How Java compilation and the JVM work

Java Compilation Pipeline

Java source code does not run directly on your machine. It goes through two stages before execution.

Stage 1 — Compile: javac reads your .java file and produces a .class file containing platform-neutral bytecode. No machine-specific instructions exist yet. The bytecode is an intermediate representation designed for the JVM, not for any real CPU.

Stage 2 — Run: The Java Virtual Machine (JVM) reads that bytecode and translates it to native machine instructions at runtime using a JIT (Just-In-Time) compiler. This is why the same .class file runs on Windows, macOS, and Linux without recompilation — the JVM on each platform handles translation.

The JDK (Java Development Kit) bundles javac, the JVM, and standard libraries — everything a developer needs. The JRE (Java Runtime Environment) contains only the JVM and libraries, enough to run but not compile Java programs. Modern JDK distributions include both.

Your First Compile-and-Run

// File: Hello.java
public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, Java");
    }
}

# Terminal
javac Hello.java   # produces Hello.class
java Hello         # JVM executes bytecode
# Output: Hello, Java

The class name must match the filename exactly, including case. main is the entry point the JVM looks for — its signature public static void main(String[] args) must be exact. Changing any modifier causes a runtime error, not a compile error.