Java 虚拟机在退出时会等待所有的平台线程线程结束, 但是不会等待虚拟线程们结束.

注意: 创建的平台线程 Thread.daemon 默认是 false, 当其被修改为 true 时也不会被等待.

如下代码

public static void main(String[] args) throws InterruptedException {System.out.println("- Hello World!");// start a platform threadThread platform = Thread.ofPlatform().start(() -> {try {TimeUnit.SECONDS.sleep(8);} catch (InterruptedException ignore) {System.out.println("platform thread Interrupted!");}long id = Thread.currentThread().threadId();System.out.println("+ Hello World from platform thread " + id);});// start a virtual threadThread virtual = Thread.ofVirtual().start(() -> {try {TimeUnit.SECONDS.sleep(10);} catch (InterruptedException ignore) {System.out.println("virtual thread Interrupted!");}long id = Thread.currentThread().threadId();System.out.println("+ Hello World from virtual thread " + id);});// wait for the threads to complete// platform.join();// virtual.join();TimeUnit.SECONDS.sleep(5);// print the thread stateSystem.out.println("- platform thread state is " + platform.getState());System.out.println("- virtual thread state is " + virtual.getState());System.out.println("- Goodbye World!");
}

总是会输出

- Hello World!
- platform thread state is TIMED_WAITING
- virtual thread state is WAITING
- Goodbye World!
+ Hello World from platform thread 21

为什么?

从Java层来看. 因为 Virtual Thead 总是为 daemon thread. 下面是 Thread#isDaemon() 在 JDK21 中的源码

/*** Tests if this thread is a daemon thread.* The daemon status of a virtual thread is always {@code true}.** @return  {@code true} if this thread is a daemon thread;*          {@code false} otherwise.* @see     #setDaemon(boolean)*/
public final boolean isDaemon() {if (isVirtual()) {return true;} else {return holder.daemon;}
}

同时, Virtual Thead 的 daemon 属性不可被修改, 尝试调用一个虚拟线程的Thread#setDaemon(boolean on) 将会抛出 'false' not legal for virtual threads

另外可以从Thead 的 State 中看出, 平台线程调用 sleep 之后 状态为 TIMED_WAITING 而 虚拟线程的状态为 WAITING

更深层的原理这里不再深究, 可以肯定的是和虚拟线程的调度器(们)的策略有关.