Java and backend interview questions

Unbounded new Thread() under load will exhaust memory and file descriptors.

Runnable.run returns void and cannot throw checked exceptions.

Collections.synchronizedMap locks the entire map on every call.

It is the Java Memory Model rule that says memory writes by thread A become visible to thread B in a defined way.

PUT of the same body to the same URL should leave the resource in the same state no matter how many times you retry.

The client sends a unique Idempotency-Key

OFFSET skips N rows

Create: 201 with Location, or 200 if the body includes the resource.

Almost all expect versioned JSON APIs, nouns as URLs, and correct verbs and statuses.

The key identifies a client intent.

Sessions need a shared store such as Redis once you have more than one app instance, or sticky load balancing which you should avoid.

localStorage is easy for XSS scripts to steal.

The access token is short-lived and sent on every API call.

XSS injects script into a page the user trusts and can steal tokens or act as the user.

The user is redirected to the identity provider, logs in, and the browser comes back with a short-lived code.

The app reads Redis first

A string GET/SET is simple for a blob you always read whole.

Invalidation messages get lost, deploys race, and some keys are never written again.

In-memory sessions die when the instance dies and they force sticky load balancing.

When a hot key expires, thousands of requests miss together and stampede the database.