Java Runtime Data Areas

Java Virtual Machine divides managed memory into distinct data regions:

// Example of memory allocation
Object obj = new Object();

1. Program Counter Register

• Thread-private
• Points to current executing bytecode instruction

2. Java Virtual Machine Stack

• Method execution memory model
• Contains stack frames with:
  • Local variable array
  • Operand stack
  • Return value
  • Dynamic linking

3. Native Method Stack

• Handles native method calls
• Thread-private

4. Method Area

• Stores class metadata, constants, static variables
• Shared among threads

5. Heap

• Stores object instances
• Shared among threads
• Divided in to:
  • Young Generation (Eden, Survivor spaces)
  • Old Generation
  • Permanent Generation (removed in JDK 8)

Garbage Collection

Reachability Analysis

1. Mark objects reachable from GC Roots
2. Unreachable objects are eligible for collection

GC Algorithms

1. Mark-Sweep
2. Copying
3. Mark-Compact
4. Generational Collection

Class Loading Mechanism

Loading Process

1. Loading
2. Linking (Verification, Preparation, Resolution)
3. Initialization

Parent Delegation Model

• Class loaders delegate upward
• Enhances security

Class Loader Types

1. Bootstrap
2. Extension
3. System/Application
4. Custom

JVM Optimization Techniques

1. Adjust heap sizes (-Xms, -Xmx)
2. Tune survivor ratios (-XX:SurvivorRatio)
3. Use parallel GC (-XX:+UseParallelGC)
4. Consider large memory pages (-XX:+LargePageSizeInBytes)
5. Monitor with tools (jps, jstack, jmap)