Java Memory Management: Heap, Stack, and Garbage Collection

Java applications rely heavily on efficient memory management to ensure performance and stability. Understanding how memory is allocated and managed is crucial for developers working on large-scale applications like AEM.

JVM Memory Parameters

When starting an AEM instance, you often see commands like:

java -XX:MaxPermSize=256m -Xmx1024M -jar cq-quickstart-6.3.0.jar

Key Parameters

-XX:MaxPermSize=256m

Sets the maximum size for the Permanent Generation (PermGen) heap.

Holds bytecode and metadata of classes.
Separate from the object heap containing actual instances.
Note: PermGen was removed in Java 8, replaced by Metaspace.

-Xmx1024M : Sets the maximum Java heap size (where objects live).

-Xms : Sets the Initial heap size.

-Xss : Sets Stack size per thread

Heap vs Stack Memory

Heap Memory

Used by all application threads.
Stores objects and global data.
Divided into:
- Young Generation (short-lived objects).
- Old Generation (long-lived objects).
Accessible globally.
Lives until the application ends.
When full → OutOfMemoryError.

Stack Memory

Used by a single thread.
Stores:
- Local primitive variables
- References to objects in heap
Very small and extremely fast.
Short-lived (exists only during method execution).
Organized in LIFO (Last-In-First-Out) manner.
When full → StackOverflowError.

Garbage Collection (GC)

Java automatically manages memory using Garbage Collection:

Types of GC

Minor GC

Works on Young Generation.
Fast and frequent.
Removes short-lived objects.

Major GC (Full GC)

Works on Old Generation.
Slower and can make the application pause.
Cleans long-lived objects.

Memory Management in Java 8 and Later

PermGen Removed: Replaced by Metaspace, which stores class metadata.
Metaspace grows dynamically, reducing OutOfMemoryError risk.
Tune Metaspace using: -XX:MetaspaceSize (initial size), -XX:MaxMetaspaceSize (maximum size).