Virtual Threads (Java 21) trong Spring Boot 3.2 hoạt động thế nào? Khác gì WebFlux?

Virtual Threads (Project Loom, Java 21) là thread cực nhẹ do JVM quản lý — không phải OS thread.

Bật trong Spring Boot 3.2:

# application.yml — một dòng là xong
spring:
  threads:
    virtual:
      enabled: true

Spring Boot tự dùng Virtual Thread cho Tomcat, @Async, @Scheduled.

Tại sao Virtual Threads tốt:
- OS thread: tốn ~1MB stack, context switch nặng → giới hạn ~few thousands.
- Virtual Thread: tốn ~KB, triệu cái đồng thời → idle khi block I/O (DB call, HTTP).

// Code BLOCKING thông thường — giờ scale tốt hơn với VT
@RestController
class UserController {
  @GetMapping("/users/{id}")
  User get(@PathVariable Long id) {
    return userRepo.findById(id).orElseThrow();  // blocking DB call
    // Virtual Thread block → unmount khỏi carrier thread
    // carrier thread phục vụ request khác trong lúc chờ DB
  }
}

So sánh với WebFlux (Reactive):

Lưu ý quan trọng:
- MDC context bị leak với Virtual Threads (thread-local reuse) — dùng Micrometer Observation API thay raw MDC.
- synchronized block pin Virtual Thread vào OS thread → triệt tiêu lợi ích, dùng ReentrantLock thay.
- Không cải thiện CPU-bound task — chỉ hiệu quả với I/O-bound.

Virtual Threads (Project Loom, Java 21) are ultra-lightweight threads managed by the JVM — not OS threads.

Enable in Spring Boot 3.2:

# application.yml — one line is all it takes
spring:
  threads:
    virtual:
      enabled: true

Spring Boot automatically uses Virtual Threads for Tomcat, @Async, and @Scheduled.

Why Virtual Threads are beneficial:
- OS threads: ~1MB stack each, heavy context switching → limited to a few thousand.
- Virtual Threads: ~KB each, millions can coexist → unmount from the carrier thread when blocking on I/O (DB calls, HTTP).

// Normal BLOCKING code — now scales much better with VT
@RestController
class UserController {
  @GetMapping("/users/{id}")
  User get(@PathVariable Long id) {
    return userRepo.findById(id).orElseThrow();  // blocking DB call
    // Virtual Thread blocks → unmounts from carrier thread
    // carrier thread serves another request while waiting for DB
  }
}

Comparison with WebFlux (Reactive):

Important caveats:
- MDC context leaks with Virtual Threads (thread-local reuse) — use the Micrometer Observation API instead of raw MDC.
- synchronized blocks pin the Virtual Thread to an OS thread → eliminates the benefit. Use ReentrantLock instead.
- No improvement for CPU-bound tasks — only effective for I/O-bound workloads.