leetcode.1114-按序打印-多线程案例

题目

1114. 按序打印 - 力扣（Leetcode）

给你一个类：

public class Foo {
  public void first() { print("first"); }
  public void second() { print("second"); }
  public void third() { print("third"); }
}

三个不同的线程 A、B、C 将会共用一个 Foo 实例。

• 线程 A 将会调用 first() 方法
• 线程 B 将会调用 second() 方法
• 线程 C 将会调用 third() 方法

请设计修改程序，以确保 second() 方法在 first() 方法之后被执行，third() 方法在 second() 方法之后被执行。

提示：

• 尽管输入中的数字似乎暗示了顺序，但是我们并不保证线程在操作系统中的调度顺序。
• 你看到的输入格式主要是为了确保测试的全面性。

解法一 

先声明两个AtomicInteger变量并初始化为0，使用他来控制first()和second(),second() 和third()之间的顺序关系。

在进入second和third方法时，提前进入循环去检验变量是否被增加为1，如果是，则进入方法，否则空转。

class Foo {

  private AtomicInteger firstJobDone = new AtomicInteger(0);
  private AtomicInteger secondJobDone = new AtomicInteger(0);

  public Foo() {}

  public void first(Runnable printFirst) throws InterruptedException {
    // printFirst.run() outputs "first".
    printFirst.run();
    // mark the first job as done, by increasing its count.
    firstJobDone.incrementAndGet();
  }

  public void second(Runnable printSecond) throws InterruptedException {
    while (firstJobDone.get() != 1) {
      // waiting for the first job to be done.
    }
    // printSecond.run() outputs "second".
    printSecond.run();
    // mark the second as done, by increasing its count.
    secondJobDone.incrementAndGet();
  }

  public void third(Runnable printThird) throws InterruptedException {
    while (secondJobDone.get() != 1) {
      // waiting for the second job to be done.
    }
    // printThird.run() outputs "third".
    printThird.run();
  }
}

方法二

CountDownLatch(int count); //构造方法，创建一个值为count 的计数器。
​
//阻塞当前线程，将当前线程加入阻塞队列。
//当计数器的值变为0时，在CountDownLatch上await()的线程就会被唤醒
await();
​
await(long timeout, TimeUnit unit);//在timeout的时间之内阻塞当前线程,时间一过则当前线程可以执行，
​
countDown();//对计数器进行递减1操作，当计数器递减至0时，当前线程会去唤醒阻塞队列里的所有线程。

在构造函数中使用 CountDownLatch（）来说设置两个值为1的计数器，然后在second和third函数中加入awati（）函数进行等待,当计数器为0时被唤醒

代码：

 class Foo {

    private final CountDownLatch firstDone;
    private final CountDownLatch secondDone;

    public Foo() {
        firstDone = new CountDownLatch(1);
        secondDone = new CountDownLatch(1);
    }

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

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

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

方法三

使用信号量Semaphore ，先初始化信号量，不允许second和third函数运行，在first函数末尾调用release函数使得计数值加一，second函数可以运行；同理，在second函数结束后调用release函数增肌计数值，使得third()函数可以运行。

class Foo {

    private Semaphore two = new Semaphore(0);
    private Semaphore three = new Semaphore(0);

    public Foo() {

    }

    public void first(Runnable printFirst) throws InterruptedException {

        // printFirst.run() outputs "first". Do not change or remove this line.
        printFirst.run();
        two.release();
    }

    public void second(Runnable printSecond) throws InterruptedException {
        two.acquire();
        // printSecond.run() outputs "second". Do not change or remove this line.
        printSecond.run();
        three.release();
    }

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

方法四

使用object和synchronized进行原子操作

public class Foo {
    private  int flag = 1;
    private final Object object = new Object();

    public Foo() {

    }

    public void first(Runnable printFirst) throws InterruptedException {
        synchronized (object) {
            while (flag != 1) object.wait();
            printFirst.run();
            flag = 2;
            object.notifyAll();
        }
    }

    public void second(Runnable printSecond) throws InterruptedException {
        synchronized (object) {
            while (flag != 2) object.wait();
            printSecond.run();
            flag = 3;
            object.notifyAll();
        }
    }

    public void third(Runnable printThird) throws InterruptedException {
        synchronized (object) {
            while (flag != 3) object.wait();
            printThird.run();
        }
        
    }
}