Java Compilation & Execution Flow

This document explains how Java source code is compiled and executed, from writing a .java file to running it inside the JVM.
This understanding is critical for Java fundamentals, JVM internals, and interviews.

1. High-Level Overview

Java execution happens in two major phases:

1. Compilation phase – Source code → Bytecode
2. Execution phase – Bytecode → Native machine instructions

Java follows the principle:

Write Once, Run Anywhere (WORA)

This is possible because Java compiles code into platform-independent bytecode, not native machine code.

2. Compilation Phase (javac)

Step 1: Write Source Code

Employee.java

This file contains human-readable Java code.

Step 2: Compile Using javac

javac Employee.java

What happens internally:

• Syntax checking
• Type checking
• Bytecode generation

Output:

Employee.class

📌 The .class file contains bytecode, not machine code.

3. Execution Phase (java)

Step 3: Run Using java

java Employee

This starts the Java Virtual Machine (JVM).

4. Real Execution Command (IDE Example)

When running from IntelliJ, the command looks like:

"C:\Program Files\Java\jdk-21\bin\java.exe"
-javaagent:idea_rt.jar=49735
-Dfile.encoding=UTF-8
-classpath out\production\javafoundry
com.ajay.javafoundry.foundation.intro.Employee

This command tells the JVM:

• Which Java runtime to use
• Which classpath contains compiled classes
• Which class contains the main() method

5. Key Components Involved

5.1 java.exe

• Native launcher that starts the JVM
• Allocates memory (Heap, Stack, Metaspace)
• Initializes classloaders

5.2 Classpath (-classpath or -cp)

Classpath tells JVM where to find compiled .class files.

Example:

out/production/javafoundry

JVM searches classes in this order:

1. Bootstrap ClassLoader
2. Platform ClassLoader
3. Application ClassLoader

5.3 Fully Qualified Class Name

com.ajay.javafoundry.foundation.intro.Employee

This maps to:

com/ajay/javafoundry/foundation/intro/Employee.class

The class must contain:

public static void main(String[] args)

6. JVM Execution Steps (In Order)

1. JVM starts
2. ClassLoader loads the class
3. Bytecode verification
4. Memory allocation
5. Static blocks execute
6. main() method starts
7. Program runs

7. Role of Bytecode

• Bytecode is platform-independent
• JVM converts bytecode into machine code using:
  • Interpreter
  • JIT (Just-In-Time Compiler)

This is why Java is portable across OS and CPU architectures.

8. Why Java Needs Both javac and java

📌 Compilation happens once, execution can happen many times.

9. Common Interview Questions

Q: Is Java interpreted or compiled?

Answer: Java is both compiled and interpreted. It is compiled to bytecode and interpreted/JIT-compiled at runtime.

Q: What happens if main() is missing?

Answer: JVM throws:

Error: Main method not found

Q: Can JVM run without JDK?

Answer: Yes. JVM is part of JRE, JDK is needed only for compilation.

10. Key Takeaways

• Java source code is compiled into bytecode
• JVM executes bytecode, not source code
• Classpath is critical for class loading
• JVM provides portability and runtime optimizations
• Understanding this flow is essential for JVM, LLD, and system design

11. Mapping to JavaFoundry

This topic connects directly to:

• JVM internals
• Class loading
• Memory management
• Debugging and performance tuning

This knowledge will be reused in:

• Multithreading
• Garbage collection
• Backend system design