Java Virtual Machine (JVM)

Jvm

The JVM (Java Virtual Machine) is a virtual computer designed to run Java applications.

• It reads and interprets .class files (which are Java bytecode) and executes them.
• It is the core component that allows Java to run independently of the operating system.

How Java works

1. Write java source code (.java)
2. The java compiler (javac) compiles the source code into bytecode (.class)
3. The JVM specific to your OS executes the bytecode
   • As long as the JVM for that platform knows how to interpret the bytecode, your program will run the same everywhere

How it works

• Regular Programs
  • Programs request resources like CPU, memory, and input/output devices from the operating system
  • But since each operating system handles these requests differently, most programming languages are dependent on the OS
• How Java Works
  • Instead of interacting with the operating system directly, Java uses the JVM to communicate indirectly with the OS
  • Java Program → JVM → Operating System
  • Thanks to this, Java code can run on any OS without modification
• JVM has separate versions for different OS
  • Windows JVM, macOS JVM, Linux JVM, etc
• In the past
  • Ppl actually tuned the JVM (wtf?)

Architecture

• https://www.notion.so/JVM-1fb096ba270c8191a256cdd0973533a7?pvs=4
  • actually just read this
• heap memory
  • all the objects are created here
    • new keyword objects, like new Person() is in here
    • the object’s address will be stored in the stack
• stack
  • Stores method call frames (also called stack frames)
  • Each method call creates a new frame on the stack ⇒ every method has its own stack
  • Stores:
    • Local variables (like method parameters)
    • References (addresses) to objects in the heap
• static methods are not stored in either heap or stack

public class Main {
    public static void main(String[] args) {
        Person p = new Person(); // 👈 p is in the stack, new Person() is in the heap
    }
}