Suspending - Resuming and Stopping Threads

Suspending - Resuming, and Stopping Threads

‘• Stopping a thread: A thread can be stopped from running further by issuing the following statement -

th.stop();

By this statement the thread enters in a dead state. From stopping state a thread can never return to a runnable state.

‘• Blocking a thread: A thread can be temporarily stopped from running. This is called blocking or suspending of a thread. Following are the ways by which thread can be blocked

1. sleep()

By sleep method a thread can be blocked for some specific time. When the specified time gets elapsed then the thread can return to a runnable state.

2. suspend

By suspend method the thread can be blocked until further request comes. When the resume() method is invoked then the thread returns to a runnable state.

3. wait

The thread can be made suspended for some specific conditions. When the notify method is called then the blocked thread returns to the runnable state.

• The difference between the suspending and stopping thread is that if a thread is suspended then its execution is stopped temporarily and it can return to a runnable state. But in case, if a thread is stopped then it goes to a dead state and can never return to runnable state.

Resuming a thread: The resume() method is only used with suspend() method. This method is only to resume a thread which was suspended using suspend() method. The syntax is -

public final resume()

Ex. 3.17.1 Write a java program for inventory problem to illustrates the usage of thread synchronized keyword and inter thread communication process. They have three classes called consumer, producer and stock.

Sol.:

class Consumer implements Runnable

{

Stock obj1;

Thread t;

Consumer (Stock obj1)

{

this.obj1 = obj1;

t = new Thread(this, "Consumer thread");

t.start();

}

public void run().

{

while (true)

{

try {

t.sleep(900);

} catch (InterruptedException e) { }

obj1.getStock((int)(Math.random()*100));

}

}

void stop()

{

t.stop();

}

}

class Producer implements Runnable

{

Stock obj2;

Thread t;

Producer(Stock obj2)

{

this.obj2 = obj2;

t = new Thread(this, "Producer thread");

t.start();

}

public void run()

{

while(true)

{

try {

t.sleep(900);

} catch (InterruptedException e) { }

obj2.addStock((int)(Math.random()*100));

}

}

void stop()

{

t.stop();

}

}

class Stock

{

int items = 0;

public synchronized void addStock(int i)

{

items = items + i;

System.out.println("Stock added:" + i);

System.out.println("present stock :" + items);

notify();

}

public synchronized int getStock(int j)

{

while(true)

{

if(items >= i)

{

items = items - j;

System.out.println("Item is removed from stock :" + j);

System.out.println("Current stock is :" + items);

break;

}

else

{

System.out.println("\tStock is not enough!!!" );

System.out.println ("\t Waiting for items to get added...");

try {

wait();

}catch(InterruptedException e) { }

}

}

return items;

}

public static void main(String args[])

{

Stock j = new Stock();

Producer p = new Producer(j);

Consumer c = new Consumer(j);

try

{

Thread.sleep(10000);

p.stop();

c.stop();

p.t.join();

c.t.join();

System.out.println("Thread stopped");

} catch (InterruptedException e) { }

System.exit(0);

}