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题目
有N个哲学家围坐在一张圆桌旁，桌上只有N把叉子，每对哲学家中间各有一把。
哲学家的两种行为：
一、思考
二、吃意大利面
哲学家只能拿起手边左边或右边的叉子
吃饭需要两把叉子
正确地模仿哲学家的行为
方法一
一次只允许四个人抢叉子
import java.util.concurrent.Semaphore;
class 方法一 {public static class PhilosopherTest {//一次只允许四个人抢叉子static final Semaphore count = new Semaphore(4);//五只叉子static final Semaphore[] mutex = {new Semaphore(1),new Semaphore(1),new Semaphore(1),new Semaphore(1),new Semaphore(1)};static class Philosopher extends Thread {Philosopher(int name) {super.setName(String.valueOf(name));}@Overridepublic void run() {do {try {//只有四个人有抢叉子的资格count.acquire();Integer i = Integer.parseInt(super.getName());//规定都先拿左手边的叉子，于是四个人左手都有叉子mutex[i].acquire();//大家开始抢右边的叉子mutex[(i + 1) % 5].acquire();//谁先抢到谁第一个开吃System.out.println("哲学家" + i + "号吃饭！");//吃完放下左手的叉子，对于左边人来说，就是他的右叉子，直接开吃mutex[i].release();//再放下右手的叉子mutex[(i + 1) % 5].release();//吃完了，开始思考，由于放下了右手的叉子，相当于给一个叉子没有的哲学家一个左叉子count.release();//模拟延迟Thread.sleep(2000);} catch (InterruptedException e) {System.out.println("异常");}} while (true);}}public static void main(String[] args) {Philosopher[] threads=new Philosopher[5];for (int i = 0; i < 5; i++) {threads[i] = new Philosopher(i);}for (Philosopher i : threads) {i.start();}}}
}
count每次acquire就会减一，使得第五个来访问的哲学家被阻塞
下面是将think和eat方法分离出来的改进版本：
import java.util.concurrent.Semaphore;
public class 方法一改进 {public static class PhilosopherTest {// 一次只允许四个人抢叉子static final Semaphore count = new Semaphore(4);// 五只叉子static final Semaphore[] mutex = {new Semaphore(1),new Semaphore(1),new Semaphore(1),new Semaphore(1),new Semaphore(1)};static class Philosopher extends Thread {Philosopher(int name) {super.setName(String.valueOf(name));}@Overridepublic void run() {do {try {think();eat();} catch (InterruptedException e) {System.out.println("异常");}} while (true);}public void think() throws InterruptedException {// 模拟思考System.out.println("哲学家" + super.getName() + "号正在思考");Thread.sleep(2000); // 模拟延迟}public void eat() throws InterruptedException {// 只有四个人有抢叉子的资格count.acquire();Integer i = Integer.parseInt(super.getName());// 规定都先拿左手边的叉子，于是四个人左手都有叉子mutex[i].acquire();// 大家开始抢右边的叉子mutex[(i + 1) % 5].acquire();// 谁先抢到谁第一个开吃System.out.println("哲学家" + i + "号吃饭！");// 吃完放下左手的叉子，对于左边人来说，就是他的右叉子，直接开吃mutex[i].release();// 再放下右手的叉子mutex[(i + 1) % 5].release();// 吃完了，开始思考，由于放下了右手的叉子，相当于给一个叉子没有的哲学家一个左叉子count.release();}}public static void main(String[] args) {PhilosopherTest.Philosopher[] threads=new PhilosopherTest.Philosopher[5];for (int i = 0; i < 5; i++) {threads[i] = new PhilosopherTest.Philosopher(i);}for (PhilosopherTest.Philosopher i : threads) {i.start();}}}}
本质没区别。
方法二
先获取左筷子，一段时间内申请不到右筷子就将左筷子释放
import java.util.concurrent.Semaphore;public class 方法二 {//先获取左筷子，一段时间内申请不到右筷子就将左筷子释放// Five forksstatic final Semaphore[] mutex = {new Semaphore(1),new Semaphore(1),new Semaphore(1),new Semaphore(1),new Semaphore(1)};static class Philosopher extends Thread {Philosopher(int name) {super.setName(String.valueOf(name));}@Overridepublic void run() {do {try {Integer i = Integer.parseInt(super.getName());//尝试获取左筷子if (mutex[i].tryAcquire()) {//尝试获取右筷子if (mutex[(i + 1) % 5].tryAcquire()) {System.out.println("哲学家" + i + "号吃饭!");mutex[i].release();mutex[(i + 1) % 5].release();Thread.sleep(2000);} else {//如果获取不到右筷子，就把左筷子扔了mutex[i].release();}}//这里没有else，获取不到左筷子就一直尝试} catch (InterruptedException e) {System.out.println("异常");}} while (true);}}public static void main(String[] args) {Philosopher[] threads=new Philosopher[5];for (int i = 0; i < 5; i++) {threads[i] = new Philosopher(i);}for (Philosopher i : threads) {i.start();}}
