I am trying to code solution of Producer-Consumer problem with the help of Semaphores. Below is the code I have written, on execution it is getting stuck after producing one number.
I have checked the assignment of both the Semaphore and it look fine. The sequence of acquire and release methods being called also looks correct.
public class Q {
int n;
Semaphore consumerSem = new Semaphore(0);
Semaphore producerSem = new Semaphore(1);
synchronized void get(){
try {
consumerSem.acquire();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Got Data : " + n);
producerSem.release();
}
synchronized void put(int n){
try {
producerSem.acquire();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Putting Data : " + n);
this.n=n;
consumerSem.release();
}
}
public class Producer implements Runnable{
Q q;
Producer(Q q){
this.q = q;
}
@Override
public void run() {
for (int i=0; i<20;i++){
q.put(i);
}
}
}
public class Consumer implements Runnable{
Q q;
Consumer(Q q){
this.q = q;
}
@Override
public void run() {
for (int i =0;i<20;i++){
q.get();
}
}
}
public class PCRun {
public static void main(String[] args) {
Q q = new Q();
new Thread(new Producer(q),"Producer").start();
new Thread(new Consumer(q),"Consumer").start();
}
}
You've made get
and put
synchronized
. So the producer goes in, locks q
, uses the one permit of producerSem
and blocks at the next put
call. Unfortunately, q
is still locked by the producer, so the consumer won't be able to enter get
. To fix this, remove both synchronized
s.
Now to synchronize the access on n
, use synchronized
only where n
is accessed, not for both whole methods.
int n;
final Semaphore consumerSem = new Semaphore(0);
final Semaphore producerSem = new Semaphore(1);
void get() {
try {
consumerSem.acquire();
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (this) {
System.out.println("Got Data : " + n);
}
producerSem.release();
}
void put(int n) {
try {
producerSem.acquire();
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (this) { // invariant print + write, atomic
System.out.println("Putting Data : " + n);
this.n = n;
}
consumerSem.release();
}