信号量解决哲学家就餐问题(GUI动态演示)

Posted 2020-08-09 littleschemer

问题描述：餐桌前坐着5位哲学家，两个人中间有一只筷子，桌子中央有面条。哲学家思考问

题，当饿了的时候拿起左右两只筷子吃饭，必须同时拿到两只筷子才能吃饭。当5个哲学家都

拿起自己右手边的筷子，准备拿左手边的筷子时可能产生死锁现象。

为了解决互斥问题，必须将筷子资源作为同步资源。在同一时刻，对于一个筷子对象，只能由

一个哲学家（线程）对其进行访问。由于Java程序语言提供了同步机制，可以轻松解决线程互

斥问题。

算法设计思路：将哲学家、筷子、房间分别定义为三个独立的类。其中，哲学家继承自线程，

拥有think()和eat()两个同步方法。筷子属于共享资源，定义takeup()和putdown()为PV操作。

算法的伪代码如下：

var chopstick:array[0..4] of semaphore(:=1);
   i:integer;
	room:semaphore(:=4);
procedure philosopher(i:integer, chopstick left, chopstick right)
begin 
	repeat
		think();
		room.enter();
		left.takeup();
		right.takeup();
		eat();
		right.putdown();
		left.putdown();
		room.exit();
	forever;
end;
begin(*main)
	parbegin
	philosopher(1).start();
	philosopher(2).start();
	philosopher(3).start();
	philosopher(4).start();
	philosopher(5).start();
	parend
end.

1. 定义筷子类。筷子属于同步资源，被持有者持有，同时被索取者竞争。因此，取筷子和放筷子的操作必须作好同步。

public class Chopstick
{

	private boolean avilable;	//筷子是否可用标记
	private Philosopher taker;	//索取者
	private Philosopher  owner;//占有者
	private Dinner.DinnerTable table;

	public Chopstick(int id, Dinner.DinnerTable table) {
		this.table = table;
		avilable = true;
	}

	//取筷子,相当于P操作
	public synchronized void takeup(Philosopher taker) {

		this.setTaker(taker);
		while (!avilable) {
			try {
				//System.out.println("哲学家等待另一根筷子");
				taker.setFace(2);
				wait();
			}
			catch (InterruptedException e) {
			}
		}

		avilable = false;
		this.setOwner(taker);
		table.repaint();
	}
	
	//放下筷子，相当于V操作
	public synchronized void putdown() {

		avilable = true;
		this.setOwner(null);
		notify();  //唤醒其他等待的线程
		table.repaint();
	}
	
	public int getOwnerId(){
		return this.getOwner().getPersonId();
	}

	public Philosopher getTaker() {
		return taker;
	}

	public void setTaker(Philosopher taker) {
		this.taker = taker;
	}

	public Philosopher getOwner() {
		return owner;
	}

	public void setOwner(Philosopher owner) {
		this.owner = owner;
	}
}

2.定义哲学家类。哲学家参与资源的竞争，申明为一个线程，其run()方法为一个死循环操作，

不断切换思考和吃饭两个动作（思考和吃饭设置一个随机时间）。

import java.util.Random;

import javax.swing.ImageIcon;


public  class Philosopher extends Thread
{
	public static final  byte STATUS_OF_NORMAL  = 0;		//初始表情
	public static final byte STATUS_OF_THINK =1;			//思考表情
	public static final byte STATUS_OF_HUNGRY = 2;		//饥饿表情
	public static final byte STATUS_OF_EAT = 3;			//就餐表情
	
	private Room room;
	private Chopstick left, right;
	private int personId;
	private ImageIcon init, eat, think, hungry;
	private ImageIcon currentFace;
	private Random rand = new Random();		//随机休眠时间
	
