JVM architecture

Learn the JVM's class loader, runtime and execution engine.

JVM architecture

The JVM (Java Virtual Machine) is an abstract machine that runs compiled Java code. Source code is first compiled to bytecode with javac; the JVM then runs this bytecode by interpreting or compiling it.

The JVM has three main components: the class loader loads classes into memory, the runtime data areas hold data, and the execution engine runs the bytecode. Java 21 LTS uses this structure.

This architecture lets Java code run independently of the operating system. The same bytecode runs the same way on Windows, Linux or macOS.

Compile and run flow

Java's flow has two stages. First javac turns the .java source file into a .class bytecode file. Then the java command runs the bytecode inside the JVM.

javac Main.java   # produces Main.class
java Main          # JVM loads and runs Main.class

Bytecode is OS-independent; it runs on any JVM. This is the basis of the 'write once, run anywhere' principle.

Exercise Print the Java version

Define a Main class. In the main method, print the value of Runtime.version().feature() with System.out.println. This value is the Java LTS major version number (21 for Java 21).

Show example solution

public class Main {
    public static void main(String[] args) {
        System.out.println(Runtime.version().feature());
    }
}

Test yourself

What does the JVM do?
- It compiles source code
- It runs bytecode
- It manages the operating system
- It deletes classes
Show answer

It runs bytecode — The JVM runs compiled bytecode.

Complete the javac input

javac Main.____  # produces Main.class

Show answer

javac Main.java  # produces Main.class

Match the JVM components

Class loader

Loads classes
Runtime data areas

Holds data
Execution engine

Runs bytecode
javac

Compiles source

Next lesson: Class loaders

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