LeetCode【多线程】—— 按序打印

一、题目

我们提供了一个类：

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。

示例 1:

输入: [1,2,3]
输出: “firstsecondthird”
解释:
有三个线程会被异步启动。
输入 [1,2,3] 表示线程 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。
正确的输出是 “firstsecondthird”。

示例 2:

输入: [1,3,2]
输出: “firstsecondthird”
解释:
输入 [1,3,2] 表示线程 A 将会调用 first() 方法，线程 B 将会调用 third() 方法，线程 C 将会调用 second() 方法。
正确的输出是 “firstsecondthird”。

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。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/print-in-order
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

二、解答

1、解法1

package day01;

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

public class Foo {

    private ReentrantLock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();
    private int count = 1;
    public Foo() {}

    public void first(Runnable printFirst) throws InterruptedException {
        lock.lock();
        // printFirst.run() outputs "first". Do not change or remove this line.
        printFirst.run();
        count = 2;
        //其实这里调用signal是有问题
        //假设这里唤醒第三个线程
        //第三个线程会再次调用await,进入等待池等待唤醒
        //第二个线程此时也在等待池中
        //这样就会导致第一个线程和第二个线程都在等待池中
        //condition.signal();
        condition.signalAll();
        lock.unlock();
    }

    public void second(Runnable printSecond) throws InterruptedException {
        lock.lock();
        //为什么这里可以使用if,而下面必须使用while呢
        //如果线程一运行完,线程三拿到了锁,则需要再次判断count值让其进入等待池
        if(count != 2) {
            condition.await();
        }
        // printSecond.run() outputs "second". Do not change or remove this line.
        printSecond.run();
        count = 3;
        //这里可以调用signal方法,因为只剩下一个线程在等待了
        condition.signal();
        lock.unlock();
    }

    public void third(Runnable printThird) throws InterruptedException {
        lock.lock();
        while(count != 3) {
            condition.await();
        }
        // printThird.run() outputs "third". Do not change or remove this line.
        printThird.run();
        lock.unlock();
    }
}

2、解法2

package day01;

import java.util.concurrent.atomic.AtomicInteger;

class Foo2 {

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

    public Foo2() {}

    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();
    }
}

3、解法3

class Foo {

    private AtomicInteger firstJobDone = 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.
        firstJobDone.incrementAndGet();
    }

    public void third(Runnable printThird) throws InterruptedException {
        while (firstJobDone.get() != 2) {
            // waiting for the second job to be done.
        }
        // printThird.run() outputs "third".
        printThird.run();
    }
}