NIO  Any time you or I or anyone else gets   Please I need that horribly   it is almost universal precursor to near total destruction of what you are trying to work on      write us a small sample code  sanitizing from it any actual code used and show us your question   Do not get frustrated  Often what this resolves to is your dba is using a package bought somewhere and the original design is not tweaked for massive data structures    That is very common

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gc is a candidate for elimination in future releases - a Sun Engineer once commented that maybe fewer than twenty people in the world actually know how to use  gc   - I did some work last night for a few hours on a central   critical data-structure using SecureRandom generated data  at somewhere just past 40 000 objects the vm would slow down as though it had run out of pointers  Clearly it was choking down on 16-bit pointer tables and exhibited classic  failing machinery  behavior   I tried -Xms and so on  kept bit twiddling until it would run to about 57 xxx something  Then it would run gc going from say 57 127 to 57 128 after a gc   - at about the pace of code-bloat at camp Easy Money   Your design needs fundamental re-work  probably a sliding window approach

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If you are using JUnit and Spring  try adding this in every test class    DirtiesContext classMode   DirtiesContext ClassMode AFTER CLASS

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To manually Request GC  not from System gc         Go To   bin folder in JDK eg -C  Program Files Java jdk1 6 0 31 bin Open jconsole exe Connect to the desired local Process  Go To memory tab and click perform GC

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I would like to add some thing here  Please not that Java runs on Virtual Machine and not actual Machine  The virtual machine has its own way of communication with the machine  It may varry from system to system  Now When we call the GC we ask the Virtual Machine of Java to call the Garbage Collector    Since the Garbage Collector is with Virtual Machine   we can not force it to do a cleanup there and then  Rather that we queue our request with the Garbage Collector  It depends on the Virtual Machine  after particular time  this may change from system to system  generally when the threshold memory allocated to the JVM is full  the actual machine will free up the space   D

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Another options is to not create new objects   Object pooling is away to reduce the need GC in Java   Object pooling is generally not going to be faster than Object creation  esp for lightweight objects  but it is faster than Garbage Collection  If you created 10 000 objects and each object was 16 bytes  That s 160 000 bytes GC has to reclaim  On the other hand  if you don t need all 10 000 at the same time  you can create a pool to recycle reuse the objects which eliminates the need to construct new objects and eliminates the need to GC old objects   Something like this  untested   And if you want it to be thread safe you can swap out the LinkedList for a ConcurrentLinkedQueue   public abstract class Pool lt T gt        private int mApproximateSize      private LinkedList lt T gt  mPool   new LinkedList lt  gt          public Pool int approximateSize            mApproximateSize   approximateSize             public T attain             T item   mPool poll            if  item    null                item   newInstance                      return item             public void release T item            int approxSize   mPool size       not guaranteed accurate         if  approxSize  lt  mApproximateSize                recycle item               mPool add item             else if  approxSize  gt  mApproximateSize                decommission mPool poll                          public abstract T newInstance         public abstract void recycle T item        public void decommission T item

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Using the Java    Virtual Machine Tool Interface  JVM TI   the function  jvmtiError ForceGarbageCollection jvmtiEnv  env    will  Force the VM to perform a garbage collection   The JVM TI is part of the JavaTM Platform Debugger Architecture  JPDA

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How to Force Java GC Okay  here are a few different ways to force Java GC   Click JConsole s Perform GC button Use JMap s jmap -histo live 7544 command where 7544 is the pid Call the Java Diagnostic Console s jcmd 7544 GC run command Call System gc    in your code Call Runtime getRuntime   gc    in your code   None of these work Here s the dirty little secret  None of these are guaranteed to work  You really can t force Java GC  The Java garbage collection algos are non-deterministic  and while all of these methods can motivate the JVM to do GC  you can t actually force it  If the JVM has too much going on and a stop-the-world operation is not possible  these commands will either error out  or they will run but GC won t actually happen  if  input equalsIgnoreCase  quot gc quot          System gc        result    quot Just some GC  quot      if  input equalsIgnoreCase  quot runtime quot          Runtime getRuntime   gc        result    quot Just some more GC  quot      Fix the darn problem If you ve got a memory leak or object allocation problem  then fix it  Sitting around with your finger on Java Mission Control s Force Java GC button only kicks the can down the road  Profile your app with Java Flight Recorder  view the results in VisualVM or JMC  and fix the problem  Trying to force Java GC is a fools game

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You can trigger a GC from the command line   This is useful for batch crontab   jdk1 7 0 bin jcmd  lt pid gt  GC run   See     https   docs oracle com javase 8 docs technotes guides troubleshoot tooldescr006 html

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The best  if not only  way to force a GC would be to write a custom JVM   I believe the Garbage collectors are pluggable so you could probably just pick one of the available implementations and tweak it   Note   This is NOT an easy answer

