Java Virtual Machine (JVM): Internal Architecture and Key Concepts Explained

By Ashwani Kumar Last updated: 20 Jun 2025

The Java Virtual Machine (JVM) is the engine that enables Java programs to run on any platform. It acts as a runtime environment for Java bytecode, handling memory, execution, and platform independence.

It’s not hardware — it’s a virtual, abstract machine that runs Java applications.

🔍 JVM vs JRE vs JDK

When you install Java, you get:

JDK (Java Development Kit): Tools for compiling and building Java applications (includes JRE).
JRE (Java Runtime Environment): Everything needed to run Java applications (includes JVM).
JVM: The part that actually runs your Java .class files.

JVM is platform-dependent (specific to OS), but Java is platform-independent because the same bytecode runs on any JVM.

🧠 Responsibilities of JVM

JVM handles the complete lifecycle of a Java application:

Loads bytecode from compiled .class files
Verifies the bytecode for safety
Executes the bytecode
Manages memory and runtime resources

🏗️ Internal Architecture of JVM

JVM Architecture

The JVM is composed of several critical components:

1. ClassLoader

Loads .class files at runtime
Divides loading into Bootstrap, Extension, and Application classloaders

2. Method Area

Stores metadata about loaded classes (method info, constants, etc.)

3. Heap

Runtime memory where all objects are stored
Shared across all threads

4. Java Stack

Each thread has its own stack
Holds method frames, local variables, and partial results

5. Program Counter (PC) Register

Stores the address of the current instruction being executed by the thread

6. Native Method Stack

Holds native (non-Java) code, often interfacing with system libraries via JNI

7. Execution Engine

Runs the bytecode using:
- A virtual processor
- An interpreter (line-by-line execution)
- A JIT (Just-In-Time) compiler for high performance (compiles hot code into native machine code)

✅ Quick Q&A

Q1. What is the difference between JDK and JRE?
JDK includes development tools + JRE. JRE only includes what’s needed to run Java apps.

Q2. Can Java source code be written in Notepad?
Yes.

Q3. Does Java create .class files after compilation?
Yes.

Q4. Can one .java file contain multiple classes?
Yes, but only one can be public.

Q5. What is a compilation unit?
It’s a single .java file.

Q6. Is an empty .java file valid?
Yes, it's syntactically valid.

JVM is what makes Java the "Write Once, Run Anywhere" language. Understanding its architecture is key to mastering Java performance and memory management.