Introduction

• One Java program = one process
• A process can have many threads (doing different execution**)**
• Threads share:
  • Heap memory
  • Static variables
• Threads have their own:
  • Stack
  • Program counter

Process (JVM)
 ├─ Thread A
 ├─ Thread B
 └─ Thread C

Platform Threads (Traditional)

• Backed by OS threads (1 Java thread = 1 OS Thread)
• Heavy (~1MB stack per thread)
• Limited (~thousands max)
• Context switching is expensive
• Each blocked thread = wasted OS thread

new Thread(() -> {}).start();

Thread lifecycle

• start() → creates new thread
• run() → task logic (DO NOT call directly)
• sleep(ms) → pause