What are Runtime getRuntime totalMemory and freeMemory

Question

I ve been wondering what the exact meaning of  Runtime getRuntime   totalMemory    Runtime getRuntime   freeMemory    and Runtime getRuntime   maxMemory   is   My understanding is  Runtime getRuntime   totalMemory   returns the total memory my process is using  Is that correct    How about freeMemory   and maxMemory

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The names and values are confusing  If you are looking for the total free memory you will have to calculate this value by your self  It is not what you get from freeMemory      See the following guide   Total designated memory  this will equal the configured -Xmx value      Runtime getRuntime   maxMemory      Current allocated free memory  is the current allocated space ready for new objects  Caution this is not the total free available memory      Runtime getRuntime   freeMemory      Total allocated memory  is the total allocated space reserved for the java process      Runtime getRuntime   totalMemory      Used memory  has to be calculated      usedMemory   Runtime getRuntime   totalMemory   - Runtime getRuntime   freeMemory      Total free memory  has to be calculated      freeMemory   Runtime getRuntime   maxMemory   - usedMemory    A picture may help to clarify

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You can see the results in MB format  with the division of 1024 x 1024 which is equal to 1 MB   int dataSize   1024   1024   System out println  Used Memory          Runtime getRuntime   totalMemory   - Runtime getRuntime   freeMemory    dataSize     MB    System out println  Free Memory         Runtime getRuntime   freeMemory   dataSize     MB    System out println  Total Memory        Runtime getRuntime   totalMemory   dataSize     MB    System out println  Max Memory          Runtime getRuntime   maxMemory   dataSize     MB

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Codified version of all other answers  at the time of writing    import java io            This class is based on  lt a href  http   stackoverflow com users 2478930 cheneym  gt cheneym lt  a gt  s     lt a href  http   stackoverflow com a 18375641 253468  gt awesome interpretation lt  a gt     of the Java   link Runtime  s memory query methods  which reflects intuitive thinking     Also includes comments and observations from others on the same question  and my own experience      lt p gt      lt img src  https   i stack imgur com GjuwM png  alt  Runtime s memory interpretation  gt      lt p gt      lt b gt JVM memory management crash course lt  b gt      Java virtual machine process  heap size is bounded by the maximum memory allowed     The startup and maximum size can be configured by JVM arguments     JVMs don t allocate the maximum memory on startup as the program running may never require that     This is to be a good player and not waste system resources unnecessarily     Instead they allocate some memory and then grow when new allocations require it     The garbage collector will be run at times to clean up unused objects to prevent this growing     Many parameters of this management such as when to grow shrink or which GC to use    can be tuned via advanced configuration parameters on JVM startup         see  lt a href  http   stackoverflow com a 42567450 253468  gt         What are Runtime getRuntime   totalMemory   and freeMemory    lt  a gt      see  lt a href  http   www oracle com technetwork java javase memorymanagement-whitepaper-150215 pdf  gt         Memory Management in the Sun Java HotSpot    Virtual Machine lt  a gt      see  lt a href  http   docs oracle com javase 8 docs technotes tools windows java html  gt         Full VM options reference for Windows lt  a gt      see  lt a href  http   docs oracle com javase 8 docs technotes tools unix java html  gt         Full VM options reference for Linux  Mac OS X and Solaris lt  a gt      see  lt a href  http   www oracle com technetwork articles java vmoptions-jsp-140102 html  gt         Java HotSpot VM Options quick reference lt  a gt      public class SystemMemory           can be white-box mocked for testing     private final Runtime runtime   Runtime getRuntime                     lt b gt Total allocated memory lt  b gt   space currently reserved for the JVM heap within the process          lt p gt          lt i gt Caution lt  i gt   this is not the total memory  the JVM may grow the heap for new allocations              public long getAllocatedTotal             return runtime totalMemory                           lt b gt Current allocated free memory lt  b gt   space immediately ready for new objects          lt p gt          lt i gt Caution lt  i gt   this is not the total free available memory         the JVM may grow the heap for new allocations              public long getAllocatedFree             return runtime freeMemory                           lt b gt Used memory lt  b gt          Java heap currently used by instantiated objects           lt p gt          lt i gt Caution lt  i gt   May include no longer referenced objects  soft references  etc         that will be swept away by the next garbage collection              public long getUsed             return getAllocatedTotal   - getAllocatedFree                           lt b gt Maximum allocation lt  b gt   the process  allocated memory will not grow any further          lt p gt          lt i gt Caution lt  i gt   This may change over time  do not cache it         There are some JVMs   garbage collectors that can shrink the allocated process memory          lt p gt          lt i gt Caution lt  i gt   If this is true  the JVM will likely run GC more often              public boolean isAtMaximumAllocation             return getAllocatedTotal      getTotal                 return getUnallocated      0                         lt b gt Unallocated memory lt  b gt   amount of space the process  heap can grow              public long getUnallocated             return getTotal   - getAllocatedTotal                           lt b gt Total designated memory lt  b gt   this will equal the configured   code -Xmx  value          lt p gt          lt i gt Caution lt  i gt   You can never allocate more memory than this  unless you use native code              public long getTotal             return runtime maxMemory                           lt b gt Total free memory lt  b gt   memory available for new Objects         even at the cost of growing the allocated memory of the process              public long getFree             return getTotal   - getUsed                 return getAllocatedFree     getUnallocated                           lt b gt Unbounded memory lt  b gt   there is no inherent limit on free memory              public boolean isBounded             return getTotal      Long MAX VALUE                        Dump of the current state for debugging or understanding the memory divisions          lt p gt          lt i gt Caution lt  i gt   Numbers may not match up exactly as state may change during the call              public String getCurrentStats             StringWriter backing   new StringWriter            PrintWriter out   new PrintWriter backing  false           out printf  Total  allocated   d    1f    out of possible   d   s   s   d n                   getAllocatedTotal                     float getAllocatedTotal      float getTotal     100                  getTotal                    isBounded     bounded     unbounded                   isAtMaximumAllocation     maxed out     can grow                   getUnallocated                      out printf  Used    d    1f   of total    d     1f   of allocated    d  n                   getUsed                     float getUsed      float getTotal     100                  getTotal                     float getUsed      float getAllocatedTotal     100                  getAllocatedTotal                      out printf  Free    d    1f    out of   d total    d    1f    out of   d allocated n                   getFree                     float getFree      float getTotal     100                  getTotal                    getAllocatedFree                     float getAllocatedFree      float getAllocatedTotal     100                  getAllocatedTotal                      out flush            return backing toString               public static void main String    args            SystemMemory memory   new SystemMemory            System out println memory getCurrentStats

