Concurrency & Executors

Introduction

Beyond raw threads, Java provides a rich concurrency toolkit: Executors, CompletableFuture, locks, concurrent collections and (Java 21+) structured concurrency.

Informative example

CompletableFuture — async pipelines without callback hell:

import java.util.concurrent.*;

CompletableFuture<String> user  = CompletableFuture.supplyAsync(() -> fetchUser(42));
CompletableFuture<String> perms = CompletableFuture.supplyAsync(() -> fetchPerms(42));

user.thenCombine(perms, (u, p) -> u + " has " + p)
    .thenAccept(System.out::println)
    .exceptionally(ex -> { System.err.println(ex); return null; });

Real-world use

At Netflix, almost every microservice composes 10-30 downstream calls per request. CompletableFuture (or Reactor on top of it) is what stitches them together so the page returns in 200 ms instead of 2 s of serial waits.

Best practices

• Always supply your own Executor to async methods — the default ForkJoinPool is small.
• Use ConcurrentHashMap, never Collections.synchronizedMap.
• Time-box async calls with .orTimeout(2, SECONDS).

Purpose of this lesson

Master Concurrency & Executors so you can apply it confidently in production Java code, technical interviews, and code reviews.

Step-by-step explanation

1. Understand the core idea behind Concurrency & Executors.
2. Walk through the runnable example and tweak it in the playground.
3. Apply the pattern in a small Spring Boot or CLI exercise of your own.
4. Re-read the common mistakes and interview Q&A to lock the concept in.

Interactive workflow diagram

Debugging tips

• RejectedExecutionException → queue is full or executor is shut down.
• Use CompletableFuture + thenApply chains instead of blocking .get().

Optimization strategies

• Profile before optimizing — JFR (Java Flight Recorder) and async-profiler reveal real hotspots.
• Prefer immutable data and stream pipelines over hand-rolled loops when readability matters.
• Reach for the right JDK collection (ArrayList vs LinkedList vs ArrayDeque) before writing custom data structures.

Enterprise example

Teams at Netflix, Uber and Goldman Sachs apply Concurrency & Executors daily — usually wrapped behind Spring Boot services with observability hooks (Micrometer + OpenTelemetry).

Interview questions & answers

Summary

In this lesson you learned Concurrency & Executors — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.