Java Architecture

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Java Architecture defines how the Java programming language works internally — from writing code to running it on any device.
It explains how Java achieves its famous feature:

“Write Once, Run Anywhere” (WORA)

2. Main Components of Java Architecture

Java Architecture mainly consists of three key parts:

A. Java Virtual Machine (JVM)

  • JVM is the engine that runs Java programs.
  • It converts bytecode (intermediate code) into machine code for the current operating system.
  • It makes Java platform-independent.

Functions of JVM:

  • Loads code
  • Verifies code
  • Executes code
  • Manages memory (Garbage Collection)

B. Java Runtime Environment (JRE)

  • JRE = JVM + Libraries + Other components to run applications.
  • It provides all the necessary runtime environments for Java programs.

Think of JRE as:
The setup needed to run a Java program (but not to develop one).

C. Java Development Kit (JDK)

  • JDK = JRE + Development tools.
  • It’s used by developers to write, compile, and run Java programs.

Includes:

  • Compiler (javac)
  • Debugger
  • JavaDoc tool
  • JRE

Think of JDK as:
The complete toolkit for Java developers.

3. Flow of Execution in Java

Here’s how a Java program runs step-by-step:

  1. You write code → MyProgram.java
  2. Compiler (javac) → converts code to Bytecode → MyProgram.class
  3. Classloader → loads .class files into JVM
  4. Bytecode Verifier → checks code safety
  5. JIT Compiler (Just-In-Time) → converts bytecode to native machine code
  6. JVM executes the program

4. Diagram: Java Architecture

          ┌──────────────────────────────┐
          │       Java Application       │
          └──────────────┬───────────────┘
                         │
                (1) Compiled by javac
                         │
          ┌──────────────▼───────────────┐
          │        Bytecode (.class)     │
          └──────────────┬───────────────┘
                         │
                (2) Loaded by Classloader
                         │
          ┌──────────────▼───────────────┐
          │             JVM              │
          │  ┌────────────────────────┐  │
          │  │ Class Loader Subsystem │  │
          │  │ Bytecode Verifier      │  │
          │  │ Interpreter/JIT        │  │
          │  │ Garbage Collector      │  │
          │  └────────────────────────┘  │
          └──────────────┬───────────────┘
                         │
               (3) Machine Code Executed

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