LeetCode 1114. Print in Order（并发）

题目来源：https://leetcode.com/problems/print-in-order/

问题描述

1114. Print in Order

Easy

Suppose we have a class:

public class Foo {

  public void first() { print("first"); }

  public void second() { print("second"); }

  public void third() { print("third"); }

}

The same instance of Foo will be passed to three different threads. Thread A will call first(), thread B will call second(), and thread C will call third(). Design a mechanism and modify the program to ensure that second() is executed after first(), and third() is executed after second().

 

Example 1:

Input: [1,2,3]

Output: "firstsecondthird"

Explanation: There are three threads being fired asynchronously. The input [1,2,3] means thread A calls first(), thread B calls second(), and thread C calls third(). "firstsecondthird" is the correct output.

Example 2:

Input: [1,3,2]

Output: "firstsecondthird"

Explanation: The input [1,3,2] means thread A calls first(), thread B calls third(), and thread C calls second(). "firstsecondthird" is the correct output.

 

Note:

We do not know how the threads will be scheduled in the operating system, even though the numbers in the input seems to imply the ordering. The input format you see is mainly to ensure our tests' comprehensiveness.

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题意

编写线程间同步的代码，使得三个并行执行的线程按固定顺序输出

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思路

用两个共享变量first_flag和second_flag，分别控制第二个线程和第三个线程的执行，使用wait()和notifyAll()进行线程间通信。注意notify/notifyAll唤醒的线程是从wait之后开始执行而不是从同步块的首部开始执行。

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代码

class Foo {
    private Boolean first_flag = false;
    private Boolean second_flag = false;

    public Foo() {
        
    }

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

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

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