深度揭秘：JDK 21 虚拟线程原理与性能调优实战

虚拟线程 (数量巨大) 
    ↓ (由JVM调度)
载体线程 (Carrier Thread, 数量=CPU核心数)
    ↓ (1:1)
OS内核线程

java

// jdk/internal/vm/Continuation.java
private class Continuation {
    // 栈帧被冻结为堆上的对象数组
    private Object[] stackFrames;
    private int framePointer;
}

java

// 源码：java.lang.VirtualThread
private static final ForkJoinPool DEFAULT_SCHEDULER = 
    createDefaultScheduler();

-Djdk.virtualThreadScheduler.parallelism=8
-Djdk.virtualThreadScheduler.maxPoolSize=16

java

// 正确创建方式
Thread vthread = Thread.startVirtualThread(() -> {
    System.out.println("虚拟线程运行");
});

// 使用工厂
ThreadFactory factory = Thread.ofVirtual()
    .name("worker-", 0)
    .factory();

// 千万注意：不要使用线程池包装虚拟线程！
// Executors.newVirtualThreadPerTaskExecutor() 每次任务都新建虚拟线程
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    for (int i = 0; i < 100_000; i++) {
        executor.submit(() -> {
            Thread.sleep(1000); // 模拟IO
            return;
        });
    }
} // 自动等待所有虚拟线程完成

java

// 压测代码核心
HttpClient client = HttpClient.newHttpClient();
List<CompletableFuture<Void>> futures = new ArrayList<>();
for (int i = 0; i < 20000; i++) {
    var req = HttpRequest.newBuilder(URI.create("http://localhost:8080/delay?ms=100"))
        .GET().build();
    var future = client.sendAsync(req, BodyHandlers.ofString())
        .thenAccept(resp -> {});
    futures.add(future);
}

bash

# 启动参数示例
java -XX:+UseZGC \
     -Djdk.virtualThreadScheduler.parallelism=16 \
     -Djdk.tracePinnedThreads=short \
     -Djdk.defaultScheduler.parallelism=16 \
     -jar myapp.jar

java

// 错误示例：synchronized会固定虚拟线程到载体线程
synchronized(lock) {
    Thread.sleep(1000); // 此时载体线程被阻塞，无法调度其他虚拟线程
}

// 正确：改用ReentrantLock
lock.lock();
try {
    Thread.sleep(1000);
} finally {
    lock.unlock();
}

java

try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    Future<String> user = scope.fork(() -> fetchUser());
    Future<Integer> order = scope.fork(() -> fetchOrder());
    
    scope.join();           // 等待所有fork任务
    scope.throwIfFailed();  // 任一失败则传播异常
    
    return new Response(user.resultNow(), order.resultNow());
} //

...CompletableFuture@thenApply...
...AbstractExecutorService@submit...
... (丢失业务上下文)

java.base/java.lang.VirtualThread.run
    com.example.Service.handleRequest (Service.java:42)
    com.example.Service.fetchUser (Service.java:58)
    java.net.SocketInputStream.read (native)