Java Memory Management

Java Heap Vs Stack Memory

Java Heap Space:

  • Java Heap space is used by java runtime to allocate memory to Objects and JRE classes.
  • Whenever we create any object, it’s always created in the Heap space.
  • Garbage Collection runs on the heap memory to free the memory used by objects that doesn’t have any reference.
  • Any object created in the heap space has global access and can be referenced from anywhere of the application.

Java Stack Memory:

  • Java Stack memory is used for execution of a thread. They contain method specific values that are short-lived and references to other objects in the heap that are getting referred from the method.
  • Stack memory is always referenced in LIFO (Last-In-First-Out) order.
  • Whenever a method is invoked, a new block is created in the stack memory for the method to hold local primitive values and reference to other objects in the method. As soon as method ends, the block becomes unused and become available for next method.
  • Stack memory size is very less compared to Heap memory.

 

Difference btw Stack & Heap Memory

  • We can use -Xms and -Xmx JVM option to define the startup size and maximum size of heap memory. We can use -Xss to define the stack memory size.
  • When stack memory is full, Java runtime throws java.lang.StackOverFlowError whereas if heap memory is full, it throws java.lang.OutOfMemoryError: Java Heap Space error.
  • Whenever an object is created, it’s always stored in the Heap space and stack memory contains the reference to it. Stack memory only contains local primitive variables and reference variables to objects in heap space.
  • Heap memory is used by all the parts of the application whereas stack memory is used only by one thread of execution.
  • Memory management in stack is done in LIFO manner whereas it’s more complex in Heap memory because it’s used globally. Heap memory is divided into Young-Generation, Old-Generation etc.
  • Stack memory size is very less when compared to Heap memory. Because of simplicity in memory allocation (LIFO), stack memory is very fast when compared to heap memory.

 

Detailed Explanation at this post.

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