JVM Internals & Java Memory Management

Understanding the internal architecture of the Java Virtual Machine (JVM) is crucial for building high-performance, low-latency applications.

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1. JVM Architecture Overview

The JVM consists of three main subsystems:

1. Class Loader Subsystem: Loading, Linking, and Initializing .class files.
2. Runtime Data Areas: Memory allocated to run Java programs.
3. Execution Engine: Bytecode interpreter, JIT (Just-In-Time) compiler, and Garbage Collector.

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2. Runtime Data Areas

┌─────────────────────────────────────────────────────────────┐
│                    Runtime Data Areas                       │
│                                                             │
│  ┌──────────────────────┐        ┌───────────────────────┐  │
│  │   Method / Metaspace │        │       Heap Memory     │  │
│  │   (Class metadata,   │        │   (Young Gen, Old Gen)│  │
│  │    Static variables) │        │                       │  │
│  └──────────────────────┘        └───────────────────────┘  │
│  ────────────────────── Shared Across Threads ─────────────  │
│  ────────────────────── Per-Thread Memory ────────────────  │
│  ┌──────────────┐  ┌──────────────────┐  ┌───────────────┐  │
│  │ JVM Stack    │  │ Program Counter  │  │ Native Method │  │
│  │ (Frames, LV) │  │ (PC Register)    │  │ Stack         │  │
│  └──────────────┘  └──────────────────┘  └───────────────┘  │
└─────────────────────────────────────────────────────────────┘

Stack vs Heap Memory

Feature	Stack Memory	Heap Memory
Scope	Per-thread allocation	Shared across all threads
Contents	Primitive types & Object references	Actual object instances
Lifecycle	Automatic (freed when method exits)	Managed by Garbage Collector
Error Type	java.lang.StackOverflowError	java.lang.OutOfMemoryError: Java heap space

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3. Generational Garbage Collection

Java splits heap memory into generations based on the weak generational hypothesis:

• Young Generation:
  • Eden Space: Where new objects are initially allocated.
  • Survivor Spaces (S0 & S1): Objects that survive minor GC cycles move here.
• Old Generation (Tenured):
  • Objects that survive multiple GC cycles (default threshold: 15) are promoted to Old Gen.

# Useful JVM Flags for Tuning
-Xms4g -Xmx4g            # Initial and Max Heap Size
-XX:+UseG1GC            # Use Garbage-First Collector
-XX:+UseZGC             # Low-latency ZGC (Java 15+)
-XX:+HeapDumpOnOutOfMemoryError