Chapter 6: Multithreading & Concurrency

Concurrency enables modern high-throughput Java applications to exploit multi-core CPUs and manage asynchronous tasks. Understanding OS thread lifecycles, memory visibility, synchronization monitors, executor pools, coordinate synchronizers, and modern Java 21 virtual threads is essential for building scalable backend systems.

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🧭 Subtopics & Curriculum

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Foundations & Lifecycle6.1

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6.1 Thread Creation & 6 Lifecycle States

• 1. Process vs Thread
• 2. Ways to Define and Launch Threads
  • 2.1. Approach 1: Extending Thread class
  • 2.2. Approach 2: Implementing Runnable (Preferred over extending Thread)
  • 2.3 Approach 3: Callable<V> with Future<V> (Returns a value, throws checked exceptions)
• 3. Daemon vs User Threads
• 4. Runnable, Callable, & Future
  • Runnable -
  • Callable -
  • Future -
• 5. The 6 JVM Thread Lifecycle States (Thread.State)

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Locking & Mutual Exclusion6.2

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6.2 Thread Synchronization & Locks

• Synchronization
• Problems that Synchronization solves
  • 1. Race Condition — the biggest problem ✅
  • 2. Data Inconsistency ✅
  • 3. Lost Update ✅
  • 4. Visibility Problem ✅
  • 5. Atomicity Problem ✅
  • 6. Deadlock — Synchronization doesn't solve it ❌
  • 7. Starvation ❌
  • 8. Livelock ❌
• Locks
  • What does a lock actually provide?
• Lock vs synchronized
  • Inter-Thread Signaling: wait(), notify(), and notifyAll()
  • Explicit Locks: java.util.concurrent.locks
  • ReentrantLock Capabilities
  • ReentrantReadWriteLock

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Hardware & Memory6.3

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6.3 Volatile & Atomic Variables

• 1. The volatile Keyword: Visibility & Reordering
  • What volatile Guarantees:
  • What volatile Does NOT Guarantee:
• 2. Atomicity (java.util.concurrent.atomic)
  • How do we solve the atomicity problem?
  • How Compare-And-Swap (CAS) Works
• volatile vs Atomic

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Thread Pooling & Sizing6.4

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6.4 Executors & Thread Pools

• What is Thread Pool?
• What is ExecutorService?
  • Why we need Thread Pool? Why can't we simply do new Thread()?
• Different common Thread Pools :
  • 1. newFixedThreadPool()
  • 2. newCachedThreadPool()
  • 3. newSingleThreadExecutor()
• How is this related to Spring Boot?
• Thread Pool Sizing Formulas
• 4. Graceful Shutdown Protocol

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Coordination Utilities6.5

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6.5 Synchronizers & Concurrent Tools

• 1. CountDownLatch: One-Time Countdown Gate
• 2. CyclicBarrier: Multi-Thread Meeting Point
• 3. Semaphore: Resource Rate Limiting & Permits
• 4. BlockingQueue: Producer-Consumer Engine

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Next-Gen Concurrency6.6

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6.6 Virtual Threads & Structured Concurrency

• 1. Platform (OS) Threads vs Virtual Threads
• 2. Launching Virtual Threads
• 3. The Thread Pinning Caveat

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Concurrent Collections6.7

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6.7 Thread-Safe Collections

• What are thread-safe collection?
• Thread-Safe collection and Corresponding normal collection
• CopyOnWriteArrayList ("ArrayList")
  • When to use CopyOnWriteArrayList?
• ConcurrentHashMap ("Hashmap")
• CopyOnWriteArraySet ("HashSet")
• ConcurrentSkipListMap ("TreeMap")
• ConcurrentSkipListSet ("TreeSet")
• ConcurrentLinkedQueue ("LinkedList")
• ConcurrentLinkedDeque ("ArrayDeque")
• What about Vector?
  • Problem with Vector
• Collections.synchronizedList()
  • Problem with Synchronized Collection

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