Tuesday, January 22, 2013

Difference between Stack and Heap memory in Java

Difference between stack and heap memory is common programming question asked by beginners learning Java or any other programming language. Stack and heap memory are two terms programmers starts hearing once they started programming but without any clear and definite explanation. Lack of knowledge on what is heap in Java and what is stack memory in Java, results in misconcepts related to stack and heap. To add to this confusion, stack is also a data structure which is used to store elements in LIFO(Last In First out) order and available in Java API as java.util.Stack. In general both stack and heap are part of memory, a program is allocated and used for different purposes. Java program runs inside JVM which is launched as a process by "java" command. Java also uses both stack and heap memory for different needs. In our last article 10 points on Java heap space I have touched base on Java heap space and in this article we will see difference between stack and heap memory in Java.

Difference between Stack vs Heap in Java

Difference between heap and statck in Java - Interview Question
Here are few differences between stack and heap memory in Java:

1) Main difference between heap and stack is that stack memory is used to store local variables and function call, while heap memory is used to store objects in Java. No matter, where object is created in code e.g. as member variable, local variable or class variable,  they are always created inside heap space in Java.

2) Each Thread in Java has there own stack which can be specified using -Xss JVM parameter, similarly you can also specify heap size of Java program using JVM option -Xms and -Xmx where -Xms is starting size of heap and -Xmx is maximum size of java heap. to learn more about JVM options see my post 10 JVM option Java programmer should know.

3) If there is no memory left in stack for storing function call or local variable, JVM will throw java.lang.StackOverFlowError, while if there is no more heap space for creating object, JVM will throw java.lang.OutOfMemoryError: Java Heap Space. Read more about how to deal with java.lang.OutOfMemoryError  in my post 2 ways to solve OutOfMemoryError in Java.

4) If you are using Recursion, on which method calls itself, You can quickly fill up stack memory. Another difference between stack and heap is that size of stack memory is lot lesser than size of  heap memory in Java.

5) Variables stored in stacks are only visible to the owner Thread, while objects created in heap are visible to all thread. In other words stack memory is kind of private memory of Java Threads, while heap memory is shared among all threads.

That's all on difference between Stack and Heap memory in Java. As I said, It’s important to understand what is heap and what is stack in Java and which kind of variables goes where, how you can run out of stack and heap memory in Java etc. Let us know if you are familiar with any other difference between stack and heap memory in java.


Javin @ Garbage collection in Java said...

Good question Keul. I think Garbage collector only collect Heap memory. Since object is only created in heap and stack mostly contains local variable which gets wiped off once they lost scope.

nlotz said...

The stack contains only values for integral types and references to objects, which are always stored in the heap. Since it doesn't contain any objects itself, there would be nothing to garbage-collect.

Erwin Müller said...

For me it would be interesting to know where variables are stored that are references to objects. For example: I create a new Object:
foo = new Object()

The object itself is stored on the heap. But what about the reference to the object?

Like in C, you have the memory of the object and the pointer or reference to that memory:
foo = malloc(sizeof(Struc));

It wouldn't make much sense to store the reference on the heap, if the reference is a local variable or a parameter.

Keul said...

@nlotz i think not only integral type but all predefined data type or we can say primitive types and all method calls are stored in Stack..

Anonymous said...

"No matter, where object is created in code e.g. as member variable, local variable or class variable, they are always created inside heap space in Java."

This is inaccurate. Escape analysis was introduced in 6u14 and enabled by default in 6u23.

"The -XX:+DoEscapeAnalysis option directs HotSpot to look for objects that are created and referenced by a single thread within the scope of a method compilation. Allocation is omitted for such non-escaping objects, and their fields are treated as local variables, often residing in machine registers. Synchronization on non-escaping objects is also elided."


"Escape analysis is supported and enabled by default in Java SE 6u23 and later."


Vivek Hingorani said...

Great Article.. Your blog is full of useful information

Pavan S said...

Very informative and as someone said above it is good refresher.. thanks for sharing information.

pranathichennuru said...

Very informative ....thank u :) I have one doubt regarding stack. How Runtime stacks (stack created by each thread) share same memory?

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