Memory Management In Java – Stack And Heap

Whenever you trigger a java command, it divides allocated memory into two parts – Stack and Heap. Stack is used only for execution purpose. Heap is used for storage purpose. consider the following program and we will see how it uses stack and heap memory through diagram.

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class StackAndHeapMemory
{
     static void methodOne()
     {
          System.out.println("From Method One");
          methodTwo();
     }

     static void methodTwo()
     {
          System.out.println("From Method Two");
     }

     public static void main(String[] args)
     {
          System.out.println("Main Method Started");
          methodOne();
          System.out.println("Main Method Ended");
     }
}

When you trigger >java StackAndHeapMemory, java command divides allocated memory into two parts. one is stack and another one is heap. As already said, stack is used for execution purpose and heap is used for storage purpose. Here is the steps involved in execution of above program.

Step 1 :

First java command enters the stack memory for execution. First it checks whether the class StackAndHeapMemory is loaded in heap memory or not. If it is not loaded, loading operation of class StackAndHeapMemory starts.

Step 2 :

Randomly one object is created in the heap memory. It is also called Class Memory.  After object creation, all static members are loaded into class memory. You know that execution of every java program start with main() method. So, java commands calls main() method for execution.

Step 3 :
main() method enters stack memory for execution. First statement in main() method (Line 16) is executed. It prints “Main Method Started” on the console. In the second statement (Line 17), it calls methodOne() for execution.

Step 4 :

methodOne() enters the stack for execution. First statement (Line 5) of methodOne() is executed first. It prints “From Method One” on the console. In the second statement (Line 6), it calls methodTwo() for execution.

Step 5 :

methodTwo() enters the stack for execution. In the methodTwo(), there is only one statement (Line 11). This statement is executed. It prints “From Method Two” on the console. There is no other statements in methodTwo(). So, methodTwo() leaves stack memory.

Step 6 :

Now, again control comes back to methodOne(). Second statement (Line 6) of methodOne() is already executed in step 4. There are no other statements left in methodOne(). So, methodOne() also leaves stack.

Step 7 :

Now, control comes back to main() method. second statement (Line 17) of main() is already executed is in step 3. So, now third statement (Line 18) is executed. It prints “Main Method Ended” on the console. As there are no other statements left in main() method, it also leaves stack after clearing the heap memory.

Step 8 :

java command also leaves stack memory and gives back the allocated memory to OS.

Here is the diagrammatic representation of memory allocation of the above program.
Memory Management In Java

Output :

Main Method Started
From Method One
From Method Two
Main Method Ended