How to free memory in Java

Question

Is there a way to free memory in Java  similar to C s free   function  Or is setting the object to null and relying on GC the only option

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Althrough java provides automatic garbage collection sometimes you will want to know how large the object is and how much of it is left  Free memory using programatically  import java lang  and Runtime r Runtime getRuntime     to obtain values of memory using mem1 r freeMemory    to free memory call the r gc    method and the call freeMemory

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No one seems to have mentioned explicitly setting object references to null  which is a legitimate technique to  freeing  memory you may want to consider   For example  say you d declared a List lt String gt  at the beginning of a method which grew in size to be very large  but was only required until half-way through the method   You could at this point set the List reference to null to allow the garbage collector to potentially reclaim this object before the method completes  and the reference falls out of scope anyway    Note that I rarely use this technique in reality but it s worth considering when dealing with very large data structures

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System gc          Runs the garbage collector     Calling the gc method suggests that the Java Virtual Machine expend effort toward recycling unused objects in order to make the memory they currently occupy available for quick reuse  When control returns from the method call  the Java Virtual Machine has made a best effort to reclaim space from all discarded objects     Not recommended   Edit  I wrote the original response in 2009   It s now 2015   Garbage collectors have gotten steadily better in the  20 years Java s been around   At this point  if you re manually calling the garbage collector  you may want to consider other approaches    If you re forcing GC on a limited number of machines  it may be worth having a load balancer point away from the current machine  waiting for it to finish serving to connected clients  timeout after some period for hanging connections  and then just hard-restart the JVM   This is a terrible solution  but if you re looking at System gc    forced-restarts may be a possible stopgap  Consider using a different garbage collector   For example  the  new in the last six years  G1 collector is a low-pause model  it uses more CPU overall  but does it s best to never force a hard-stop on execution   Since server CPUs now almost all have multiple cores  this is A Really Good Tradeoff to have available  Look at your flags tuning memory use   Especially in newer versions of Java  if you don t have that many long-term running objects  consider bumping up the size of newgen in the heap   newgen  young  is where new objects are allocated   For a webserver  everything created for a request is put here  and if this space is too small  Java will spend extra time upgrading the objects to longer-lived memory  where they re more expensive to kill    If newgen is slightly too small  you re going to pay for it    For example  in G1    XX G1NewSizePercent  defaults to 5  probably doesn t matter   XX G1MaxNewSizePercent  defaults to 60  probably raise this    Consider telling the garbage collector you re not okay with a longer pause   This will cause more-frequent GC runs  to allow the system to keep the rest of it s constraints   In G1    XX MaxGCPauseMillis  defaults to 200

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Recommendation from JAVA is to assign to null From https   docs oracle com cd E19159-01 819-3681 abebi index html  Explicitly assigning a null value to variables that are no longer needed helps the garbage collector to identify the parts of memory that can be safely reclaimed  Although Java provides memory management  it does not prevent memory leaks or using excessive amounts of memory  An application may induce memory leaks by not releasing object references  Doing so prevents the Java garbage collector from reclaiming those objects  and results in increasing amounts of memory being used  Explicitly nullifying references to variables after their use allows the garbage collector to reclaim memory  One way to detect memory leaks is to employ profiling tools and take memory snapshots after each transaction  A leak-free application in steady state will show a steady active heap memory after garbage collections

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For example  say you d declared a List at the beginning of a method which grew in size to be very large  but was only required until half-way through the method  You could at this point set the List reference to null to allow the garbage collector to potentially reclaim this object before the method completes  and the reference falls out of scope anyway       This is correct  but this solution may not be generalizable   While setting a List object reference to null -will- make memory available for garbage collection  this is only true for a List object of primitive types   If the List object instead contains reference types  setting the List object   null will not dereference -any- of the reference types contained -in- the list   In this case  setting the List object   null will orphan the contained reference types whose objects will not be available for garbage collection unless the garbage collection algorithm is smart enough to determine that the objects have been orphaned

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If you really want to allocate and free a block of memory you can do this with direct ByteBuffers  There is even a non-portable way to free the memory   However  as has been suggested  just because you have to free memory in C  doesn t mean it a good idea to have to do this   If you feel you really have a good use case for free    please include it in the question so we can see what you are rtying to do  it is quite likely there is a better way

