Java Memory Management Explained: 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).

Important Points

• Objects are always created in Heap; stack only holds references.
• Heap is large but slower, stack is tiny but fast.
• Proper tuning of ‘-Xmx’, ‘-Xms’, and ‘-Xss’ is critical for performance.
• Monitor GC logs to avoid long pauses during Major GC.