哲学家就餐问题-Java语言实现死锁避免
我死锁预防是至少破坏死锁产生的四个必要条件之一,带来的问题就是系统资源利用率低且不符合开发习惯,而死锁避免不是事先釆取某种限制措施破坏死锁的必要条件,只是注意避免死锁的最终发生。
哲学家就餐问题
5 个沉默寡言的哲学家围坐在圆桌前,每人面前一盘意面。叉子放在哲学家之间的桌面上。(5 个哲学家,5 根筷子)
所有的哲学家都只会在思考和进餐两种行为间交替。哲学家只有同时拿到左边和右边的筷子才能吃到面,而同一根筷子在同一时间只能被一个哲学家使用。每个哲学家吃完面后都需要把筷子放回桌面以供其他哲学家吃面。只要条件允许,哲学家可以拿起左边或者右边的筷子,但在没有同时拿到左右筷子时不能进食。
产生死锁的四个必要条件:
(1) 互斥条件:一个资源每次只能被一个进程使用。
(2) 请求与保持条件:一个进程因请求资源而阻塞时,对已获得的资源保持不放。
(3) 不可剥夺条件:进程已获得的资源,在末使用完之前,不能强行剥夺。
(4) 环路等待条件:若干进程之间形成一种头尾相接的循环等待资源关系。
只要系统发生了死锁,这些条件必然成立,而只要上述条件之一不满足,就不会发生死锁。
解除死锁:从死锁进程处剥夺资源;终止部分或全部进程
Java语言实现死锁避免Demo
加锁前检查需要的资源是否足够,只有哲学家两边的筷子都没人用时同时拿起
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
class DiningPhilosophers {
//5根筷子的状态,0代表未使用,1代表已使用
public static int[] chopsticks = new int[5];
public static ReentrantLock lock = new ReentrantLock();
public static Condition condition = lock.newCondition();
public static void main(String[] args) {
for (int i = 0; i < 5; i++) {
new Thread(new Philosophers(i)).start();
}
}
static class Philosophers implements Runnable {
public int No;
public Philosophers(int no) {
No = no;
}
private void eat() {
System.out.println("哲学家 " + No + " is eating");
try {
Thread.currentThread().sleep(1500);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private void think() {
System.out.println("哲学家 " + No + " is thinking");
try {
Thread.currentThread().sleep(1500);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private void takeChopsticks() {
lock.lock();
//如果右边的筷子和左边的筷子都没有被使用
if (chopsticks[No] == 0 && chopsticks[(No + 4) % 5] == 0) {
chopsticks[No] = 1;
chopsticks[(No + 4) % 5] = 1;
} else {
try {
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
lock.unlock();
}
private void putdownChopsticks() {
lock.lock();
chopsticks[No] = 0;
chopsticks[(No + 4) % 5] = 0;
condition.signalAll();
lock.unlock();
}
@Override
public void run() {
while (true) {
this.think();
this.takeChopsticks();
this.eat();
this.putdownChopsticks();
}
}
}
}
/* 哲学家 0 is thinking
哲学家 4 is thinking
哲学家 1 is thinking
哲学家 3 is thinking
哲学家 2 is thinking
哲学家 0 is eating
哲学家 2 is eating
哲学家 2 is thinking
哲学家 0 is thinking
哲学家 1 is eating
哲学家 3 is eating
哲学家 4 is eating
哲学家 0 is eating
哲学家 3 is thinking
哲学家 2 is eating
哲学家 1 is thinking
……………………………… */
给锁添加时限:先拿起右边的筷子,再尝试拿左边的筷子,如果一段时间后依然没有拿到左边的筷子,则放下右边的筷子
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
class DiningPhilosophers2 {
public static ReentrantLock[] chopsticks = new ReentrantLock[5];
public static Condition[] conditions = new Condition[5];
public static void main(String[] args) {
for (int i = 0; i < 5; i++) {
chopsticks[i] = new ReentrantLock();
conditions[i] = chopsticks[i].newCondition();
}
for (int i = 0; i < 5; i++) {
new Thread(new Philosophers(i)).start();
}
}
static class Philosophers implements Runnable {
public int No;
public Philosophers(int no) {
No = no;
}
private void eat() {
System.out.println("哲学家 " + No + " is eating");
try {
Thread.currentThread().sleep(1500);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private void think() {
System.out.println("哲学家 " + No + " is thinking");
try {
Thread.currentThread().sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private void takeChopsticks() {
//是否获取了两个筷子
boolean acquire = false;
while (true) {
if (chopsticks[No].tryLock()) {
while (true) {
if (!chopsticks[(No + 4) % 5].tryLock()) {
chopsticks[No].unlock();
try {
Thread.currentThread().sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
} else {
acquire = true;
}
break;
}
if (acquire) {
chopsticks[(No + 4) % 5].unlock();
chopsticks[No].unlock();
break;
}
}
}
}
private void putdownChopsticks() {
chopsticks[(No + 4) % 5].lock();
chopsticks[No].lock();
conditions[No].signalAll();
conditions[(No + 4) % 5].signalAll();
chopsticks[(No + 4) % 5].unlock();
chopsticks[No].unlock();
}
@Override
public void run() {
while (true) {
this.think();
this.takeChopsticks();
this.eat();
this.putdownChopsticks();
}
}
}
}
/* 哲学家 0 is thinking
哲学家 2 is thinking
哲学家 4 is thinking
哲学家 1 is thinking
哲学家 3 is thinking
哲学家 0 is eating
哲学家 2 is eating
哲学家 1 is eating
哲学家 3 is eating
哲学家 4 is eating
哲学家 0 is thinking
哲学家 1 is thinking
哲学家 2 is thinking
哲学家 0 is eating
哲学家 3 is thinking
哲学家 4 is thinking
哲学家 2 is eating
哲学家 1 is eating
哲学家 3 is eating
哲学家 0 is thinking
哲学家 2 is thinking
哲学家 1 is thinking
哲学家 4 is eating
哲学家 0 is eating
哲学家 3 is thinking
哲学家 1 is eating
哲学家 4 is thinking
哲学家 2 is eating
哲学家 3 is eating*/