JVM

Heap Size

From The Java Developers Almanac: Getting the Size of the Java Memory Heap

The heap is the area in memory in which objects are created:

// Get current size of heap in bytes
long heapSize = Runtime.getRuntime().totalMemory();

// Get maximum size of heap in bytes. The heap cannot grow beyond this size.
// Any attempt will result in an OutOfMemoryException.
long heapMaxSize = Runtime.getRuntime().maxMemory();

// Get amount of free memory within the heap in bytes. This size will increase
// after garbage collection and decrease as new objects are created.
long heapFreeSize = Runtime.getRuntime().freeMemory();

To investigate the results from these methods, I ran an application which uses a significant amount of heap memory. The maximum heap memory was set at 1GB (-Xmx1024m). The first line is soon after the application starts, the second just before it exits:

maxMemory [1,065,484,288] totalMemory   [2,031,616] freeMemory  [1,405,304]
maxMemory [1,065,484,288] totalMemory [133,517,312] freeMemory [70,297,464]

Reading the JavaDocs for these methods we would say:

• freeMemory cannot exceed totalMemory.

• Total free memory can be calculated as follows:

  total = maxMemory - totalMemory + freeMemory
  

Miscellaneous

Do you know your data size? - Don’t pay the price for hidden class fields.