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PrintInOrder.md

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Suppose we have a class:

public class Foo {
  public void one() { print("one"); }
  public void two() { print("two"); }
  public void three() { print("three"); }
}

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

Example 1:

Input: [1,2,3]
Output: "onetwothree"
Explanation: There are three threads being fired asynchronously. The input [1,2,3] means thread A calls one(), thread B calls two(), and thread C calls three(). "onetwothree" is the correct output.

Example 2:

Input: [1,3,2]
Output: "onetwothree"
Explanation: The input [1,3,2] means thread A calls one(), thread B calls three(), and thread C calls two(). "onetwothree" is the correct output.

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。

解析

多线程顺按顺序输出1,2,3。

两种方式：

  • 采用信号量，线程1 线程2 执行完分别释放自己的信号量。
  • 采用计数形式，每个线程执行完毕计数器自增一，每个线程必须拿到跟自己线程对应的序号才可以正常执行。

代码

import java.util.concurrent.Semaphore;
class Foo {
  private int flag = 1;
  public Foo() {

  }
  
  private Semaphore semaphore = new Semaphore(0);
  private Semaphore semaphore2 = new Semaphore(0);

  public void first(Runnable printFirst) throws InterruptedException {

//    synchronized (this) {
//      while (flag != 1) {
//        this.wait();
//      }
//
//      printFirst.run();
//      flag = 2;
//      this.notifyAll();
//    }
    // printFirst.run() outputs "first". Do not change or remove this line.
      printFirst.run();
      semaphore.release();
  }

  public void second(Runnable printSecond) throws InterruptedException {

    // printSecond.run() outputs "second". Do not change or remove this line.
//    synchronized (this) {
//      while (flag != 2) {
//        this.wait();
//      }
//
//      printSecond.run();
//      flag = 3;
//      this.notifyAll();
//    }
    semaphore.acquire();
    printSecond.run();
    semaphore2.release();

  }

  public void third(Runnable printThird) throws InterruptedException {

//    synchronized (this) {
//      while (flag != 3) {
//        this.wait();
//      }
//      printThird.run();
//      flag = 1;
//      this.notifyAll();
//    }
    semaphore2.acquire();
    printThird.run();
  }
}