Condition 线程通信

• Condition 接口描述了可能会与锁有关联的条件变量。这些变量在用法上与使用 Object.wait 访问的隐式监视器类似，但提供了更强大的功能。需要特别指出的是，单个 Lock 可能与多个 Condition 对象关联。为了避免兼容性问题，Condition 方法的名称与对应的 Object 版本中的不同。
• 在 Condition 对象中，与 wait、notify 和 notifyAll 方法对应的分别是await、signal 和 signalAll。
• Condition 实例实质上被绑定到一个锁上。要为特定 Lock 实例获得Condition 实例，请使用其 newCondition() 方法

//使用区别如下
class Clerk2 {
    private int product = 0;

    private Lock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();

    // 进货
    public void get() {
        lock.lock();

        try {
            while (product >= 1) { // 为了避免虚假唤醒，应该总是使用在循环中。
                System.out.println("产品已满！");

                try {
                    condition.await();
                } catch (InterruptedException e) {
                }

            }
            System.out.println(Thread.currentThread().getName() + " : "
                    + ++product);

            condition.signalAll();
        } finally {
            lock.unlock();
        }

    }

    // 卖货
    public void sale() {
        lock.lock();

        try {
            while (product <= 0) {
                System.out.println("缺货！");

                try {
                    condition.await();
                } catch (InterruptedException e) {
                }
            }

            System.out.println(Thread.currentThread().getName() + " : "
                    + --product);

            condition.signalAll();

        } finally {
            lock.unlock();
        }
    }
}

线程按序交替

编写一个程序，开启 3 个线程，这三个线程的 ID 分别为 A、B、C，每个线程将自己的 ID 在屏幕上打印 10 遍，要求输出的结果必须按顺序显示
如：ABCABCABC…… 依次递归

public class TestABCAlternate {

    public static void main(String[] args) {
        AlternateDemo ad = new AlternateDemo();

        new Thread(new Runnable() {
            @Override
            public void run() {

                for (int i = 1; i <= 20; i++) {
                    ad.loopA(i);
                }

            }
        }, "A").start();

        new Thread(new Runnable() {
            @Override
            public void run() {

                for (int i = 1; i <= 20; i++) {
                    ad.loopB(i);
                }

            }
        }, "B").start();

        new Thread(new Runnable() {
            @Override
            public void run() {

                for (int i = 1; i <= 20; i++) {
                    ad.loopC(i);

                    System.out.println("-----------------------------------");
                }

            }
        }, "C").start();
    }

}

class AlternateDemo {

    private int number = 1; //当前正在执行线程的标记

    private Lock lock = new ReentrantLock();
    private Condition condition1 = lock.newCondition();
    private Condition condition2 = lock.newCondition();
    private Condition condition3 = lock.newCondition();

    /**
     * @param totalLoop : 循环第几轮
     */
    public void loopA(int totalLoop) {
        lock.lock();

        try {
            //1. 判断
            if (number != 1) {
                condition1.await();
            }

            //2. 打印
            for (int i = 1; i <= 1; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + i + "\t" + totalLoop);
            }

            //3. 唤醒
            number = 2;
            condition2.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void loopB(int totalLoop) {
        lock.lock();

        try {
            //1. 判断
            if (number != 2) {
                condition2.await();
            }

            //2. 打印
            for (int i = 1; i <= 1; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + i + "\t" + totalLoop);
            }

            //3. 唤醒
            number = 3;
            condition3.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void loopC(int totalLoop) {
        lock.lock();

        try {
            //1. 判断
            if (number != 3) {
                condition3.await();
            }

            //2. 打印
            for (int i = 1; i <= 1; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + i + "\t" + totalLoop);
            }

            //3. 唤醒
            number = 1;
            condition1.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

}