Java Memory Management

1
Topic 7

• Java Memory Management
• (before Recursion)

2
Chapter Objectives
3
Memory Allocation in General

• When a program is executed, its memory area
  typically consists of
• the code segment a storage area for a program or
  the part of the program that is executing
• the runtime stack used when calling methods, to
  hold local variables, parameters, returned
  values, etc.
• the heap used to hold dynamic data (e.g. objects)

4
Memory Allocation in Java

• separate areas of memory are allocated for each
  class, interface, object, and running method
• runtime stack used for method information
• heap used for static information (interfaces and
  classes) and instance information (objects)
• example what happens when an object is created
  by new?
• a reference variable is put on the runtime stack
• an object is created using memory in the heap

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Runtime Stack

• runtime stack is the memory space used when a
  method is executed
• keeps track of method invocations, holds local
  variables, parameters, etc.
• runtime stack is also called program stack or
  system stack
• when a method is invoked, an activation record
  that represents the invocation is pushed onto the
  runtime stack

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Activation Record

• an activation record ( or call frame ) contains
• address to return to after method ends
• copy of the methods local variables
• copy of the methods parameters for that
  invocation

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Example

• when the main method is invoked, an activation
  record for main is created and pushed onto the
  runtime stack
• (diagram)

8
Example (contd)

• suppose main calls the method m2
• an activation record for m2 is created and pushed
  onto the runtime stack
• suppose m2 calls m3
• an activation record for m2 is created and pushed
  onto the runtime stack
• when m3 terminates, its activation record is
  popped off and control returns to m2
• when m2 terminates, its activation record is
  popped off and control returns to main
• when main terminates, its activation record is
  popped off and control returns to the operating
  system

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Heap

• static space contains one copy of each class and
  interface named in the program
• contains their static variables and methods
• object space
• information is stored about each object
• value of its instance variables
• type of object (i.e. name of class)
• memory address of object (unique)

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Object Creation

• memory is allocated in the heap area when an
  object is created using new
• the reference variable is put in the call frame
  on the runtime stack
• an object is created using memory in the heap

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Memory Deallocation

• what happens when a method returns?
• on the runtime stack, the call frame is
  automatically popped when the method returns
• so, that memory is deallocated

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Garbage Collection

• what happens to objects on the heap?
• an object stays on the heap even if there is no
  longer a variable referencing it
• so, Java has automatic garbage collection it
  regularly identifies objects which no longer have
  a variable referencing them, and deallocates the
  memory

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Parameter Passing in Java

• pass by value used with simple variables
  (integers, doubles, etc.)
• a copy of the contents of the actual parameter is
  used by the method
• the method can change the copy
• can it change the original?

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Parameter Passing (contd)

• pass by reference used with reference variables
• a copy of the contents of the actual parameter is
  used by the method
• but, what is that? a reference to the object
• so, the method can change the object, since it
  knows where it is
• note that in fact, the reference is being passed
  by value

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Examples