Difference Between Stack and Heap in Java

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

 

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.

 

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.

 

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.