Java Program Structure, JVM Execution Pipeline & Top-Level Main

1Concept

Java programs run on the JVM (Java Virtual Machine). Source code (.java) is compiled by javac into platform-independent bytecode (.class files), which the JVM JIT (HotSpot Just-In-Time) compiler converts into native machine code. In Java 21, main() can be declared with instance main methods or standard public static void main(String[] args).

2Architecture Diagram

[App.java] ──► (javac) ──► [App.class Bytecode] ──► [JVM ClassLoader] ──► [JIT HotSpot] ──► Native CPU Code

3Code Example

public class Application {
    public static void main(String[] args) {
        System.out.println("=== Java 21 LTS Runtime Pipeline ===");
        System.out.println("Java Version: " + System.getProperty("java.version"));
        System.out.println("Available CPU Cores: " + Runtime.getRuntime().availableProcessors());
        System.out.println("Max Heap Memory: " + (Runtime.getRuntime().maxMemory() / (1024 * 1024)) + " MB");
    }
}

4Expected Output

=== Java 21 LTS Runtime Pipeline ===
Java Version: 21.0.2
Available CPU Cores: 8
Max Heap Memory: 4096 MB

5Key Takeaways

• ✓Java source filename must match the public class name (e.g. Application.java).
• ✓Bytecode enables 'Write Once, Run Anywhere' (WORA) across all OS platforms.
• ✓The JVM JIT compiler dynamically optimizes frequently executed bytecode into native assembly.