java - 编写一个有 2 个线程的程序，交替打印

Question

我最近在面试中被问到这个问题。

Write a program with two threads (A and B), where A prints 1 , B prints 2 and so on until 50 is reached.

我们该怎么做？

Answer 1

赋值的本质是演示一个线程如何向另一个线程发送信号。最常见的方式是使用阻塞队列，但这里一个信号不携带任何信息，所以一个信号量就足够了。

创建用 2 个信号量参数化的线程类:输入和输出:

class ThreadPrinter implements Runnable {
    int counter;
    Semaphore ins, outs;

    ThreadPrinter(int counter, Semaphore ins, Semaphore outs) {
        this.counter = counter;
        this.ins = ins;
        this.outs = outs;
    }

    @Override
    public void run() {
        for (int i = 0; i < 25; i++) {
            ins.aquire(); // wait for permission to run
            System.out.println("" + counter);
            outs.release();  // allow another thread to run
            counter += 2;
        }
    }

创建 2 个 Semaphore 并将它们传递给 2 个线程:

Semaphore a = new Semaphore(1);  // first thread is allowed to run immediately
Semaphore b = new Semaphore(0); // second thread has to wait
ThreadPrinter tp1 = new ThreadPrinter(1, a, b);
ThreadPrinter tp2 = new ThreadPrinter(2, b, a); 

注意信号量 a 和 b 以不同的顺序传递。