}
下面是将eat和think分出来的版本：
import java.util.concurrent.Semaphore;
class 方法二改进 {//先获取左筷子，一段时间内申请不到右筷子就将左筷子释放public static class Philosopher extends Thread {private static Semaphore[] chopsticks = {new Semaphore(1), new Semaphore(1), new Semaphore(1), new Semaphore(1), new Semaphore(1)};private int id;public Philosopher(int id) {this.id = id;}@Overridepublic void run() {while (true) {think();eat();}}public void think() {System.out.println("哲学家_" + this.id + "正在思考");//思考一秒时间try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}}public void eat() {try {if (chopsticks[this.id].tryAcquire()) { // 获取左筷子if (chopsticks[(this.id + 1) % chopsticks.length].tryAcquire()) { // 获取右筷子System.out.println("哲学家_" + this.id + "正在吃饭");// 吃饭花一秒时间try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();} finally {chopsticks[this.id].release(); // 放下左筷子chopsticks[(this.id + 1) % chopsticks.length].release(); // 放下右筷子}} else {chopsticks[this.id].release(); // 如果不能获取右筷子，释放左筷子}}} catch (Exception e) {e.printStackTrace();}}public static void main(String[] args) {Philosopher[] threads=new Philosopher[5];for (int i = 0; i < 5; i++) {threads[i] = new Philosopher(i);}for (Philosopher i : threads) {i.start();}}}}
下面是用可重入锁实现的版本：
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;class Philosopher extends Thread {private Lock leftFork;private Lock rightFork;private int philosopherId;private int eatCount; // 计数器public Philosopher(int philosopherId, Lock leftFork, Lock rightFork) {this.philosopherId = philosopherId;this.leftFork = leftFork;this.rightFork = rightFork;this.eatCount = 0;}private void think() throws InterruptedException {System.out.println("Philosopher " + philosopherId + " is thinking.");Thread.sleep((long) (Math.random() * 1000));}private void eat() throws InterruptedException {System.out.println("Philosopher " + philosopherId + " is eating.");Thread.sleep((long) (Math.random() * 1000));eatCount++;}@Overridepublic void run() {try {while (eatCount < 1) { // 表示每个哲学家吃了1次think();/* 使用ReentrantLock锁, 该类中有一个tryLock()方法, 在指定时间内获取不到锁对象, 就从阻塞队列移除,不用一直等待。当获取了左手边的筷子之后, 尝试获取右手边的筷子, 如果该筷子被其他哲学家占用, 获取失败, 此时就先把自己左手边的筷子,给释放掉. 这样就避免了死锁问题 */if (leftFork.tryLock()) {System.out.println("Philosopher " + philosopherId + " picked up left fork.");if (rightFork.tryLock()) {System.out.println("Philosopher " + philosopherId + " picked up right fork.");eat();rightFork.unlock();System.out.println("Philosopher " + philosopherId + " put down right fork.");}leftFork.unlock();System.out.println("Philosopher " + philosopherId + " put down left fork.");}}} catch (InterruptedException e) {e.printStackTrace();}}
}class DiningPhilosophers {public static void main(String[] args) {int numPhilosophers = 5;Philosopher[] philosophers = new Philosopher[numPhilosophers];Lock[] forks = new ReentrantLock[numPhilosophers];for (int i = 0; i < numPhilosophers; i++) {forks[i] = new ReentrantLock();}for (int i = 0; i < numPhilosophers; i++) {philosophers[i] = new Philosopher(i, forks[i], forks[(i + 1) % numPhilosophers]);philosophers[i].start();}// 等待所有哲学家线程结束for (Philosopher philosopher : philosophers) {try {philosopher.join();} catch (InterruptedException e) {e.printStackTrace();}}System.out.println("All philosophers have finished eating. Program ends.");}
