Map code to physical memory in Java

Operating System Perspective (OS) : From Operating System perspective, a process representation in memory is represented as

Take aways :

• Operating System has the same perspective for all the processes, irrespective of which language has been used to create the binary for the process.
• Its the same for JAVA , C , C++ , PHP, Perl , Python as its how the OS perceives the process .
• The representation is on RAM , so what we call stack , heap , data or code is a logical binding , as underlying storage is RAM.
• Stack is known to grow down with the criteria , one can add data onto the top and remove from the top.
• Stack elements can be removed starting from top , no random deletion of elements are possible.
• Heap is the logical structure where allocation and deallocation can happen randomly . (Heap is not queue )
• Both stack and Heap differs only in the logical way of accessing the data (insertion , retrieval and deletion) and both are mapped on same Map.

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What is so special about Java processes.

• Java classes runs on JVM which internally manages HEAP.
• The management guarantees that any allocation of memory on the JVM manged HEAP will be deallocated automatically.
• To deallocate JVM runs garbage collection threads along with the Code you write.
• All Java does is manages a part of the HEAP , you are free to allocate memory(JAVA provides API) outside the Java managed HEAP.
• If one allocates memory outside JVM manged heap , deallocation is the responsibility of the programmer.

Lets take a sample Java class

public class SampleClass {

    private int state;

    public SampleClass(int input){
        state = input;
    }

    public int getState(){
        return  state;
    }

    public static void main(String[] args) {
        SampleClass var = new SampleClass(10);
        System.out.println(var.getState());
    }

}

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Java code is compiled and executed , during code execution Java class files is consumed by JVM binary which generates instructions which are executed by CPU.

Most important instruction in code

SampleClass var = new SampleClass(10);

KEY TAKE AWAYS :

• All the functions of a class is stored in Code area
• The references are stored in stack
• The actual class ( the instance variables ) are stored in Heap
• Its not possible to instantiate Class on Stack in Java .
• This is representation of Java code From OS process memory model .
• All Java does is manages a part of the HEAP , you are free to allocate memory(JAVA provides API) outside the Java managed HEAP.
• If one allocates memory outside JVM manged heap , deallocation is the responsibility of the programmer.

Lets take a deep dive in How JVM manages the memory .

Abstract view of JVM memory management.

JVM  specification provides flexibility on what to place where

Heap is divided into :   Young Generation |  OLD(Tenured) Generation |    Meta Generation (perm Gen in JDK 7 )

JDK 8 documentation