Thread Safety Fundamentals

Shared mutable state, races, and the patterns that fix them.

Advanced11 min read

Read these first

Middleware Chain of Responsibility

Why this matters

Race conditions — detailed pointwise breakdown in the section below.
Immutability — detailed pointwise breakdown in the section below.
Confinement — detailed pointwise breakdown in the section below.
Synchronised blocks — detailed pointwise breakdown in the section below.
Concurrency appears in elevator, booking, cache, and feed problems.

Unsynchronised — update lost

A reads 5

B reads 5Before A writes

A writes 6

B writes 6Expected 7

Synchronised — correct

A acquires the lock

A reads 5, writes 6

A releases, B acquires

B reads 6, writes 7

count++ is read, add, write — three steps. Two threads can read the same value before either writes, so one increment disappears. A lock makes the three steps atomic.

Overview

Shared mutable state, races, and the patterns that fix them. Study each section below in order — every list is interview-ready reference material.

Race conditions

Points to cover

Definition — what Race conditions means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Immutability

Points to cover

Definition — what Immutability means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Confinement

Points to cover

Definition — what Confinement means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Synchronised blocks

Points to cover

Definition — what Synchronised blocks means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Java

// CampusOS — apply "Race conditions" on paper first, then in code
public final class ThreadSafetyFundamentalsExample {
    // 1. Name entities from the problem statement
    // 2. Draw relationships before writing methods
    // 3. Introduce patterns only when a force appears
}

Quick recall

Everything you need if you only revisit this box.

Race conditions — review this slice before mock interviews.
Immutability — review this slice before mock interviews.
Confinement — review this slice before mock interviews.
Synchronised blocks — review this slice before mock interviews.

Test yourself

Answer these before moving on — recall is what makes it stick.