6.6 Java 21 Virtual Threads & Structured Concurrency (Project Loom)

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Java 21 introduced Virtual Threads as an official LTS feature under Project Loom, fundamentally modernizing high-throughput server architecture.

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1. Platform (OS) Threads vs Virtual Threads

  Traditional Platform Threads (1:1 with OS Kernel Threads):
  JVM Thread 1 ──────► Linux Kernel pthread (1 MB fixed stack memory)
  JVM Thread 2 ──────► Linux Kernel pthread (1 MB fixed stack memory)
  * Limited to ~5,000–10,000 total threads before OS context-switch thrashing

  Java 21 Virtual Threads (M:N User-Space Schedulers):
  [ Millions of Virtual Threads ] (few hundred bytes on JVM Heap)
                │ (Dynamic unmounting during blocking I/O)
                ▼
  [ Carrier OS Threads ] (Equal to number of available CPU cores)
                ▼
  [ CPU Hardware Cores ]

Dimension	Platform Threads (Thread)	Virtual Threads (VirtualThread)
OS Mapping	1:1 with OS kernel pthread	M:N (millions mapped to a few carrier threads)
Memory Footprint	~1 MB fixed OS stack space	~几百 bytes starting memory (dynamically sized on heap)
Creation Cost	Expensive kernel system call	Cheap Java heap object allocation
Blocking I/O	OS thread stays blocked, wasting RAM	Thread unmounts from carrier thread, freeing carrier for others
Best Used For	CPU-bound calculations	I/O-bound microservices (HTTP, DB, Sockets)

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2. Launching Virtual Threads

// Method 1: Dedicated Executor (One virtual thread per task)
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    IntStream.range(0, 10_000).forEach(i -> {
        executor.submit(() -> {
            Thread.sleep(Duration.ofSeconds(1)); // Unmounts carrier thread!
            return i;
        });
    });
} // Blocks until all 10,000 virtual tasks finish

// Method 2: Fluent Thread Builder
Thread vThread = Thread.ofVirtual().name("payment-worker-", 1).start(() -> {
    System.out.println("Executing on carrier: " + Thread.currentThread());
});

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3. The Thread Pinning Caveat

A virtual thread cannot unmount from its carrier thread during blocking I/O if it is pinned:

• Cause 1: Running inside a synchronized block or method.
• Cause 2: Executing native code via JNI.
• Solution: Replace synchronized blocks in I/O-intensive code with java.util.concurrent.locks.ReentrantLock.