Print in Order

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.

分析

semaphore，只有后两个需要semaphore，初始都设置为1，因为不能运行。

import java.util.concurrent.*;
class Foo {
    Semaphore run2;
    Semaphore run3;

    public Foo() {
        run2 = new Semaphore(0);
        run3 = new Semaphore(0);
        
        
    }

    public void first(Runnable printFirst) throws InterruptedException {
        
        // printFirst.run() outputs "first". Do not change or remove this line.
        printFirst.run();
        run2.release();
    }

    public void second(Runnable printSecond) throws InterruptedException {
        run2.acquire();
        // printSecond.run() outputs "second". Do not change or remove this line.
        printSecond.run();
        run3.release();
    }

    public void third(Runnable printThird) throws InterruptedException {
        run3.acquire();
        // printThird.run() outputs "third". Do not change or remove this line.
        printThird.run();
        
    }
}

用synchronized, volatile 可不用在这里。好像synchronized 和wait, notifyall一起用？

class Foo {
    private volatile boolean v1;
    private volatile boolean v2;

    public Foo() {
        v1 = false;
        v2 = false;
        
    }

    public synchronized void first(Runnable printFirst) throws InterruptedException {
        
        
        // printFirst.run() outputs "first". Do not change or remove this line.
        printFirst.run();
        v1 = true;
        notifyAll();
        
    }

    public synchronized void second(Runnable printSecond) throws InterruptedException {
       while(!v1){
           wait();
       }
        // printSecond.run() outputs "second". Do not change or remove this line.
        printSecond.run();
        v2 = true;
        notifyAll();
    }

    public synchronized void third(Runnable printThird) throws InterruptedException {
        while(!v2){
            wait();
        }
        // printThird.run() outputs "third". Do not change or remove this line.
        printThird.run();
            
        
    }
}

AtomicInteger 3个函数都要判断while

import java.util.concurrent.atomic.AtomicInteger;
class Foo {
    AtomicInteger count = new AtomicInteger();
    
    public Foo() {
        count.set(0);
    }

    public void first(Runnable printFirst) throws InterruptedException {
       while (!count.compareAndSet(0,1)){
            
        }
        // printFirst.run() outputs "first". Do not change or remove this line.
        printFirst.run();
        
    }

    public void second(Runnable printSecond) throws InterruptedException {
         while (!count.compareAndSet(1,2)){
            
        }
        // printSecond.run() outputs "second". Do not change or remove this line.
        printSecond.run();
        
    }

    public void third(Runnable printThird) throws InterruptedException {
         while (!count.compareAndSet(2,3)){
          
        }
        // printThird.run() outputs "third". Do not change or remove this line.
        printThird.run();
    }
}