哲学家就餐问题

        哲学家就餐问题是指5个哲学家坐成一圈，每两个之间有一把叉子，只有当同时握有两把叉子才能用餐，哲学家除了吃饭还有思考时间，假如5个哲学家同时拿起左手的叉子，这时候又准备拿右手的叉子，那么将陷入死锁状态；假如规定如果左手叉子拿到之后一段时间没拿到右手叉子，那么将放下左手叉子，然后再继续尝试，那么这样就有可能陷入死循环的状态。本文主要通过给定叉子的状态，每次取叉子需要判断左右两边的叉子是否未被使用，如果是的则取叉子，如果任意其中一个被使用了，则等待；同时用完叉子之后，需要将叉子放下；为避免线程安全问题，对取叉子和放叉子加锁。代码如下：

import java.util.*;

public class PhilosopherProblem {
	
	class Philosopher extends Thread{
		private String name;
		private Fork fork;
		public Philosopher(String name,Fork fork){
			super(name);
			this.name=name;
			this.fork=fork;
			
		}
		public void run() {
			while(true)
			{
				fork.take();
				eating();
				fork.put();
				thinking();
			}
			
		}
		public void eating()
		{
			System.out.println("Philosopher"+name+" says:"+"I am eating now!");
			try {
				Thread.sleep(1000);
			}
			catch(InterruptedException e)
			{
				e.printStackTrace();
			}
		}
		public void thinking()
		{
			System.out.println("Philosopher"+name+" says:"+"I am full and want to think for some time!");
			try {
				Thread.sleep(1000);
			}
			catch(InterruptedException e)
			{
				e.printStackTrace();
			}
		}
	}
	class Fork {
		
		private boolean state[];
		private int n;
		public Fork(int n)
		{
			this.state=new boolean[n];
			this.n=n;
			
		}
		
		
		//为了保证线程同步，take和put两个动作都用synchronized修饰，防止相邻两个人同取一个筷子的情况
		public synchronized void take() {
			String name = Thread.currentThread().getName();
	        int i = Integer.parseInt(name);
	        while(state[i]||state[(i+1)%n]){
	            try {
	                wait();//如果左右手有一只正被使用，等待
	            } catch (InterruptedException e) {
	                // TODO Auto-generated catch block
	                e.printStackTrace();
	            }
	        }
	        //当左右两边筷子都未被使用，则拿起两边的筷子
	        System.out.println("Philosopher"+i+"get two forks and is ready to eat!");
	        state[i]= true;
	        state[(i+1)%n]=true;
		}
		public synchronized void put() {
			String name = Thread.currentThread().getName();
	        int i = Integer.parseInt(name);
	        //使用完之后放下筷子
	        System.out.println("Philosopher"+i+"put two forks and is ready to think!");
	        state[i]= false;
	        state[(i+1)%n]=false;
	        notifyAll();
		}
	}


	public static void main(String[] args) {
		// TODO Auto-generated method stub
		System.out.println("Please write the number of philosopher: ");
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		PhilosopherProblem pp=new PhilosopherProblem();
		Fork fork=pp.new Fork(n);
		for(int i=0;i<n;i++)
		{
			pp.new Philosopher(""+i,fork).start();
		}
		
		
		
		

	}

}

运行结果如下：

Please write the number of philosopher: 
5
Philosopher0get two forks and is ready to eat!
Philosopher2get two forks and is ready to eat!
Philosopher2 says:I am eating now!
Philosopher0 says:I am eating now!
Philosopher0put two forks and is ready to think!
Philosopher0 says:I am full and want to think for some time!
Philosopher4get two forks and is ready to eat!
Philosopher4 says:I am eating now!
Philosopher2put two forks and is ready to think!
Philosopher2 says:I am full and want to think for some time!
Philosopher1get two forks and is ready to eat!
Philosopher1 says:I am eating now!
Philosopher1put two forks and is ready to think!
Philosopher1 says:I am full and want to think for some time!
Philosopher2get two forks and is ready to eat!
Philosopher2 says:I am eating now!
Philosopher4put two forks and is ready to think!
Philosopher4 says:I am full and want to think for some time!
Philosopher0get two forks and is ready to eat!
Philosopher0 says:I am eating now!
Philosopher2put two forks and is ready to think!
Philosopher2 says:I am full and want to think for some time!
Philosopher3get two forks and is ready to eat!
Philosopher3 says:I am eating now!
Philosopher0put two forks and is ready to think!
Philosopher0 says:I am full and want to think for some time!
Philosopher1get two forks and is ready to eat!
Philosopher1 says:I am eating now!
Philosopher3put two forks and is ready to think!
Philosopher3 says:I am full and want to think for some time!
Philosopher4get two forks and is ready to eat!
Philosopher4 says:I am eating now!
Philosopher1put two forks and is ready to think!
Philosopher1 says:I am full and want to think for some time!
Philosopher2get two forks and is ready to eat!
Philosopher2 says:I am eating now!
Philosopher2put two forks and is ready to think!
Philosopher2 says:I am full and want to think for some time!
Philosopher4put two forks and is ready to think!
Philosopher4 says:I am full and want to think for some time!
Philosopher1get two forks and is ready to eat!
Philosopher1 says:I am eating now!
Philosopher3get two forks and is ready to eat!
Philosopher3 says:I am eating now!
Philosopher1put two forks and is ready to think!
Philosopher1 says:I am full and want to think for some time!
Philosopher0get two forks and is ready to eat!
Philosopher0 says:I am eating now!
Philosopher3put two forks and is ready to think!
Philosopher3 says:I am full and want to think for some time!
Philosopher2get two forks and is ready to eat!
Philosopher2 says:I am eating now!
Philosopher2put two forks and is ready to think!
Philosopher2 says:I am full and want to think for some time!
Philosopher0put two forks and is ready to think!
Philosopher0 says:I am full and want to think for some time!
Philosopher3get two forks and is ready to eat!
Philosopher3 says:I am eating now!
Philosopher1get two forks and is ready to eat!
Philosopher1 says:I am eating now!
Philosopher3put two forks and is ready to think!
Philosopher3 says:I am full and want to think for some time!
Philosopher1put two forks and is ready to think!
Philosopher1 says:I am full and want to think for some time!
Philosopher2get two forks and is ready to eat!
Philosopher2 says:I am eating now!
Philosopher0get two forks and is ready to eat!
Philosopher0 says:I am eating now!
Philosopher2put two forks and is ready to think!
Philosopher2 says:I am full and want to think for some time!
Philosopher3get two forks and is ready to eat!
Philosopher3 says:I am eating now!
Philosopher0put two forks and is ready to think!
Philosopher0 says:I am full and want to think for some time!
Philosopher1get two forks and is ready to eat!
Philosopher1 says:I am eating now!

        由结果可以看出，使用了叉子是否被使用这一信号来对哲学家的动作进行控制，并且将注意叉子的使用与安放都需要加锁。因此结果中如果5个人用餐，每次最多两个人用餐，并且是不相邻的两个人，这样就不会出现死锁以及死循环的问题。