	public Philosopher(Room room, int id, Chopstick left, Chopstick right) {

		this.room = room;
		this.personId = id;
		this.left = left;
		this.right = right;
		eat = new ImageIcon("eat.png");
		think = new ImageIcon("think.png");
		hungry = new ImageIcon("hungry.png");
		init = new ImageIcon("init.png");
		currentFace = init;
		setFace(Philosopher.STATUS_OF_NORMAL);
	}

	public void eat() {

		left.takeup(Philosopher.this);
		right.takeup(Philosopher.this);
		setFace(Philosopher.STATUS_OF_EAT);
		//  System.out.println("哲学家" + this.philoId + "正在用餐");
	}

	public void think() {

		left.putdown();
		right.putdown();
		setFace(Philosopher.STATUS_OF_THINK);
		//  System.out.println("哲学家" + this.philoId + "正在思考");
	}

	/**
	 *  设置表情图片
	 */
	public void setFace(int faceId) {
		switch (faceId) {
		case Philosopher.STATUS_OF_NORMAL:
			currentFace = init;
			break;
		case Philosopher.STATUS_OF_THINK:
			currentFace = think;
			break;
		case Philosopher.STATUS_OF_HUNGRY:
			currentFace = hungry;
			break;
		case Philosopher.STATUS_OF_EAT:
			currentFace = eat;
			break;
		}

	}

	public ImageIcon getFace() {
		return currentFace;
	}

	public void run() {

		while (true) {
			think();//哲学家思考
			try {
				sleep((int) ((rand.nextFloat() + 0.5) * 3000));
			}
			catch (InterruptedException e) {
			}
//			room.enter();//进入房间
			eat();//哲学家就餐
			try {
				sleep((int) ((rand.nextFloat() + 0.5) * 4000));
			}
			catch (InterruptedException e) {
			}
//			room.exit();//离开房间
		}
	}

	public int getPersonId() {
		return personId;
	}

	public void setPersonId(int personId) {
		this.personId = personId;
	}
}

3. 用Swing实现图形界面，将界面和业务逻辑代码分开。

import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.BoxLayout;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class Dinner extends JFrame
{
	private static final int personCount = 20;	//哲学家数量
	private static Room room = new Room(personCount - 1);//房间锁变量,用于控制同时就餐的人数dd
	private JButton startBtn = new JButton("开始");
	private JButton stopBtn = new JButton("暂停");
	private JButton continueBtn = new JButton("继续");
	private JButton endBtn = new JButton("结束");
	private boolean started = false;
	private boolean stopped = false;
	final static Chopstick[] chopstick = new Chopstick[personCount];
	final static Philosopher[] philos = new Philosopher[personCount];
	ImageIcon normalFace, eatFace, thinkFace, hungryFace;  //表情图片

	// 因Table(餐桌)类与Dinner类联系较为紧密，故将其作为内部类，方便消息的传递
	class DinnerTable extends JPanel
	{
		private int counts;
		private int r1 = 20;
		private int CHOPS_LENGTH = 35;	//筷子的长度
		private int DISTANCE = 100;		//筷子与人的距离
		private int VERTICAL_DIS = -20;		//表情与名字的距离
		private int r2 = r1 + CHOPS_LENGTH;
		private int r3 = r2 + DISTANCE;
		private int delta;
		private int delta0 = -18;

		public DinnerTable(int counts) {
			setOpaque(false);
			this.setPreferredSize(new Dimension(550, 400));
			this.counts = counts;
			this.delta = 360 / counts;

		}

		public void paintComponent(Graphics page) {
			super.paintComponent(page);

			int x1, y1, x2, y2, x3, y3;
			page.fillOval(200, 150, 150, 150);
			
			//画餐桌上的筷子
			for (int i = 0; i < counts; i++) {
				page.setColor(Color.YELLOW);
				
