Lc 1114.按序打印

我们提供了一个类：

public class Foo {
public void one() { print(“one”); }
public void two() { print(“two”); }
public void three() { print(“three”); }
}
三个不同的线程将会共用一个 Foo 实例。

线程 A 将会调用 one() 方法
线程 B 将会调用 two() 方法
线程 C 将会调用 three() 方法
请设计修改程序，以确保 two() 方法在 one() 方法之后被执行，three() 方法在 two() 方法之后被执行。

class Foo {

    boolean firstFinished = false;
    boolean secondFinished = false;
    final Object obj = new Object();

    public Foo() {

    }

    public void first(Runnable printFirst) throws InterruptedException {
        synchronized (obj) {
            // printFirst.run() outputs "first". Do not change or remove this line.
            printFirst.run();
            firstFinished = true;
        }
    }

    public void second(Runnable printSecond) throws InterruptedException {
        synchronized (obj) {
            while (firstFinished) {
                // printSecond.run() outputs "second". Do not change or remove this line.
                printSecond.run();
                secondFinished = true;
            }

        }
    }

    public void third(Runnable printThird) throws InterruptedException {
        synchronized (obj) {
            while (secondFinished) {
                // printThird.run() outputs "third". Do not change or remove this line.
                printThird.run();
            }
        }
    }
}

提交结果：超时

原因：while循环，不停抢占CPU资源来判断自身是否结束循环（自旋）。

优化

使用wait+notify避免了自旋

class Foo {

    final Object obj = new Object();
    boolean firstFinished = false;
    boolean secondFinished = false;

    public Foo() {

    }

    public void first(Runnable printFirst) throws InterruptedException {
        synchronized (obj) {
            // printFirst.run() outputs "first". Do not change or remove this line.
            printFirst.run();
            firstFinished = true;
            obj.notifyAll();
        }
    }

    public void second(Runnable printSecond) throws InterruptedException {
        synchronized (obj) {
            while (!firstFinished) {
                obj.wait();
            }
            // printSecond.run() outputs "second". Do not change or remove this line.
            printSecond.run();
            secondFinished = true;
            obj.notifyAll();
        }
    }

    public void third(Runnable printThird) throws InterruptedException {
        synchronized (obj) {
            while (!secondFinished) {
                obj.wait();
            }
            // printThird.run() outputs "third". Do not change or remove this line.
            printThird.run();
        }
    }
}

优化：使用CountDownLatch（Semaphore同理）

class Foo {

    CountDownLatch c2;
    CountDownLatch c3;

    public Foo() {
        c2 = new CountDownLatch(1);
        c3 = new CountDownLatch(1);
    }

    public void first(Runnable printFirst) throws InterruptedException {
        // printFirst.run() outputs "first". Do not change or remove this line.
        printFirst.run();
        c2.countDown();
    }

    public void second(Runnable printSecond) throws InterruptedException {
        c2.await();
        // printSecond.run() outputs "second". Do not change or remove this line.
        printSecond.run();
        c3.countDown();
    }

    public void third(Runnable printThird) throws InterruptedException {
        c3.await();
        // printThird.run() outputs "third". Do not change or remove this line.
        printThird.run();
    }
}