java - java中join()方法的JVM内部工作

Question

简单地说，Thread 的 join() 方法内部是如何工作的，我知道使用 join() 方法可以将一个线程加入到另一个线程的末尾。

例如，如果我们在主线程中使用thread.join()，那么主线程应该等待，直到Thread线程的run方法完全完成。但是它在内部是如何发生的，这会将主线程的堆栈附加到线程堆栈的末尾吗？我很困惑，因为两个线程不能共享同一个堆栈。如果我错了，请纠正我。

Answer 1

这是 Oracle JDK 的实现(如 jdk1.8.0_05/src.zip 中所示)

/**
 * Waits at most {@code millis} milliseconds for this thread to
 * die. A timeout of {@code 0} means to wait forever.
 *
 * <p> This implementation uses a loop of {@code this.wait} calls
 * conditioned on {@code this.isAlive}. As a thread terminates the
 * {@code this.notifyAll} method is invoked. It is recommended that
 * applications not use {@code wait}, {@code notify}, or
 * {@code notifyAll} on {@code Thread} instances.
 *
 * @param  millis
 *         the time to wait in milliseconds
 *
 * @throws  IllegalArgumentException
 *          if the value of {@code millis} is negative
 *
 * @throws  InterruptedException
 *          if any thread has interrupted the current thread. The
 *          <i>interrupted status</i> of the current thread is
 *          cleared when this exception is thrown.
 */
public final synchronized void join(long millis)
throws InterruptedException {
    long base = System.currentTimeMillis();
    long now = 0;

    if (millis < 0) {
        throw new IllegalArgumentException("timeout value is negative");
    }

    if (millis == 0) {
        while (isAlive()) {
            wait(0);
        }
    } else {
        while (isAlive()) {
            long delay = millis - now;
            if (delay <= 0) {
                break;
            }
            wait(delay);
            now = System.currentTimeMillis() - base;
        }
    }
}

如您所见，没有发生任何奇特的堆栈操作。他们只是使用 wait 和 notification 方法让一个线程等待，直到另一个线程通知它终止。