				//根据坐标系 x = r*cos(delta) , y = r*sin(delta) 计算点
				x1 = 275 + (int) (r1 * Math.cos(((delta * i)+ delta0*(-5)  ) * Math.PI / 180));
				y1 = 225 + (int) (r1 * Math.sin(((delta * i) + delta0*(-5)  ) * Math.PI / 180));
				x2 = 275 + (int) (r2 * Math.cos(((delta * i) + delta0 *(-5) ) * Math.PI / 180));
				y2 = 225 + (int) (r2 * Math.sin(((delta * i) + delta0 *(-5) ) * Math.PI / 180));
				x3 = 275 + (int) (r3 * Math.cos(((delta * i) + delta0 ) * Math.PI / 180));
				y3 = 225 + (int) (r3 * Math.sin(((delta * i) + delta0 ) * Math.PI / 180));
				
				if (chopstick[i].getOwner() != null && philos[i].getPersonId() == chopstick[i].getOwnerId()) {
					//检查哲学家是否拿到左边筷子
					page.drawLine(x3 - 20, y3, x3 - 20, y3 + CHOPS_LENGTH);
				}

				if (chopstick[(i + 1) % personCount].getOwner() != null && philos[i].getPersonId()  == chopstick[(i + 1) % personCount].getOwnerId() ) {
					//检查哲学家是否拿到右边筷子
					page.drawLine(x3 + 45, y3, x3 + 45, y3 + CHOPS_LENGTH);
				}

				if (chopstick[i].getOwner() == null) {
					//筷子在餐桌上
					page.drawLine(x1, y1, x2, y2);
				}

				philos[i].getFace().paintIcon(this, page, x3, y3);   //画表情
				page.setColor(Color.BLUE);
				page.drawString("哲学家" + (i + 1), x3 - 5, y3 + VERTICAL_DIS);//画名字

			}
		}
	}

	 /**
	  *  设置窗口居中
	  */
	private void putUIInCenter(){
		int windowWidth = this.getWidth();                 	 	   //获得窗口宽
		int windowHeight = this.getHeight();                 	  //获得窗口高
		Toolkit kit = Toolkit.getDefaultToolkit();             	 //定义工具包
		Dimension screenSize = kit.getScreenSize();             //获取屏幕的尺寸
		int screenWidth = screenSize.width;                     	//获取屏幕的宽
		int screenHeight = screenSize.height;                  	 //获取屏幕的高
		this.setLocation(screenWidth / 2 - windowWidth / 2, screenHeight / 2 - windowHeight / 2);
	}
	
	public Dinner() {
		setTitle("哲学家就餐");
		setPreferredSize(new Dimension(600, 650));
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		this.pack();
		setVisible(true);
		setResizable(false);

		putUIInCenter();

		normalFace = new ImageIcon("init.png");
		eatFace = new ImageIcon("eat.png");
		thinkFace= new ImageIcon("think.png");
		hungryFace = new ImageIcon("hungry.png");


		JPanel btnPanel = new JPanel();
		btnPanel.setBounds(160, 20, 300, 200);
		btnPanel.setOpaque(false);
		btnPanel.setLayout(new FlowLayout());
		btnPanel.add(startBtn);
		btnPanel.add(stopBtn);
		btnPanel.add(continueBtn);
		btnPanel.add(endBtn);

		//各种按钮增加点击事件
		startBtn.addActionListener(new startListener());
		continueBtn.addActionListener(new continueListener());
		stopBtn.addActionListener(new stopListener());
		endBtn.addActionListener(new endListener());

		JPanel mainPanel = new JPanel();
		mainPanel.setLayout(null);
		mainPanel.setBackground(Color.black);
		mainPanel.add(btnPanel);
		DinnerTable table = new DinnerTable(personCount);
		table.setBounds(20, 50, 500, 420);
		
		for (int i = 0; i < chopstick.length; i++) {
			chopstick[i] = new Chopstick(i, table);
		}
		for (int i = 0; i < philos.length; i++) {
			philos[i] = new Philosopher(room, i,
							chopstick[i], chopstick[(i + 1) % personCount]);
		}

		mainPanel.add(table);