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To understand it better  run this following program  in jdk1 7 x         java -Xms1025k -Xmx1025k -XshowSettings vm  MemoryTest    This will print jvm options and the used  free  total and maximum memory available in jvm   public class MemoryTest           public static void main String args                      System out println  Used Memory           Runtime getRuntime   totalMemory   - Runtime getRuntime   freeMemory        bytes                    System out println  Free Memory         Runtime getRuntime   freeMemory       bytes                    System out println  Total Memory        Runtime getRuntime   totalMemory       bytes                    System out println  Max Memory          Runtime getRuntime   maxMemory       bytes

User · Answer

JVM heap size can be growable and shrinkable by the Garbage-Collection mechanism  But  it can t allocate over maximum memory size  Runtime maxMemory  This is the meaning of maximum memory  Total memory means the allocated heap size  And free memory means the available size in total memory   example  java -Xms20M -Xmn10M -Xmx50M      This means that jvm should allocate heap 20M on start ms   In this case  total memory is 20M  free memory is 20M-used size  If more heap is needed  JVM allocate more but can t over 50M mx   In the case of maximum  total memory is 50M  and free size is 50M-used size  As for minumum size mn   if heap is not used much  jvm can shrink heap size to 10M   This mechanism is for efficiency of memory  If small java program run on huge fixed size heap memory  so much memory may be wasteful

User · Answer

According to the API  totalMemory     Returns the total amount of memory in the Java virtual machine  The value returned by this method may vary over time  depending on the host environment  Note that the amount of memory required to hold an object of any given type may be implementation-dependent   maxMemory     Returns the maximum amount of memory that the Java virtual machine will attempt to use  If there is no inherent limit then the value Long MAX VALUE will be returned   freeMemory     Returns the amount of free memory in the Java Virtual Machine  Calling the gc method may result in increasing the value returned by freeMemory   In reference to your question  maxMemory   returns the -Xmx value   You may be wondering why there is a totalMemory   AND a maxMemory     The answer is that the JVM allocates memory lazily   Lets say you start your Java process as such   java -Xms64m -Xmx1024m Foo   Your process starts with 64mb of memory  and if and when it needs more  up to 1024m   it will allocate memory   totalMemory   corresponds to the amount of memory currently available to the JVM for Foo   If the JVM needs more memory  it will lazily allocate it up to the maximum memory   If you run with -Xms1024m -Xmx1024m  the value you get from totalMemory   and maxMemory   will be equal   Also  if you want to accurately calculate the amount of used memory  you do so with the following calculation    final long usedMem   totalMemory   - freeMemory

User · Answer

Runtime totalMemory - the memory that the JVM has allocated thus far  This isn t necessarily what is in use or the maximum   Runtime maxMemory - the maximum amount of memory that the JVM has been configured to use  Once your process reaches this amount  the JVM will not allocate more and instead GC much more frequently   Runtime freeMemory - I m not sure if this is measured from the max or the portion of the total that is unused  I am guessing it is a measurement of the portion of total which is unused

[java] What are Runtime.getRuntime().totalMemory() and freeMemory()?

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