Jvm architecture

🧠 JVM Architecture (Java Virtual Machine)¶

Think of the JVM as a tiny, disciplined city inside your computer. Java code walks in as raw text and walks out as executable instructions, guided through different “districts” that each have a job.

🧩 1. Class Loader Subsystem¶

This is the entry gate 🚪

What it does:

Loads .class files into memory
Follows Delegation Model (parent-first approach)

Types of Class Loaders:

Bootstrap ClassLoader → loads core Java classes (java.lang.*)
Extension ClassLoader
Application ClassLoader

Phases:

Loading → bytecode enters JVM
Linking
- Verification ✔
- Preparation ⚙
- Resolution 🔗
- Initialization → static variables assigned

🗂️ 2. Runtime Data Areas (Memory Structure)¶

This is the city’s storage system 🏙️

🔹 Heap (Shared)¶

Stores objects
Managed by Garbage Collector
Example: new User()

🔹 Method Area (Shared)¶

Stores class metadata, static variables, constants

🔹 Stack (Thread-specific)¶

Each thread gets its own stack
Stores:
- Method calls
- Local variables

🔹 PC Register¶

Keeps track of current executing instruction

🔹 Native Method Stack¶

Used for native (C/C++) methods

⚙️ 3. Execution Engine¶

This is the brain + muscles combo 💪🧠

Components:

🟢 Interpreter¶

Reads bytecode line by line
Slower but simple

🔵 JIT Compiler (Just-In-Time)¶

Converts bytecode → native machine code
Improves performance 🚀

🧹 Garbage Collector¶

Automatically removes unused objects
Keeps memory clean

🔌 4. Native Interface (JNI)¶

Bridge between Java world and native world 🌉

Allows Java to call C/C++ code
Useful for system-level operations

📚 5. Native Method Libraries¶

External libraries (like .dll, .so)
Used by JNI

🔄 JVM Flow (End-to-End)¶

.java → (javac) → .class (bytecode)
       ↓
Class Loader
       ↓
Runtime Data Areas
       ↓
Execution Engine
       ↓
Machine Code → Output

🎯 Quick Memory Trick¶

Class Loader → loads
Memory (Heap/Stack) → stores
Execution Engine → runs
JNI → connects