信号灯不起作用

Question

我试图创建一个演示信号量用法的小程序。 我创建了2个线程，它们运行Farmer的两个实例：一个以字符串“ north”作为参数，另一个以“ south”。 他们似乎没有同时完成第一个线程和第二个线程，而是看起来都同时完成了（如输出所示：

农夫越过桥，往北走
农夫穿过桥，往南走
农夫已经过桥，现在正向北
农夫已经过桥，现在正向南

谁能告诉我我在做什么错？

import java.util.concurrent.Semaphore;
public class Farmer implements Runnable
{
    private String heading;
    private final Semaphore bridge = new Semaphore(1);
    public Farmer(String heading)
    {
        this.heading = heading;
    }

    public void run() 
    {
        if (heading == "north")
        {
            try 
            {
                //Check if the bridge is empty
                bridge.acquire();
                System.out.println("Farmer going over the bridge, heading north");
                Thread.sleep(1000);
                System.out.println("Farmer has crossed the bridge and is now heading north");
                bridge.release();
            } 
            catch (InterruptedException e) 
            {
                e.printStackTrace();
            }
            //Farmer crossed the bridge and "releases" it

        }
        else if (heading == "south")
        {
            try 
            {
                //Check if the bridge is empty
                bridge.acquire();
                System.out.println("Farmer going over the bridge, heading south");
                Thread.sleep(1000);
                //Farmer crossed the bridge and "releases" it
                System.out.println("Farmer has crossed the bridge and is now heading south");
                bridge.release();
            } 
            catch (InterruptedException e) 
            {
                e.printStackTrace();
            }
        }
    }
}

Answer 1

每个农民都在创建自己的信号量，这意味着它可以独立于任何其他人获取和释放它。

更改此：

private final Semaphore bridge = new Semaphore(1);

对此：

private final static Semaphore bridge = new Semaphore(1);

然后，该信号灯将在所有Farmer实例之间共享。