🚀 Java Architecture (Step-by-Step)

1. What is Java?

Java is a high-level, object-oriented, platform-independent programming language.
Its biggest feature = Write Once, Run Anywhere (WORA).

👉Example:
If you write a Java program on Windows, the same program can run on Linux or Mac without changes.

2. Java Program Execution Flow

Java works in these main steps:

Write code in .java file (human-readable).
Example:

class Hello {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

Compile code with javac (Java Compiler).
- Converts .java → .class (bytecode).
- Bytecode is platform-independent.
Run code with java command.
- JVM loads .class file.
- JVM executes bytecode using Execution Engine + JIT.

3. Main Components of Java Architecture

Java Architecture has 3 core parts:

(A) JVM (Java Virtual Machine)

Acts as a virtual computer inside your machine.
Executes bytecode and converts it into machine code.
Provides Memory Management + Security + Garbage Collection.

👉Interview Point:

JVM is platform dependent (Windows JVM ≠ Linux JVM).
But bytecode is the same → Java is platform independent.

(B) JRE (Java Runtime Environment)

Provides the environment to run Java applications.
Includes:
- JVM
- Libraries (API)
- Supporting files

👉Interview Tip:
If you only need to run Java programs (not write/compile), you just need JRE.

(C) JDK (Java Development Kit)

Complete package for developers.
Includes:
- JRE (to run programs)
- Compiler (javac) to compile code
- Debugger + Development tools

👉Interview Tip:

JDK = JRE + Development Tools
Developers install JDK, normal users can use JRE.

4. Detailed JVM Architecture

JVM itself has sub-components:

1. ClassLoader

Loads .class files into memory.
3 types:
- Bootstrap ClassLoader → loads core classes (java.lang.*).
- Extension ClassLoader → loads classes from ext libraries.
- Application ClassLoader → loads user-defined classes.

👉Example:
When you write System.out.println, java.lang.System is loaded by Bootstrap ClassLoader.

2. Memory Areas in JVM

JVM divides memory into parts when running a program:

Method Area: Stores class-level info (fields, methods, constants).
Heap: Stores objects (created with new).
Stack: Stores method calls, local variables.
PC Register: Keeps track of current instruction.
Native Method Stack: Stores non-Java methods (like C/C++).

👉Example Flow:

Student s = new Student();

s (reference) stored in Stack.
new Student() object stored in Heap.

3. Execution Engine

Runs bytecode with two parts:
- Interpreter → executes bytecode line by line (slower).
- JIT Compiler (Just-In-Time) → compiles frequently used code into machine code (faster).

👉Example:
If a method is called 1000 times, JIT compiles it into native code so JVM doesn't re-interpret every time.

4. Garbage Collector (GC)

Frees memory by removing unused objects from Heap.
Runs automatically.
Uses algorithms like Mark and Sweep.

👉Example:

Student s1 = new Student();
s1 = null; // old object becomes unreachable → Garbage Collector cleans it

5. Step-by-Step Flow of Program Execution

Source Code (.java) → written by developer.
Compiler (javac) → converts to Bytecode (.class).
ClassLoader → loads .class file into JVM.
Bytecode Verifier → checks security (no illegal code).
Execution Engine (Interpreter + JIT) → executes code.
Garbage Collector → cleans unused objects.

7. Key Interview Questions

1. Why is Java platform independent?

Because of bytecode + JVM for each OS.

2. Why is JVM platform dependent?

Each OS has its own implementation of JVM.

3. What is the role of JIT?

Improves performance by compiling frequently used bytecode into native machine code.

4. Difference between JVM, JRE, JDK?

JVM → Executes bytecode.
JRE → JVM + Libraries (runtime environment).
JDK → JRE + Development tools (for developers).

5. What happens when you run a Java program?

Code → Compiled into Bytecode → Loaded by ClassLoader → Verified → Executed by Execution Engine.