Java Condition和ReentrantLock

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class Main {
    private static final int COUNT_THRESHOLD = 5;
    private int count = 0;
    private ReentrantLock lock = new ReentrantLock();
    private Condition thresholdReachedCondition = lock.newCondition();

    public void increment() {
        lock.lock();
        try {
            count++;
            System.out.println("Incremented: " + count);
            if (count >= COUNT_THRESHOLD) {
                thresholdReachedCondition.signalAll();
            }
        } finally {
            lock.unlock();
        }
    }

    public void waitForThreshold() throws InterruptedException {
        lock.lock();
        try {
            while (count < COUNT_THRESHOLD) {
                thresholdReachedCondition.await();
            }
            System.out.println("Threshold reached!");
        } finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) {
        Main demo = new Main();

        Thread incrementThread = new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                demo.increment();
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });

        Thread waitThread = new Thread(() -> {
            try {
                demo.waitForThreshold();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });

        incrementThread.start();
        waitThread.start();

        try {
            incrementThread.join();
            waitThread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

在上述示例中，我们创建了一个名为ConditionDemo的类来管理计数和等待阈值的逻辑。increment()方法用于递增计数，并在达到阈值时调用signalAll()来唤醒等待线程。waitForThreshold()方法用于等待阈值达到，并在达到时输出"Threshold reached!"。

在main()方法中，我们创建了一个递增线程和一个等待线程，并通过调用start()方法启动它们。然后使用join()方法等待两个线程完成。

运行示例，你会看到递增线程在每次递增时输出计数值，当计数达到阈值5时，等待线程被唤醒并输出"Threshold reached!"。

这个示例演示了如何使用Condition和ReentrantLock实现线程间的等待和唤醒机制。适当地使用lock()、unlock()、await()和signalAll()方法可以确保线程在正确的时机等待和唤醒，以实现同步和通信。

可见Java中的Condition类似C++中的C++ std::condition_variable，Java中的ReentrantLock类似于C++中的std::mutex。