What is Polling and what are problems with it?
Polling is usually implemented by a loop that is used to check some condition repeatedly. Once the condition is true, appropriate action is taken. This wastes CPU time. For example, consider the classic queuing problem, where one thread is producing some data and another is consuming it. To make the problem more interesting, suppose that the producer has to wait until the consumer is finished before it generates more data. In a polling system, the consumer would waste many CPU cycles while it waited for the producer to produce. Once the producer was finished, it would start polling, wasting more CPU cycles waiting for the consumer to finish, and so on. Clearly, this situation is undesirable.
How Java multi-threading avoids Polling?
To avoid polling, Java uses three methods, namely, wait(), notify() and notifyAll().
wait()- It tells the calling thread to give up the monitor and go to sleep until some other thread enters the same monitor and callsnotify()ornotifyAll(). Additional forms ofwait()exist that allow you to specify a period of time to wait.notify()- It wakes up a thread that called wait( ) on the same object.notifyAll()- It wakes up all the threads that calledwait()on the same object. One of the threads will be granted access.
These methods are declared within Object, as shown here:
final void wait( ) throws InterruptedException
final void notify( )
final void notify All( )// An incorrect implementation of a producer and consumer.
class Q {
int n;
synchronized int get() {
System.out.println("Got: " + n);
return n;
}
synchronized void put(int n) {
this.n = n;
System.out.println("Put: " + n);
}
}class Producer implements Runnable {
Q q;
Producer(Q q) {
this.q = q;
new Thread(this, "Producer").start();
}
public void run() {
int i = 0;
while(true) {
q.put(i++);
}
}
}class Consumer implements Runnable {
Q q;
Consumer(Q q) {
this.q = q;
new Thread(this, "Consumer").start();
}
public void run() {
while(true) {
q.get();
}
}
}class PC {
public static void main(String args[]) {
Q q = new Q();
new Producer(q);
new Consumer(q);
System.out.println("Press Control-C to stop.");
}
}Put: 1
Got: 1
Got: 1
Got: 1
Got: 1
Got: 1
Put: 2
Put: 3
Put: 4
Put: 5
Put: 6
Put: 7
Got: 7
Although the put() and get() methods on Q are synchronized, nothing stops the producer from overrunning the consumer, nor will anything stop the consumer from consuming the same queue value twice. Thus, you get the erroneous output shown here (the exact output will vary with processor speed and task load):
After the producer put 1, the consumer started and got the same 1 five times in a row. Then, the producer resumed and produced 2 through 7 without letting the consumer have a chance to consume them.
The proper way to write this program in Java is to use wait() and notify() to signal in both directions, as shown here:
// A correct implementation of a producer and consumer.
class Q {
int n;
boolean valueSet = false;
synchronized int get() {
while(!valueSet){
try {
wait();
} catch(InterruptedException e) {
System.out.println("InterruptedException caught");
}
}
System.out.println("Got: " + n);
valueSet = false;
notify();
return n;
}
synchronized void put(int n) {
while(valueSet){
try {
wait();
} catch(InterruptedException e) {
System.out.println("InterruptedException caught");
}
}
this.n = n;
valueSet = true;
System.out.println("Put: " + n);
notify();
}
}class Producer implements Runnable {
Q q;
Producer(Q q) {
this.q = q;
new Thread(this, "Producer").start();
}
public void run() {
int i = 0;
while(true) {
q.put(i++);
}
}
}class Consumer implements Runnable {
Q q;
Consumer(Q q) {
this.q = q;
new Thread(this, "Consumer").start();
}
public void run() {
while(true) {
q.get();
}
}
}class PCFixed {
public static void main(String args[]) {
Q q = new Q();
new Producer(q);
new Consumer(q);
System.out.println("Press Control-C to stop.");
}
}Inside get(), wait() is called. This causes its execution to suspend until Producer notifies you that some data is ready. When this happens, execution inside get() resumes. After the data has been obtained, get() calls notify(). This tells Producer that it is okay to put more data in the queue. Inside put(), wait() suspends execution until Consumer has removed the item from the queue. When execution resumes, the next item of data is put in the queue, and notify() is called.
Put: 1
Got: 1
Put: 2
Got: 2
Put: 3
Got: 3
Put: 4
Got: 4
Put: 5
Got: 5
An interrupt is an indication to a thread that it should stop what it is doing and do something else. It's up to the programmer to decide exactly how a thread responds to an interrupt, but it is very common for the thread to terminate.
A thread sends an interrupt by invoking interrupt on the Thread object for the thread to be interrupted. For the interrupt mechanism to work correctly, the interrupted thread must support its own interruption.
Thread t1 = new Thread("Interrupt");
t1.start();
try{
t1.interrupt();
}catch(InterruptedException ex){
System.out.println(ex.getMessage());
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