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Difference Between wait() and notify() in Java

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  • Difficulty Level : Medium
  • Last Updated : 04 Apr, 2022
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The wait() and notify() are methods of the Object class. They were introduced to part ways with polling, which is the process of repeatedly checking for a condition to be fulfilled. Polling wastes CPU resources considerably, hence it is not preferred.

wait() Method

wait() method is a part of java.lang.Object class. When wait() method is called, the calling thread stops its execution until notify() or notifyAll() method is invoked by some other Thread.

The wait() method has 3 variations:

1. wait(): This is a basic version of the wait() method which does not take any argument. It will cause the thread to wait till notify is called.

public final void wait()

2. wait(long timeout): This version of the wait() method takes a single timeout argument. It will cause the thread to wait either till notify is called or till timeout (One which occurs earlier).

public final void wait(long timeout)

3. wait(long timeout, int nanoseconds): This version of the wait() method takes a timeout argument as well as a nanosecond argument for extra precision.

public final void wait(long timeout, int nanoseconds)

notify() Method

The notify() method is defined in the Object class, which is Java’s top-level class. It’s used to wake up only one thread that’s waiting for an object, and that thread then begins execution. The thread class notify() method is used to wake up a single thread.

public final void notify()

notify() Method in Java

Differences between wait() and notify()

S. No. Wait() notify()
1. When wait() is called on a thread holding the monitor lock, it surrenders the monitor lock and enters the waiting state. When the notify() is called on a thread holding the monitor lock, it symbolizes that the thread is soon going to surrender the lock.
2. There can be multiple threads in the waiting state at a time.

One of the waiting threads is randomly selected and notified about the same. The notified thread then exits the waiting state and enters the blocked state where it waits till the previous thread has given up the lock and this thread has acquired it. Once it acquires the lock, it enters the runnable state where it waits for CPU time and then it starts running.

3. The wait() method is defined in the Object class The notify() method is defined in the Object class
4. The wait() method is used for interthread communication. The notify() method is used to wake up a single thread
5. The wait() method is a part of java.lang.Object class The notify() method does not have any return type value
6. The wait() method is tightly integrated with the synchronization lock The notify() method is used to give the notification for one thread for a particular object

Below is demonstration of wait() and notify() method:

Java




// Java Program to demonstrate usage of wait() and notify()
 
class demo {
    // variable to check if part1 has returned
    // volatile used to prevent threads from
    // storing local copies of variable
    volatile boolean part1done = false;
 
    // method synchronized on this
    // i.e. current object of demo
    synchronized void part1()
    {
        System.out.println("Welcome to India");
        part1done = true;
        System.out.println(
            "Thread t1 about to surrender lock");
        // notify the waiting thread, if any
        notify();
    }
 
    // method synchronized on this
    // i.e. current object of demo
    synchronized void part2()
    {
        // loop to prevent spurious wake-up
        while (!part1done) {
            try {
                System.out.println("Thread t2 waiting");
                // wait till notify is called
                wait();
                System.out.println(
                    "Thread t2 running again");
            }
            catch (Exception e) {
                System.out.println(e.getClass());
            }
        }
        System.out.println("Do visit Taj Mahal");
    }
}
 
public class Main {
 
    public static void main(String[] args)
    {
 
        // Make an instance of demo class
        demo obj = new demo();
 
        // Thread t1 will call part1()
        Thread t1 = new Thread(new Runnable() {
            public void run() { obj.part1(); }
        });
 
        // Thread t2 will call part2()
        Thread t2 = new Thread(new Runnable() {
            public void run() { obj.part2(); }
        });
 
        // Start t2 and then t1
        t2.start();
        t1.start();
    }
}


Output

Thread t2 waiting
Welcome to India
Thread t1 about to surrender lock
Thread t2 running again
Do visit Taj Mahal

Various Exceptions:

1. wait()

  • It is mandatory to enclose wait() in a try-catch block because if a thread present in the waiting state gets interrupted, then it will throw InterruptedException.
  • The other two variations of wait housing parameters will throw IllegalArgumentException if the value of timeout is negative or the value of nanoseconds is not in the range 0 to 9,99,999.

Below is the implementation for the exception handling.

Java




// Program demonstrating occurrence of InterruptedException
 
class demo {
    volatile boolean part1done = false;
 
    synchronized void part1()
    {
        System.out.println("Welcome to India");
        part1done = true;
        // notify() has been commented, waiting
        // thread remains waiting forever notify();
    }
 
    synchronized void part2()
    {
        while (!part1done) {
            try {
                wait();
            }
            catch (Exception e) {
                System.out.println("Exception : "
                                   + e.getClass());
                // quit program after exception is thrown
                System.exit(-1);
            }
        }
        System.out.println("Do visit Taj Mahal");
    }
}
 
public class Main {
 
    public static void main(String[] args)
    {
        // Make an instance of demo class
        demo obj = new demo();
 
        // Thread t1 will call part1()
        Thread t1 = new Thread(new Runnable() {
            public void run() { obj.part1(); }
        });
 
        // Thread t2 will call part2()
        Thread t2 = new Thread(new Runnable() {
            public void run() { obj.part2(); }
        });
 
        // Start t2 and then t1
        t2.start();
        t1.start();
 
        // This is a counter which will
        // interrupt Thread t2 after 3 seconds
        long startTime = System.currentTimeMillis();
        while (true) {
            if (System.currentTimeMillis() - startTime
                > 3000)
                t2.interrupt();
        }
    }
}


Output

Welcome to India
Exception : class java.lang.InterruptedException

2. notify()

Unlike wait(), the notify method does not throw an InterruptedException hence it is not mandatory to house it inside a try-catch block

Note:

  • wait() and notify() both have a tendency to throw IllegalMonitorStateException
  • This occurs when a thread is holding the monitor lock of object A and tries to call wait or notify on object B.
  • In all the preceding examples, the methods were synchronized on “this” i.e. the object used to call those methods (obj). Also, the wait() & notify() were being called as this.wait() and this.notify() (usage of this was redundant). Hence, there was no issue.
  • In the below example, the methods part1 and part2 are now synchronized on an Integer object, but wait() & notify() are still being called on the object which called these methods (obj).
  • This causes an IllegalMonitorStateException

Below is the implementation of the exception handling.

Java




// Program to demonstrate IllegalMonitorStateException
 
class demo {
    volatile boolean part1done = false;
    // Made an Integer object a
    // and set it randomly to 5
    Integer a = 5;
 
    void part1()
    {
        // Synchronized code on a
        synchronized (a)
        {
            System.out.println("Welcome to India");
            part1done = true;
            // calling this.notify()
            notify();
        }
    }
 
    void part2()
    {
        // Synchronized code on a
        synchronized (a)
        {
            while (!part1done) {
                try {
                    // calling this.wait()
                    wait();
                }
                catch (Exception e) {
                    System.out.println("Exception: "+e.getClass());
                    System.exit(-1);
                }
            }
            System.out.println("Do visit Taj Mahal");
        }
    }
}
 
public class Main {
 
    public static void main(String[] args)
    {
 
        // Make an instance of demo class
        demo obj = new demo();
 
        // Thread t1 will call part1()
        Thread t1 = new Thread(new Runnable() {
            public void run() { obj.part1(); }
        });
 
        // Thread t2 will call part2()
        Thread t2 = new Thread(new Runnable() {
            public void run() { obj.part2(); }
        });
 
        // Start t2 and then t1
        t2.start();
        t1.start();
    }
}


Output

Exception: class java.lang.IllegalMonitorStateException

Note: To fix the above code, just replace notify() with a.notify() on line 17 and wait() with a.wait() on line 29.


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