哲學家進餐問題是一個多線程運用的經典例子，涉及到線程同步/互斥，臨界區訪問問題以及一個避免死鎖的解決方法。
有五個哲學家繞著圓桌坐，每個哲學家面前有一盤面，兩人之間有一支筷子，這樣每個哲學家左右各有一支筷子。
哲學家有2個狀態，思考或者拿起筷子吃飯。如果哲學家拿到一只筷子，不能吃飯，直到拿到2只才能吃飯，并且一次只能拿起身邊的一支筷子。一旦拿起便不會放下筷子直到把飯吃完，此時才把這雙筷子放回原處。
如果，很不幸地，每個哲學家拿起他或她左邊的筷子，那么就沒有人可以吃到飯了。這就會造成死鎖了。。這是需要堅決杜絕的，正如操作系統的死鎖問題。
代碼如下：
import java.util.Random;
public class DiningPhils
{
public static void main(String[] args)
{
int n = 10;
if( n < 1)
{
System.err.println( "DiningPils <# of philosophers>" );
System.exit(-1);
}
DiningPhils self = new DiningPhils();
self.init(n);
}
public int getCount()
{
return n;
}
public void setChopstick( int i, boolean v)
{
chops[ i ] = v;
}
public boolean getChopstick( int i )
{
return chops[i];
}
private void init( final int N)
{
r = new Random();
n = ( N < 0 || N > maxPhils ) ? maxPhils : N;
chops = new boolean[n];
phils = new Philosopher[n];
initPhils();
dumpStatus();
}
private void initPhils()
{
for( int i = 0; i< n; i++ )
{
phils[i] = new Philosopher( this, i );
phils[i].setTimeSlice( generateTimeSlice());
phils[i].setPriority( Thread.NORM_PRIORITY - 1);
/**哲學家進程降低一級，使所有哲學家進程
*全部初始化完畢前不會有哲學家進程搶占主線程*/
}
while( moreToStart() )
{
int i = Math.abs( r.nextInt()) % n;
if( !phils[i].isAlive())
{
System.out.println( " ### Philosopher " +
String.valueOf( i ) + " started.");
phils[i].start();
}
}
System.out.println( "\nPhilosophers Chopsticks"
+ "\n(1 = eating, 0 = thinking) (1 = taken, 0 = free)");
}
public int generateTimeSlice()
{
int ts = Math.abs(r.nextInt()) % (maxEat + 1);
if( ts == 0 )
ts = minEat;
return ts;
}
public void dumpStatus()
{
for( int i = 0; i < n; i++)
System.out.print(phils[i].getEat() ? 1 : 0);
for( int i = n; i < maxPhils + 4; i++ )
System.out.print(" ");
for( int i = 0; i < n; i++)
System.out.print(chops[i]? 1:0);
System.out.println();
}
private boolean moreToStart()
{
for( int i = 0; i < phils.length; i++ )
{
if( !phils[i].isAlive())
return true;
}
return false;
}
private int n;
private Philosopher[] phils;
private boolean[] chops;
private Random r;
private static final int maxPhils = 24; //最多哲學家數
private static final int maxEat = 4; //最多進餐時間
private static final int minEat = 1; //最少進餐時間
}
class Philosopher extends Thread
{
public Philosopher( DiningPhils HOST , int i )
{
host = HOST;
index = i;
}
public void setTimeSlice( int TS )
{
ts = TS;
}
public void setLeftChopstick( boolean flag )
{
host.setChopstick(index, flag);
}
public void setRightChopstick( boolean flag )
{
host.setChopstick((index + 1)% host.getCount() , flag);
}
private void releaseChopsticks()
{
setLeftChopstick(false);
setRightChopstick(false);
}
public boolean chopsticksFree()
{
return !host.getChopstick(index) &&
!host.getChopstick((index+1)%host.getCount());
}
public void run()
{
while(true)
{
grabChopsticks();
eat();
think();
}
}
private synchronized void grabChopsticks() /**臨界區函數，確保哲學家在沒有筷子或筷子不夠時思考，滿足條件后才就餐*/
{
while( !chopsticksFree())
{
try
{
wait();
}
catch( InterruptedException e){}
}
takeChopsticks();
notifyAll();
}
private void takeChopsticks()
{
setLeftChopstick( true );
setRightChopstick( true );
setEat(true);
host.dumpStatus();
}
private void eat()
{
pause();
setEat( false );
releaseChopsticks();
}
private void think()
{
pause();
}
private void pause()
{
setTimeSlice( host.generateTimeSlice());
try
{
sleep(ts*1000);
}
catch( InterruptedException e){}
}
private void setEat(boolean v)
{
isEating = v;
}
public boolean getEat()
{
return isEating;
}
private DiningPhils host;
private boolean isEating;
private int index;
private int ts;
}