Thread : Wait and Notify

Thread : Wait and Notify

We use sleep() method in Thread to pass the thread for some milliseconds/nanoseconds and resume the thread on its own. However, while waiting for the shared resource, this will not help.

In JAVA, Object class comes with wait() and notify/notifyAll methods, which helps us to mark saying, I am waiting for this object, who ever owns object monitor lock, release and let me know. While calling this methods, we have to own the object monitor lock of this

public class WaitNotifytest { 
static Object obj=new Object();

public static void main(String[] args) throws Exception{ 
new Thread()
{
 @Override
 public void run() {
  try
  {
   obj.wait();    
  }catch(Exception e)
  {
   e.printStackTrace();
  }
 }
}.start();

new Thread()
{
 @Override
 public void run() {
  obj.notify();
 }
}.start();
 
 Thread.sleep(1000);  
}
}

If the above program will throw java.lang.IllegalMonitorStateException

java.lang.IllegalMonitorStateException
 at java.lang.Object.wait(Native Method)
 at java.lang.Object.wait(Object.java:485)
 at WaitNotifytest$1.run(WaitNotifytest.java:21)
Exception in thread "Thread-1" java.lang.IllegalMonitorStateException
 at java.lang.Object.notifyAll(Native Method)
 at WaitNotifytest$2.run(WaitNotifytest.java:59)

notify and wait with Object monitor lock

notify() method awakes only one thread in the queue(no gaurantee in order), rest has to wait for awakened thread to notify().

public class WaitNotifytest {
static Object obj = new Object();

public static void main(String[] args) throws Exception {

new Thread() {
 @Override
 public void run() {
  try {
   synchronized (obj) {
    obj.wait();
    System.out.println("T1");
     obj.notify();
   }
  } catch (Exception e) {
   e.printStackTrace();
  }
 }
}.start();

new Thread() {
 @Override
 public void run() {
  try {
   synchronized (obj) {
    obj.wait();
    System.out.println("T2");
     obj.notify();
   }
  } catch (Exception e) {
   e.printStackTrace();
  }
 }
}.start();

new Thread() {
 @Override
 public void run() {
  synchronized (obj) {
   obj.notify();
  }
 }
}.start();
}
}

notifyAll

Each thread notifies after usage of object. If T1 and T2, wants to make read access of object then T3 alone notfiy to all the threads. To do this notifyAll() method get used. This method should only be called by a thread that is the owner of this object's monitor, otherwise we will get the IllegalMonitorStateException. Awakened Threads will compete each other runs/access this object.

public class WaitNotifytest {
static Object obj = new Object();

public static void main(String[] args) throws Exception {

new Thread() {
 @Override
 public void run() {
  try {
   synchronized (obj) {
    obj.wait();
    System.out.println("T1");
   }
  } catch (Exception e) {
   e.printStackTrace();
  }
 }
}.start();

new Thread() {
 @Override
 public void run() {
  try {
   synchronized (obj) {
    obj.wait();
    System.out.println("T2");
   }
  } catch (Exception e) {
   e.printStackTrace();
  }
 }
}.start();

new Thread() {
 @Override
 public void run() {
  synchronized (obj) {
   obj.notifyAll();
  }
 }
}.start();
}
}

In this program only one notifyAll method called on object to release all the lock.

Note:

• If wait() called on the object and no notify signal received then this wait called thread will wait for ever. However, there could be a chance of thread awaking without any of notificatin like notify, timeout or interrupt. This is called spurious wakeup. This will happen in rare condition.


• 
  We can even specify the time, how long maximum this thread to wait to acquire the lock, even if other thread is not signalled notify/notifyAll, however object lock is released and no other thread acquires lock of this object.

wait with max timeout

public class WaitNotifytest {
static Object obj = new Object();

public static void main(String[] args) throws Exception {

new Thread() {
 @Override
 public void run() {
  try {
   synchronized (obj) {
    obj.wait(400);
    System.out.println("T1");
//    obj.notify();
   }
  } catch (Exception e) {
   e.printStackTrace();
  }
 }
}.start();

new Thread() {
 @Override
 public void run() {
  synchronized (obj) {
   try{Thread.sleep(10000);}catch(Exception e){}
//   obj.notifyAll();
  }
 }
}.start();
}
}