Threads

🔍 What Is It?

Thread Lifecycle:

NEW -> RUNNABLE -> BLOCKED/WAITING/TIMED_WAITING -> TERMINATED

• →NEW — Thread created but not started
• →RUNNABLE — Running or ready to run
• →BLOCKED — Waiting to acquire a monitor lock
• →WAITING — Waiting indefinitely (wait(), join())
• →TIMED_WAITING — Waiting with timeout (sleep(), wait(timeout))
• →TERMINATED — Completed execution

Creating Threads:

1. Extend Thread class (not recommended)
2. Implement Runnable (better — separates task from execution)
3. Use ExecutorService (best — thread pool management)
4. Virtual threads (Java 21 — for I/O-bound tasks)

Thread Properties:

• →name — for debugging (set meaningful names!)
• →daemon — daemon threads stop when all non-daemon threads finish
• →priority — hints to scheduler (1-10, rarely useful)
• →interrupt — mechanism to request thread stop

🎯 Why Do We Need It?

Threads enable concurrency:

• →Server applications — handle thousands of simultaneous HTTP requests
• →Background tasks — send emails, process files without blocking the main flow
• →Parallel computation — use all CPU cores for CPU-intensive work
• →Responsive UIs — keep UI thread free while background work runs
• →Asynchronous I/O — don't block waiting for network/disk

🏗️ What Happens in Real Projects?

In Spring Boot, threads are mostly managed by the framework:

• →Each HTTP request runs in a thread from Tomcat's thread pool (default: 200 threads)
• →@Async methods run in a Spring-managed thread pool
• →@Scheduled tasks run in a scheduled thread pool
• →CompletableFuture.supplyAsync() runs in ForkJoinPool

You rarely create threads directly — you use ExecutorService or Spring's abstractions.

⚙️ How It Works

Thread Scheduling:

The JVM delegates thread scheduling to the OS. The OS scheduler decides which thread runs on which CPU core at any time. You cannot control exact scheduling — only set priorities as hints.

Thread Safety Problems:

1. Race condition — two threads read-modify-write the same variable concurrently; result depends on timing
2. Deadlock — Thread A holds lock1, waits for lock2; Thread B holds lock2, waits for lock1; both wait forever
3. Livelock — threads keep responding to each other but make no progress
4. Starvation — a thread never gets CPU time because others always have priority

Memory Visibility:

Without synchronization, changes made by one thread may not be visible to another thread due to CPU caches. Use volatile, synchronized, or java.util.concurrent for visibility guarantees.

💻 Example

Thread Patterns in Enterprise Java — How threads are used and managed in production Java applications.

// Option 1: Extend Thread (avoid — couples task to execution)
class MyThread extends Thread {
    @Override
    public void run() {
        System.out.println("Running in: " + Thread.currentThread().getName());
    }
}
new MyThread().start();

// Option 2: Implement Runnable (better — separates task from thread)
Runnable task = () -> System.out.println("Running in: " + 
    Thread.currentThread().getName());
Thread thread = new Thread(task, "my-worker");
thread.start();

// Option 3: ExecutorService (best for most cases)
ExecutorService executor = Executors.newFixedThreadPool(4);
executor.submit(() -> processOrder(orderId));
executor.submit(() -> sendEmail(userId));

// Always shutdown executors
executor.shutdown();
try {
    if (!executor.awaitTermination(60, TimeUnit.SECONDS)) {
        executor.shutdownNow();
    }
} catch (InterruptedException e) {
    executor.shutdownNow();
    Thread.currentThread().interrupt();
}

// Virtual threads (Java 21) — for I/O-bound tasks
// Create millions of lightweight threads
Thread.ofVirtual().start(() -> handleRequest(request));

// Virtual thread executor
try (ExecutorService vExecutor = Executors.newVirtualThreadPerTaskExecutor()) {
    for (int i = 0; i < 100_000; i++) {
        vExecutor.submit(() -> callExternalApi());
    }
}  // auto-shutdown via try-with-resources

🤔 Common Confusions

🚨 Common Production Issues

✅ Best Practices

🏛️ Architect's Note

💬 Frequently Asked Questions

🎯 Key Takeaways

🔗 Related Topics

🎯 Practice this topic in interviews

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