Java Memory Management

The Stack Memory

  • There can be multiple stacks
  • In general in a stack data structure, as new elements are added the elements are pushed down in stack, while popping the first element is pushed out first. First in Last Out datastructure.
  • Java store local variables of a class or method in stack, and are popped off the stack when it encounters end of function block or class block
  • In general the variables stored in stack have very short lifetime.

The Heap Memory

  • The heap is shared between threads
  • All primitives are stored in stack, whereas all objects are stored on heap

Rules

  • Objects are stored on the heap memory
  • Variables are a reference to the Object
  • Local variables are stored on the stack

Passing Variables/Objects

  • In java, while passing parameters to methods, its always the copy of value of variable that is passed.
  • In case of passing an object, the variables in stack store a reference to object in Heap. When the object is passed, the copy of reference is passed to the method.

Metaspace

  • general metadata of objects, functions
  • location where static variables are stored. Static primitive types are stored in metaspace, whereas static objects are created in heap but a pointer is held in metapace.

String Pool

  • The strings with same hashcode are always in same string pool bucket (or same hashmap entry)
  • Same bucket can have different string as well.
  • -XX:+PrintStringTableStatistics displays string pool data while running the program. Sample for 64 bit java 11 vm.
SymbolTable statistics:
Number of buckets       :     20011 =    160088 bytes, each 8
Number of entries       :        14 =       336 bytes, each 24
Number of literals      :        14 =       472 bytes, avg  33.714
Total footprint         :           =    160896 bytes
Average bucket size     :     0.001
Variance of bucket size :     0.001
Std. dev. of bucket size:     0.026
Maximum bucket size     :         1
StringTable statistics:
Number of buckets       :     65536 =    524288 bytes, each 8
Number of entries       :         6 =        96 bytes, each 16
Number of literals      :         6 =       304 bytes, avg  50.667
Total footprsize_t         :           =    524688 bytes
Average bucket size     :     0.000
Variance of bucket size :     0.000
Std. dev. of bucket size:     0.010
Maximum bucket size     :
  • -XX:StringTableSize=<prime number> sets string pool size. This can improve application performance if existing string density in table was very high.