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A valid reason for wanting to free memory from any programm  java or not   is to make more memory available to other programms on operating system level  If my java application is using 250MB I may want to force it down to 1MB and make the 249MB available to other apps

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To extend upon the answer and comment by Yiannis Xanthopoulos and Hot Licks  sorry  I cannot comment yet    you can set VM options like this example   -XX  UseG1GC -XX MinHeapFreeRatio 15 -XX MaxHeapFreeRatio 30   In my jdk 7 this will then release unused VM memory if more than 30  of the heap becomes free after GC when the VM is idle  You will probably need to tune these parameters   While I didn t see it emphasized in the link below  note that some garbage collectors may not obey these parameters and by default java may pick one of these for you  should you happen to have more than one core  hence the UseG1GC argument above    VM arguments  Update  For java 1 8 0 73 I have seen the JVM occasionally release small amounts with the default settings  Appears to only do it if  70  of the heap is unused though   don t know if it would be more aggressive releasing if the OS was low on physical memory

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In my case  since my Java code is meant to be ported to other languages in the near future  Mainly C     I at least want to pay lip service to freeing memory properly so it helps the porting process later on   I personally rely on nulling variables as a placeholder for future proper deletion  For example  I take the time to nullify all elements of an array before actually deleting  making null  the array itself   But my case is very particular  and I know I m taking performance hits when doing this

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Java uses managed memory  so the only way you can allocate memory is by using the new operator  and the only way you can deallocate memory is by relying on the garbage collector   This memory management whitepaper  PDF  may help explain what s going on   You can also call System gc   to suggest that the garbage collector run immediately   However  the Java Runtime makes the final decision  not your code   According to the Java documentation       Calling the gc method suggests that   the Java Virtual Machine expend effort   toward recycling unused objects in   order to make the memory they   currently occupy available for quick   reuse  When control returns from the   method call  the Java Virtual Machine   has made a best effort to reclaim   space from all discarded objects

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I have done experimentation on this   It s true that System gc    only suggests to run the Garbage Collector   But calling System gc    after setting all references to null  will improve performance and memory occupation

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Entirely from javacoffeebreak com faq faq0012 html     A low priority thread takes care of garbage collection automatically   for the user  During idle time  the thread may be called upon  and it   can begin to free memory previously allocated to an object in Java    But don t worry - it won t delete your objects on you       When there are no references to an object  it becomes fair game for   the garbage collector  Rather than calling some routine  like free in   C     you simply assign all references to the object to null  or   assign a new class to the reference       Example    public static void main String args           Instantiate a large memory using class   MyLargeMemoryUsingClass myClass   new MyLargeMemoryUsingClass 8192         Do some work   for                                 Do some processing on myClass           Clear reference to myClass   myClass   null        Continue processing  safe in the knowledge      that the garbage collector will reclaim myClass         If your code is about to request a large amount of memory  you may   want to request the garbage collector begin reclaiming space  rather   than allowing it to do so as a low-priority thread  To do this  add   the following to your code  System gc          The garbage collector will attempt to reclaim free space  and your   application can continue executing  with as much memory reclaimed as   possible  memory fragmentation issues may apply on certain platforms

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I personally rely on nulling variables as a placeholder for future proper deletion  For example  I take the time to nullify all elements of an array before actually deleting  making null  the array itself    This is unnecessary  The way the Java GC works is it finds objects that have no reference to them  so if I have an Object x with a reference   variable  a that points to it  the GC won t delete it  because there is a reference to that object   a - gt  x   If you null a than this happens   a - gt  null      x   So now x doesn t have a reference pointing to it and will be deleted  The same thing happens when you set a to reference to a different object than x   So if you have an array arr that references to objects x  y and z and a variable a that references to the array it looks like that   a - gt  arr - gt  x          - gt  y          - gt  z   If you null a than this happens   a - gt  null      arr - gt  x          - gt  y          - gt  z   So the GC finds arr as having no reference set to it and deletes it  which gives you this structure   a - gt  null      x      y      z   Now the GC finds x  y and z and deletes them aswell  Nulling each reference in the array won t make anything better  it will just use up CPU time and space in the code  that said  it won t hurt further than that  The GC will still be able to perform the way it should

[java] How to free memory in Java?

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