[英]Synchronizing multiple threads JAVA
我有200名學生正在等待進入有200個座位(25行8列)的房間。 門容量為4人。 當學生進入房間時,他會選擇隨機座位(行和列)。 如果選擇的座位排在第9排或以下,則需要1秒鍾才能坐下;排在18排及以下,則需要2秒鍾;如果座位排在18到25之間,則需要3秒鍾。 當他們中的任何一個坐下時,另一個人必須進入房間。 問題在於,當前四個人進入房間時,他們一個人坐在一個座位上,而不是一次坐下。 我該如何解決? 例如,如果有2個人在第5行選擇座位,則他們都需要坐1秒鍾,並且兩名新學生必須進入教室。
public class Student
{
int row;
int column;
volatile static int mutex;
//Generating random numbers for row and column
public Student(Seats[][] seats)
{
this.row = (int) Math.ceil(Math.random() * 25);
this.column = (int) Math.ceil(Math.random() * 8);
if (!seats[row][column].isTaken)
{
seats[row][column].isTaken = true;
} else
{
do
{
this.row = (int) Math.ceil(Math.random() * 25);
this.column = (int) Math.ceil(Math.random() * 8);
} while (!seats[row][column].isTaken);
seats[row][column].isTaken = true;
}
}
/*Check if the mutex is 4 (4 people are in the room) then wait
if someone enter the room increment mutex*/
synchronized void add() throws InterruptedException
{
while (mutex > 4)
wait();
Student.mutex++;
notifyAll();
}
/* Check if mutex is 0 (no one is in the room) then wait
if the student has sit - decrement mutex and notify*/
synchronized void takeSeat() throws InterruptedException
{
while (mutex == 0)
wait();
Student.mutex--;
notifyAll();
}
}
class Seats
{
int seat;
boolean isTaken;
public Seats(int seat)
{
this.seat = seat;
this.isTaken = false;
}
}
class StudentThread extends Thread
{
Seats[][] seats = new Seats[25][8];
StudentThread(Seats[][] seats)
{
this.seats = seats;
}
public void run()
{
try
{
Student student = new Student(seats);
synchronized (seats)
{
System.out.println("Student enter the room");
/*call the synchronized method from student
that increment the mutex*/
student.add();
if (Student.mutex == 4)
{
if (student.row <= 9)
{
sleep(1000);
student.takeSeat();
System.out.println("Student take a seat at "
+ student.row + " " + student.column);
}
if (student.row <= 18 && student.row > 9)
{
sleep(2000);
student.takeSeat();
System.out.println("Student take a seat at "
+ student.row + " " + student.column);
}
if (student.row <= 25 && student.row > 18)
{
sleep(3000);
student.takeSeat();
System.out.println("Student take a seat at "
+ student.row + " " + student.column);
}
}
}
} catch (InterruptedException e)
{
e.printStackTrace();
}
}
}
class Main
{
public static void main(String[] args)
{
Seats[][] seats = new Seats[25][8];
//Initializing the seats
for (int i = 0; i < 25; i++)
for (int j = 0; j < 8; j++)
{
seats[i][j] = new Seats(i);
}
for (int i = 0; i < 200; i++)
{
StudentThread T1 = new StudentThread(seats);
T1.start();
}
}
}
使用信號量 ,它們對於這類事情非常實用。
為了使示例更實際:假設您需要執行200個HTTP get-request,但是如果您同時運行4個以上的請求,服務器將禁止您執行該操作。 下面的示例顯示如何使用信號量限制同時運行的請求數。
import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
public class ResourceUsageLimiter {
static ExecutorService executor = Executors.newCachedThreadPool();
static int requests = 20;
static int maxRequestsConcurrent = 4;
static int maxRequestTime = 1000;
static Random randomizer = new Random();
static Semaphore openSlots = new Semaphore(maxRequestsConcurrent);
static long startTime = System.currentTimeMillis();
public static void main(String[] args) {
try {
for (int i = 0; i < requests; i++) {
openSlots.acquire();
executor.execute(new RequestRunner(i));
}
} catch (Exception e) {
e.printStackTrace();
} finally {
executor.shutdown();
}
}
static long time() {
return System.currentTimeMillis() - startTime;
}
static class RequestRunner implements Runnable {
int sleepTime, reqId;
public RequestRunner(int reqId) {
this.reqId = reqId;
sleepTime = randomizer.nextInt(maxRequestTime);
}
@Override
public void run() {
try {
System.out.println(time() + " " + reqId + " sleeping " + sleepTime);
Thread.sleep(sleepTime);
System.out.println(time() + " " + reqId + " sleep done");
} catch (Exception e) {
e.printStackTrace();
} finally {
openSlots.release();
}
}
}
}
當然,在示例中,限制同時運行的最大請求數的另一種方法是使用固定大小為4的線程池。
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