Metaspace replaced PermGen from Java 8 onwards.

PermGen had fixed memory allocation issues, whereas Metaspace uses native memory dynamically,

reducing OutOfMemory problems related to class metadata storage.

OLD (Before Java 8)

PermGen (Permanent Generation)

Used in:

· Java 7

· Java 6

· older JVMs

It stored:

· class metadata

· method metadata

· static variables

· interned strings (older versions)

Problem:

· fixed size memory

· difficult tuning

· frequent:

java.lang.OutOfMemoryError: PermGen space

NEW (Java 8 onwards)

Metaspace

From:

· Java 8+

· Java 11

· Java 17

· Java 21

Oracle REMOVED PermGen and replaced it with Metaspace.

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Main Difference

PermGen	Metaspace
fixed JVM memory	uses native OS memory
easy OutOfMemory	more scalable
difficult tuning	auto-expandable
part of JVM heap	outside heap
Java 7 and below	Java 8+

Before Java 8, class metadata was stored in PermGen.

From Java 8 onwards, PermGen was removed and replaced by Metaspace, which uses native memory and scales dynamically.

In Java 7 and earlier, JVM used PermGen to store class metadata.

In Java 8, PermGen was removed and replaced by Metaspace.

Unlike PermGen, Metaspace uses native memory outside the heap and can grow dynamically, reducing OutOfMemory issues related to class metadata.





Heap vs Stack

Heap	Stack
Objects	Method calls
Shared	Thread-specific
GC managed	Auto cleanup
Bigger	Smaller

Garbage Collection

Purpose

Automatically removes unreachable objects from heap memory.

GC Types

Collector	Usage
Serial GC	single thread
Parallel GC	throughput
G1GC	modern default
ZGC	low latency
Shenandoah	low pause

Mark and Sweep Concept

1. Mark reachable objects
2. Sweep unreachable objects
3. Compact memory

All objects are stored in Heap

Primitive values are stored in stack

You will see compiler error at c = new Customer(“Susan”);  due to final variable.

But it is valid.

Even it is cleared, still it will return values

Because getCustomers() is creating new copy of the instance.

Record.getCustomers().clear() will throw error. Because we cannot modify