16.3 In the famous dining philosophers problem, a bunch of philosophers are sitting around a circular table with one chopstick between each of them. A philosopher needs both chopsticks to eat, and always picks up the left chopstick before the right one. A deadlock could potentially occur if all the philosophers reached for the left chopstick at the same time. Using threads and locks, implement a simulation of the dining philosophers problem that prevents deadlocks.
經典的哲學家聚餐問題,說是有一堆哲學家圍着一個圓桌子坐着吃飯,每兩個人之間都有一根筷子,每個人吃飯需要都需要左右兩邊的筷子,而且是先拿起左邊的筷子,再拿右邊的筷子,那么如果當所有的哲學家都拿着左邊的筷子,那么就會產生死鎖的情況。如果我們先不考慮死鎖的問題,先來實現這個問題。我們可以把每個哲學家都當做一個線程,然后筷子被哲學家拿起后可以調用鎖,當被放下后調用解鎖,參見代碼如下:
import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; public class Chopstick { private Lock lock; public Chopstick() { lock = new ReentrantLock(); } public void pickUp() { lock.lock(); } public void putDown() { lock.unlock(); } } public class Philosopher extends Thread { private final int maxPause = 100; private int bites = 10; private Chopstick left; private Chopstick right; private int index; public Philosopher(int i, Chopstick left, Chopstick right) { this.left = left; this.right = right; } public void eat() { System.out.println("Philosopher" + index + ": start eating"); pickUp(); chew(); putDown(); System.out.println("Philosopher " + index + ": done eating"); } public void pickUp() { pause(); left.pickUp(); pause(); right.pickUp(); } public void chew() { System.out.println("Philosopher " + index + ": eating"); pause(); } public void pause() { try { int pause = (int)(Math.random() * maxPause); Thread.sleep(pause); } catch (InterruptedException e) { e.printStackTrace(); } } public void putDown() { left.putDown(); right.putDown(); } public void run() { for (int i = 0; i < bites; ++i) { eat(); } } } public class j { public static int size = 3; public static int leftOf(int i) { return i; } public static int rightOf(int i) { return (i + 1) % size; } public static void main(String[] args) { Chopstick[] chopsticks = new Chopstick[size + 1]; for (int i = 0; i < size + 1; ++i) { chopsticks[i] = new Chopstick(); } Philosopher[] philosophers = new Philosopher[size]; for (int i = 0; i < size; ++i) { Chopstick left = chopsticks[leftOf(i)]; Chopstick right = chopsticks[rightOf(i)]; philosophers[i] = new Philosopher(i, left, right); } for (int i = 0; i < size; ++i) { philosophers[i].start(); } } }
上面的代碼在執行中基本都會陷入死循環,因為發生了死鎖的情況,所以我們應該想機制來避免死鎖的發生,那么怎么做呢,我們首先想想死鎖是怎么形成的,是因為每個人都拿着左邊的筷子不放,又無法拿到右邊的筷子,所以就一直僵持着,那么我們換個思路想想,如果每個人在拿了左筷子,發現沒法取得右筷子后,就把左筷子放下,這樣就可以避免死鎖的形成。那么我們在Chopstik類中的pickUp函數中就應該使用tryLock()來代替lock,這樣只有在有左筷子的時候才能鎖上左筷子,而且在Philosopher類中的pickUp函數中,先判斷能不能拿左筷子,不能拿直接返回false,能拿的話再來看能不能拿右筷子,不能拿的話,先把左筷子放下,再返回false,能拿的話返回true。這樣在eat函數中先看pickUp是否能返回true,能返回的話再繼續運行之后的東西,參見代碼如下:
import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; public class Chopstick { private Lock lock; public Chopstick() { lock = new ReentrantLock(); } public boolean pickUp() { return lock.tryLock(); } public void putDown() { lock.unlock(); } } public class Philosopher extends Thread { private final int maxPause = 100; private int bites = 10; private Chopstick left; private Chopstick right; private int index; public Philosopher(int i, Chopstick left, Chopstick right) { index = i; this.left = left; this.right = right; } public void eat() { System.out.println("Philosopher" + index + ": start eating"); if (pickUp()) { chew(); putDown(); System.out.println("Philosopher " + index + ": done eating"); } else { System.out.println("Philosopher " + index + ": gave up on eating"); } } public boolean pickUp() { pause(); if (!left.pickUp()) { return false; } pause(); if (!right.pickUp()) { left.putDown(); return false; } pause(); return true; } public void chew() { System.out.println("Philosopher " + index + ": eating"); pause(); } public void pause() { try { int pause = (int)(Math.random() * maxPause); Thread.sleep(pause); } catch (InterruptedException e) { e.printStackTrace(); } } public void putDown() { left.putDown(); right.putDown(); } public void run() { for (int i = 0; i < bites; ++i) { eat(); } } } public class j { public static int size = 3; public static int leftOf(int i) { return i; } public static int rightOf(int i) { return (i + 1) % size; } public static void main(String[] args) { Chopstick[] chopsticks = new Chopstick[size + 1]; for (int i = 0; i < size + 1; ++i) { chopsticks[i] = new Chopstick(); } Philosopher[] philosophers = new Philosopher[size]; for (int i = 0; i < size; ++i) { Chopstick left = chopsticks[leftOf(i)]; Chopstick right = chopsticks[rightOf(i)]; philosophers[i] = new Philosopher(i, left, right); } for (int i = 0; i < size; ++i) { philosophers[i].start(); } } }