经典问题单车道桥梁问题的无饥饿解决方案

Question

我正在尝试为经典的单车道桥梁问题提供解决方案，在该问题中，单车道桥梁连接了两个村庄。它仅使用一个线程，因此如果农民同时跳到桥的任一侧，则可能会陷入僵局。到目前为止，这是我的解决方案，但是我不确定如何使它免于饥饿？

public class SingleLaneBridge {

  public static void main(String[] args)
  {
    final Bridge bridge = new Bridge();

    Thread thNorthbound = new Thread( new Runnable() {

      @Override
      public void run() {

        while(true) {
          Farmer farmer = new Farmer(bridge);
          Thread th = new Thread(farmer);
          farmer.setName("North Farmer : "+ th.getId());
          th.start();
          try {
            TimeUnit.SECONDS.sleep((long)(Math.random()*10));
          } catch(InterruptedException iex) {
            iex.printStackTrace();
          }
        }

      }
    });

    Thread thSouthbound = new Thread( new Runnable() {

      @Override
      public void run() {

        while(true) {
          Farmer farmer = new Farmer(bridge);
          Thread th = new Thread(farmer);
          farmer.setName("South Farmer : "+th.getId());
          th.start();
          try {
            TimeUnit.SECONDS.sleep((long)(Math.random()*10));
          }
          catch(InterruptedException iex)
          {
            iex.printStackTrace();
          }
        }
      }
    });

    thNorthbound.start();
    thSouthbound.start();
  }

}

class Bridge {
  private final Semaphore semaphore;

  public Bridge() {
    semaphore = new Semaphore(1);
  }
  public void crossBridge(Farmer farmer) {
    try {
      System.out.printf("Farmer trying to cross the bridge.\n",farmer.getName());
      semaphore.acquire();
      System.out.printf("Farmer crossing the bridge.\n",farmer.getName());
      long duration = (long)(Math.random() * 10);
      TimeUnit.SECONDS.sleep(duration);
    }
    catch(InterruptedException iex) {
      iex.printStackTrace();
    }
    finally {
      System.out.printf("Farmer as crossed the bridge.\n",farmer.getName());
      semaphore.release();
    }
  }
}

class Farmer implements Runnable
{
  private String name;
  private Bridge bridge;

  public Farmer(Bridge bridge)
  {
    this.bridge = bridge;
  }

  public void run() {
    bridge.crossBridge(this);
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }
}

Answer 1

>它仅使用一个线程[…]

我不确定您的意思；实际上，它为每个农民启动了一个单独的线程。