		JPanel descipPanel = new JPanel();
		descipPanel.setBounds(20, 480, 200, 200);
		descipPanel.setOpaque(false);
		descipPanel.setForeground(Color.red);
		descipPanel.setLayout(new BoxLayout(descipPanel, BoxLayout.Y_AXIS));
		JLabel label0 = new JLabel("哲学家就餐问题（信号量解决）");
		label0.setForeground(Color.red);
		JLabel label1 = new JLabel("初始状态的哲学家", normalFace, JLabel.RIGHT);
		label1.setForeground(Color.red);
		JLabel label2 = new JLabel("思考状态的哲学家", thinkFace, JLabel.RIGHT);
		label2.setForeground(Color.red);
		JLabel label3 = new JLabel("饥饿状态的哲学家", hungryFace, JLabel.RIGHT);
		label3.setForeground(Color.red);
		JLabel label4 = new JLabel("正在吃饭的哲学家", eatFace, JLabel.RIGHT);
		label4.setForeground(Color.red);

		descipPanel.add(label0);
		descipPanel.add(label1);
		descipPanel.add(label2);
		descipPanel.add(label3);
		descipPanel.add(label4);

		mainPanel.add(descipPanel);

		getContentPane().add(mainPanel);
	}

	private class startListener implements ActionListener
	{

		public void actionPerformed(ActionEvent evt) {
			if (!started) {
				started=true;
				for (int i = 0; i < personCount; i++) {
					philos[i].start();

				}
			}
		}
	}
	private class stopListener implements ActionListener
	{

		public void actionPerformed(ActionEvent evt) {

			for (int i = 0; i < personCount; i++) {
				philos[i].suspend();
			}
			stopped = true;
		}
	}
	private class continueListener implements ActionListener
	{

		public void actionPerformed(ActionEvent evt) {
			if (stopped) {
				for (int i = 0; i < personCount; i++) {
					philos[i].resume();
				}
				stopped = false;
			}
		}
	}
	private class endListener implements ActionListener
	{

		public void actionPerformed(ActionEvent evt) {
			System.exit(1);
		}
	}

	public static void main(String[] args) {
		new Dinner();
	}
}

4.程序运行结果如下：

5.若修改Dinner类设置的常量 personCount为10，重新运行程序，如下：

6.若继续添加限制，要求在一时刻只能有4个哲学家就餐，则可以再添加另外一个信号量（比如房间资源）。哲学家在竞争筷子之前，必须先进入一个房间才能参与下一步竞争。房间类的设计如下：

/*
 * 限制多个哲学家同时就餐，增设一间房间， 哲学家在准备就餐之前，必须先进入该房间， 若人数超过房间容量，则阻塞该线程
 */

public class Room
{

	private static int capacity;   //房间的容量上限

	public Room(int capacity) {
		this.capacity = capacity;
	}

	public synchronized void enter() {
		while (capacity == 0) {
			try {
				wait();
			}
			catch (InterruptedException e) {
			}
		}
		capacity--;
	}

	public synchronized void exit() {
		capacity++;
		notify();
	}
}

同时，设置房间上限（Dinner类的Room字段的初始化）

private static Room room = new Room(4);//房间锁变量,用于控制同时就餐的人数

修改哲学家线程的run()方法，竞争筷子资源前先进入一个房间，放下筷子的时候需要退出房间，代码如下：

public void run() {
		while (true) {
			think();//哲学家思考
			try {
				sleep((int) ((rand.nextFloat() + 0.5) * 3000));
			}
			catch (InterruptedException e) {
			}
			room.enter();//进入房间
			eat();//哲学家就餐
			try {
				sleep((int) ((rand.nextFloat() + 0.5) * 4000));
			}
			catch (InterruptedException e) {
			}
			room.exit();//离开房间
		}
	}

可以观察到，在同一时刻最多只有4个哲学家在就餐