From Power ON → Firmware → OS Kernel → Process Creation → JVM → Bytecode Execution, every layer matters when you’re designing:
I’ve created this visual to help junior developers understand the full journey — from hardware to Java execution.
Break down it into 4 clear stages:
Stage 1: System Boot
CPU → the "thinking" unit
RAM → temporary workspace for running programs
Hard Disk / SSD → permanent storage
When the computer is off:
RAM is empty
CPU has no instructions
OS resides on the disk, not in memory
The computer doesn’t yet know where the OS is.
Step 1 – Power ON: Firmware Starts
Firmware: a small permanent program on the motherboard (examples: BIOS/UEFI)
Why is it needed?
CPU doesn’t know where OS is
CPU doesn’t know how to use RAM or disk
Firmware acts as the CPU’s first teacher
What firmware does:
Takes control of CPU
Performs hardware checks (RAM, CPU, Disk) → POST (Power-On Self Test)
Finds a bootable device → locates the OS
Step 2 – Bootloader Loads
Firmware loads the bootloader from the disk:
Windows → Windows Boot Manager
Linux → GRUB
Bootloader loads the OS kernel into RAM
CPU control is handed to the kernel
Now the Operating System is running and Ready
OS kernel:
Manages CPU scheduling
Controls memory
Loads drivers (Disk, Network, USB)
Provides system calls
Then:
Background services start
GUI / shell runs
User login is allowed
Computer is now ready to use
Stage 2: Java Development
Step 3 – Developer writes code in Java
Saves it as a .java file (plain text, human-readable)
Example:
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello");
}
}
CPU cannot execute this directly—it only understands binary (0s & 1s)
Stage 3: JVM Compilation
Step 4 – Compile using javac
Command:
javac HelloWorld.java
OS receives the command
Searches for javac in the PATH
Loads javac executable from disk
OS creates a new process → Javac process
Javac internally: Reads .java file
Checks syntax & errors
Generates .class file → bytecode
Bytecode is platform-independent and JVM-ready
Stage 4: JVM Execution
Step 5 – Run Java
Command:
java HelloWorld
OS receives the command
Loads java executable from disk
OS creates a new process → JVM process
The JVM is a software-based virtual machine that runs on top of the operating system and provides an isolated runtime environment for executing Java bytecode.
Step 6 – JVM Runtime Memory
JVM allocates memory areas in RAM:
Area Purpose
Heap Objects storage
Stack Method calls & local variables
Metaspace Class metadata
Code Cache JIT compiled code
JVM exists only at runtime; it’s temporary
JVM Internal Components
ClassLoader → loads classes
Bytecode Verifier → ensures safety
Execution Engine → converts bytecode to CPU instructions
Garbage Collector → cleans memory
Application Classes Load
JVM loads .class files from disk
Verifies bytecode
Starts execution → runs main() method
Step 7 – Isolated Execution Environment
Java program cannot access other apps’ memory
Cannot directly manipulate hardware
JVM creates a sandbox
Checks security & permissions
Result: Java programs run safely and securely
Step 8 – JVM as an Abstraction Layer
Java program doesn’t know CPU instructions or OS system calls
JVM converts bytecode into machine-specific instructions
JVM is OS-specific (Windows, Linux, macOS)
Output is consistent across all platforms
This is why Java is “Write Once, Run Anywhere”
JDK vs JRE vs JVM
Terms Meaning
JDK Java Development Kit = JRE + Tools (javac, jar, javadoc)
JRE Java Runtime Environment = JVM + Java libraries
JVM Java Virtual Machine = Bytecode interpreter & execution engine
Next Topic: Class Loader Subsystem: jvm-classloader
Comments
Post a Comment