Mail us on [email protected] , to get more information about given services. Please mail your requirement at [email protected] Duration: 1 week to 2 week.
Java Training Java Tutorial. Abstract class Interface Abstract vs Interface. Package Access Modifiers Encapsulation. Next Topic Multithreading Java. Reinforcement Learning. R Programming. React Native. Python Design Patterns. This kind of application shares data between two threads: the producer , that creates the data, and the consumer , that does something with it.
The two threads communicate using a shared object. Coordination is essential: the consumer thread must not attempt to retrieve the data before the producer thread has delivered it, and the producer thread must not attempt to deliver new data if the consumer hasn't retrieved the old data. In this example, the data is a series of text messages, which are shared through an object of type Drop :. The producer thread, defined in Producer , sends a series of familiar messages.
The string "DONE" indicates that all messages have been sent. To simulate the unpredictable nature of real-world applications, the producer thread pauses for random intervals between messages. The consumer thread, defined in Consumer , simply retrieves the messages and prints them out, until it retrieves the "DONE" string. This thread also pauses for random intervals.
In general, a thread that uses the wait method confirms that a condition does not exist typically by checking a variable and then calls the wait method. When another thread establishes the condition typically by setting the same variable , it calls the notify method. A race condition occurs when:. The second thread calls the notify method; this goes unheard since the first thread is not yet waiting. How does this potential race condition get resolved?
This race condition is resolved by the synchronization lock discussed earlier. This is mandatory; the methods do not work properly and generate an exception condition if the lock is not held.
Furthermore, the wait method also releases the lock prior to waiting and reacquires the lock prior to returning from the wait method. The developer must use this lock to ensure that checking the condition and setting the condition is atomic, which typically means that the check or set must be within the lock scope.
Is there a race condition during the period that the wait method releases and reacquires the lock? The wait method is tightly integrated with the lock mechanism. The object lock is not actually freed until the waiting thread is already in a state in which it can receive notifications. This would have been difficult, if not impossible, to accomplish if we had needed to implement the wait and notify methods ourselves.
The system prevents any race conditions from occurring in this mechanism. If a thread receives a notification, is it guaranteed that the condition is set correctly? Simply, no. Prior to calling the wait method, a thread should always test the condition while holding the synchronization lock. Upon returning from the wait method, the thread should always retest the condition to determine if it should wait again.
This is because another thread can also test the condition and determine that a wait is not necessary — processing the valid data that was set by the notification thread.
Our animated canvas example is very simple; only one thread is actually waiting. In most programs, many threads are waiting and sending notifications. A race condition exists when multiple threads are waiting for notification. The race condition that is solved internally to the wait-and-notify mechanism prevents the loss of notifications, but it does not solve the following scenario when multiple threads are waiting:.
Thread 1 examines a state flag and determines that the data is not in the desired state. Thread 3 acquires the lock and proceeds to process the data; it sees that the data is in the desired state, so it processes the data and resets the state flag. This is a common case when multiple threads are involved in the notifications. More particularly, the threads that are processing the data can be thought of as consumers; they consume the data produced by other threads.
There is no guarantee that when a consumer receives a notification that it has not been processed by another consumer. As such, when a consumer wakes up, it cannot assume that the state it was waiting for is still valid. It may have been valid in the past, but the state may have been changed after the notify method was called and before the consumer thread woke up.
Waiting threads must provide the option to check the state and to return back to a waiting state in case the notification has already been handled.
This is why we always put calls to the wait method in a loop. Remember too that the wait method can return early if its thread is interrupted. In that case, processing is application-specific, depending on how the algorithm needs to handle the interruption. What happens when more than one thread is waiting for notification? Which threads actually get the notification when the notify method is called? Which thread actually receives the notification varies based on several factors, including the implementation of the Java virtual machine and scheduling and timing issues during the execution of the program.
There is no way to determine, even on a single processor platform, which of multiple threads receives the notification. Another method of the Object class assists us when multiple threads are waiting for a condition:.
Notifies all the threads waiting on the object that the condition has occurred. The notifyAll method is similar to the notify method except that all of the threads that are waiting on the object are notified instead of a single arbitrary thread.
Just like the notify method, the notifyAll method does not allow us to decide which thread gets the notification: they all get notified. When all the threads receive the notification, it is possible to work out a mechanism for the threads to choose among themselves which thread should continue and which thread s should call the wait method again.
Does the notifyAll method really wake up all the threads? Yes and no. All of the waiting threads wake up, but they still have to reacquire the object lock. So the threads do not run in parallel: they must each wait for the object lock to be freed. Thus, only one thread can run at a time, and only after the thread that called the notifyAll method releases its lock. Why would you want to wake up all of the threads? There are a few reasons. For example, there might be more than one condition to wait for.
Since we cannot control which thread gets the notification, it is entirely possible that a notification wakes up a thread that is waiting for an entirely different condition.
By waking up all the threads, we can design the program so that the threads decide among themselves which thread should execute next.
Another option could be when producers generate data that can satisfy more than one consumer. Since it may be difficult to determine how many consumers can be satisfied with the notification, an option is to notify them all, allowing the consumers to sort it out among themselves. I have put main thread to sleep fo 3 seconds giving time for waiter threads to start. Hi Pankaj, Can you please help me to debug it, I am getting the following error:: Exception in thread "main" java. NullPointerException at two.
Very lucid explanation. I visit this site every time I find any concept difficult to understand. Thanks a lot!!! It seems that the syschronized code block will block the thread, and there should be only 1 of the 3 threads fetch the msg object at any time. I have read that we should always use wait in loop. If I am wrong. Please let me know.. Hi Sir, Could you share or refer some code of how wait , join , notify used in project. First and main difference between notify and notifyAll method is that, if multiple thread is waiting on any lock in Java, notify only inform one of waiting thread while notifyAll informs all threads waiting on that lock.
Requirement: i have three threads. Just do the work that you want done by thread 1 followed by the work you want done by thread2 followed by the work you want done by thread 3. Thanks for this article. Really helpful for freshers and experienced people. In your next article please come up with some real time scenarios which you have faced in development in the next article through which increases in clarity of concept.
Excellent examples…. Hi Pankaj, Thank u for ur example program. Instead of calling msg. Could u explain this plz. The two threads waiter1 and waiter2 both are getting the lock on same object that is msg. Please correct this part here. In waiter class,waiter got a lock on msg object using synchronized msg.
Now waiter1 has been started also.. How can waiter1 get the lock again on msg object using synchronized msg when waiter is already holding lock on msg object. When msg. Now when notify method is called in Notifier thread, only one of the thread gets notified and other threads are in wait state. When notifyAll method is called, all the waiter threads are getting notified and then one of them gets the lock on msg and process.
If notified thread is execute first, obviously none of the threads will get notified and program will not terminate. Thanks of explanation. Could you provide example on volatile variable. The program is good but in that on miss take we have to set the wait time in waiter class other wise it will go for idel sleep time. In inter thread communication, all the thread share locks among them and every object has one lock. All these 3 final methods are related to lock and object has a lock. In Java every Object has a monitor and wait, notify methods are used to wait for the Object monitor or to notify other threads that Object monitor is free now.
Your email address will not be published. Prev Java BlockingQueue Example. Pankaj I love Open Source technologies and writing about my experience about them is my passion. Follow Author. Comments Lakshman says:. August 4, at am. Armando says:. July 3, at pm. Sachin Pradhan says:. May 17, at am.
0コメント