}使用ReentrantLock锁, 该类中有一个tryLock()方法, 在指定时间内获取不到锁对象, 就从阻塞队列移除,不用一直等待。当获取了左手边的筷子之后, 尝试获取右手边的筷子, 如果该筷子被其他哲学家占用, 获取失败, 此时就先把自己左手边的筷子给释放掉. 这样就避免了死锁问题
方法三
奇数哲学家先左后右，偶数科学家先右后左
import java.util.concurrent.Semaphore;public class 方法四 {//奇数哲学家先左后右，偶数科学家先右后左// Five forksstatic final Semaphore[] mutex = { new Semaphore(1),new Semaphore(1),new Semaphore(1),new Semaphore(1),new Semaphore(1) };static class Philosopher extends Thread {Philosopher(int name) {super.setName(String.valueOf(name));}@Overridepublic void run() {do {try {Integer i = Integer.parseInt(super.getName());if (i % 2 == 1) {// Odd-numbered philosopher// Try to acquire the left forkmutex[i].acquire();System.out.println("哲学家" + i + "号拿起左筷子");// Try to acquire the right forkmutex[(i + 1) % 5].acquire();System.out.println("哲学家" + i + "号拿起右筷子");} else {// Even-numbered philosopher// Try to acquire the right forkmutex[(i + 1) % 5].acquire();System.out.println("哲学家" + i + "号拿起右筷子");// Try to acquire the left forkmutex[i].acquire();System.out.println("哲学家" + i + "号拿起左筷子");}// EatSystem.out.println("哲学家" + i + "号吃饭!");// Release the forksmutex[i].release();mutex[(i + 1) % 5].release();// Think (sleep for simulation)Thread.sleep(2000);} catch (InterruptedException e) {System.out.println("异常");}} while (true);}}public static void main(String[] args) {Philosopher[] threads = new Philosopher[5];for (int i = 0; i < 5; i++) {threads[i] = new Philosopher(i);}for (Philosopher i : threads) {i.start();}}
}
下面是将think和eat分开的版本：
import java.util.concurrent.Semaphore;
public class 方法四改进 {public static class 方法四 {//奇数哲学家先左后右，偶数科学家先右后左// Five forksstatic final Semaphore[] mutex = { new Semaphore(1),new Semaphore(1),new Semaphore(1),new Semaphore(1),new Semaphore(1) };static class Philosopher extends Thread {Philosopher(int name) {super.setName(String.valueOf(name));}@Overridepublic void run() {do {try {think();eat();} catch (InterruptedException e) {System.out.println("异常");}} while (true);}public void think() throws InterruptedException {Integer i = Integer.parseInt(super.getName());System.out.println("哲学家" + i + "号正在思考");// Think (sleep for simulation)Thread.sleep(2000);}public void eat() throws InterruptedException {Integer i = Integer.parseInt(super.getName());if (i % 2 == 1) {// Odd-numbered philosopher// Try to acquire the left forkmutex[i].acquire();System.out.println("哲学家" + i + "号拿起左筷子");// Try to acquire the right forkmutex[(i + 1) % 5].acquire();System.out.println("哲学家" + i + "号拿起右筷子");} else {// Even-numbered philosopher// Try to acquire the right forkmutex[(i + 1) % 5].acquire();System.out.println("哲学家" + i + "号拿起右筷子");// Try to acquire the left forkmutex[i].acquire();System.out.println("哲学家" + i + "号拿起左筷子");}// EatSystem.out.println("哲学家" + i + "号吃饭!");// Release the forksmutex[i].release();mutex[(i + 1) % 5].release();}}public static void main(String[] args) {Philosopher[] threads = new Philosopher[5];for (int i = 0; i < 5; i++) {threads[i] = new Philosopher(i);}for (Philosopher i : threads) {i.start();}}}}
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