Java.Multithreading.What is the difference between a process and a thread?

🧠 What’s the Difference Between a Process and a Thread?

Feature	Process	Thread
Definition	A program in execution (independent)	A lightweight unit of execution within a process
Memory	Has its own memory space	Shares memory with other threads in same process
Isolation	Fully isolated from others	Not isolated — shares heap & static data
Communication	Expensive (IPC: sockets, pipes)	Cheap (shared memory, synchronized access)
Overhead	Heavy	Light
Crash impact	Crash of one process doesn’t affect others	Crash of one thread can affect whole process
Scheduling	Handled by OS	Also scheduled by OS (as lightweight processes)

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🔧 Real World Analogy

• A process is like a house — it has its own walls, kitchen, bathroom.
• A thread is like a person inside the house — multiple people (threads) can do things in parallel but share the same fridge, bathroom, etc.

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📦 Java-Specific View

When you run a Java app:

java MyApp

• JVM creates 1 process: the Java process.
• You can create many threads using:
  • new Thread()
  • ExecutorService
  • ForkJoinPool, etc.

All threads share the same heap (objects, static fields), but have their own call stacks (method frames, local variables).

🧪 Code Example

public class Demo {
    public static void main(String[] args) {
        new Thread(() -> System.out.println("Hello from thread")).start();
        System.out.println("Hello from main thread");
    }
}

Here:

• Both threads share the same process and memory
• They can access the same objects, but must synchronize to avoid conflicts

🔥 Summary

Topic	Thread	Process
Memory Sharing	✅ Shares with others in process	❌ Separate memory
Communication Cost	🟢 Low (shared mem)	🔴 High (IPC)
Overhead	🟢 Low	🔴 High
Failure Isolation	❌ Shared fate	✅ Isolated
Use in Java	Built-in (Thread, Executors)	Managed via ProcessBuilder