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Introduction to Java | Programming Basics

What is JVM?

The JVM (Java Virtual Machine) is a virtual machine that runs Java bytecode. It is the cornerstone of Java's platform independence, as it allows Java programs to be executed on any system that has a JVM installed, regardless of the underlying hardware and operating system.

How Does JVM Work?

The JVM performs several key functions:

Loading Bytecode: It loads compiled Java bytecode from .class files.
Bytecode Verification: It ensures that the bytecode does not violate Java’s security rules.
Execution: The JVM interprets the bytecode or uses Just-In-Time (JIT) compilation to convert the bytecode into native machine code for execution.
Memory Management: The JVM manages memory allocation for objects and performs garbage collection to free unused objects.

JVM Architecture

The architecture of the JVM can be divided into several parts:

Class Loader Subsystem: Responsible for loading class files.
Runtime Data Areas: Memory areas used by the JVM to store data during program execution, such as method area, heap, stack, and program counter.
Execution Engine: Executes the bytecode through either interpretation or JIT compilation.
Garbage Collector: Automatically manages memory by deleting objects that are no longer in use to reclaim memory.

Components of JVM

Component	Purpose
Class Loader	Loads compiled .class files into memory for execution.
Runtime Data Areas	Memory spaces used by the JVM for executing the program (e.g., method area, heap, stack).
Execution Engine	Executes the Java bytecode. It can either interpret or compile the bytecode using the JIT compiler.
Garbage Collector	Automatically frees memory by cleaning up objects that are no longer needed.

JVM vs. JRE vs. JDK

To clarify, here’s how JVM differs from JRE and JDK:

JVM is responsible for running the bytecode.
JRE includes the JVM and other components to run Java applications (but doesn't have development tools).
JDK includes the JRE and development tools like the Java compiler for creating Java applications.

Code Example: JVM in Action

When you compile a Java program, it gets converted into bytecode (a .class file) that is executed by the JVM. Below is an example of a simple Java program:

Code Example


                public class JVMExample {
                    public static void main(String[] args) {
                        System.out.println("This code is being executed by the JVM!");
                    }
                }

Output

This code is being executed by the JVM!

Pro Tip:

💡 Pro Tip

JVM makes Java platform-independent by enabling Java programs to run on any device with the JVM installed, thus adhering to the "Write Once, Run Anywhere" philosophy.