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There is some indirect way for forcing garbage collector  You just need to fill heap with temporary objects until the point when garbage collector will execute  I ve made class which forces garbage collector in this way   class GarbageCollectorManager        private static boolean collectionWasForced      private static int refCounter   0       public GarbageCollectorManager             refCounter                Override     protected void finalize             try               collectionWasForced   true              refCounter--              super finalize                 catch  Throwable ex                Logger getLogger GarbageCollectorManager class getName    log Level SEVERE  null  ex                        public int forceGarbageCollection             final int TEMPORARY ARRAY SIZE FOR GC   200 000          int iterationsUntilCollected   0          collectionWasForced   false           if  refCounter  lt  2               new GarbageCollectorManager             while   collectionWasForced                iterationsUntilCollected                int   arr   new int TEMPORARY ARRAY SIZE FOR GC               arr   null                     return iterationsUntilCollected             Usage   GarbageCollectorManager manager   new GarbageCollectorManager    int iterationsUntilGcExecuted   manager forceGarbageCollection      I don t know how much this method is useful  because it fills heap constantly  but if you have mission critical application which MUST force GC - when this may be the Java portable way to force GC

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YES it is almost possible to forced you have to call to methods in the same order and at the same time this ones are   System gc     System runFinalization       even if is just one object to clean the use of this two methods at the same time force the garbage collector to use the finalise   method of unreachable object freeing the memory assigned and doing what the finalize   method states   HOWEVER it is a terrible practice to use the garbage collector because the use of it could introduce an over load to the software that may be even worst than on the memory  the garbage collector has his own thread which is not possible to control plus depending on the algorithm used by the gc could take more time and is consider very inefficient  you should check your software if it worst with the help of the gc because it is definitely broke  a good solution must not depend on the gc   NOTE  just to keep on mind this will works only if in the finalize method is not a reassignment of the object  if this happens the object will keep alive an it will have a resurrection which is technically possible

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On OracleJDK 10 with G1 GC  a single call to System gc   will cause GC to clean up the Old Collection   I am not sure if GC runs immediately   However  GC will not clean up the Young Collection even if System gc   is called many times in a loop   To get GC to clean up the Young Collection  you must allocate in a loop  e g  new byte 1024   without calling System gc     Calling System gc   for some reason prevents GC from cleaning up the Young Collection

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The jlibs library has a good utility class for garbage collection  You can force garbage collection using a nifty little trick with WeakReference objects   RuntimeUtil gc   from the jlibs                This method guarantees that garbage collection is       done unlike  lt code gt   link System gc    lt  code gt            public static void gc          Object obj   new Object         WeakReference ref   new WeakReference lt Object gt  obj        obj   null       while ref get      null           System gc

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FYI  The method call System runFinalizersOnExit true  guarantees that finalizer methods are called before Java shuts down  However  this method is inherently unsafe and has been deprecated  An alternative is to add    shutdown hooks    with the method Runtime addShutdownHook   Masarrat Siddiqui

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You can try using Runtime getRuntime   gc   or use utility method System gc   Note  These methods do not ensure GC  And their scope should be limited to JVM rather than programmatically handling it in your application

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It would be better if you would describe the reason why you need garbage collection  If you are using SWT  you can dispose resources such as Image and Font to free memory  For instance   Image img   new Image Display getDefault    16  16   img dispose      There are also tools to determine undisposed resources

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Under the documentation for OutOfMemoryError it declares that it will not be thrown unless the VM has failed to reclaim memory following a full garbage collection   So if you keep allocating memory until you get the error  you will have already forced a full garbage collection    Presumably the question you really wanted to ask was  how can I reclaim the memory I think I should be reclaiming by garbage collection

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If you need to force garbage collection  perhaps you should consider how you re managing resources  Are you creating large objects that persist in memory  Are you creating large objects  e g   graphics classes  that have a Disposable interface and not calling dispose   when done with it  Are you declaring something at a class level that you only need within a single method

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JVM specification doesn t say anything specific about garbage collection  Due to this  vendors are free to implement GC in their way   So this vagueness causes uncertainty in garbage collection behavior  You should check your JVM details to know about the garbage collection approaches algorithms  Also there are options to customize behavior as well

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If you are running out of memory and getting an OutOfMemoryException you can try increasing the amount of heap space available to java by starting you program with java -Xms128m -Xmx512m instead of just java  This will give you an initial heap size of 128Mb and a maximum of 512Mb  which is far more than the standard 32Mb 128Mb

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Your best option is to call System gc   which simply is a hint to the garbage collector that you want it to do a collection   There is no way to force and immediate collection though as the garbage collector is non-deterministic

[java] How to force garbage collection in Java?

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