Thread Signaling in Java

Java has a set of thread signaling features that enable one Java thread to coordinate its actions with another. These basic thread signaling features are provided via the Object class wait() , notify() and notifyAll() methods.
When implementing thread signaling in Java you need to be aware of the problem of missed signals and spurious wakeups. I cover both in this video too.
Chapters:
0:00 Java Thread signaling introduction - wait(), notify() and notifyAll()
1:50 Calling wait(), notify() or notifyAll() must be done from within a synchronized block
2:31 Calling wait() releases the synchronization lock
5:50 Designing a thread signaling class in Java
9:17 Missed signals
17:24 notify() vs. notifyAll()
19:24 Spurious wakeups
Java Thread Signaling - text:
jenkov.com/tutorials/java-con...
Java Concurrency - text:
jenkov.com/tutorials/java-con...
Java Concurrency - playlist:
• Java Concurrency and M...

[@RahulYadav-dd7qe]

This is easily the BEST EVERY PLAYLIST of any tutorial I've ever seen. Trust me when I say this, I've watched ~500-1K tutorial videos over youtube of various kinds, but the satisfaction I got from this playlist is beyond my words. I would've easily paid $100 equivalent in my currency for this.

[@JakobJenkov]

Thank you very much for your kind words !! ... you can always share the playlist with your colleeagues... that would be helpful 😊

[@wassimalsafwi3091]

Outstanding series, Jakob. It was clear and concise, with many examples, yet rich in theory. Thank you very much for your work. I am looking forward to your next video about multithreading, hoping you will include ForkJoin, CompletableFuture, and virtual threads.

[@JakobJenkov]

You are very welcome!! :-) ... I have a few ideas for more videos about Java concurrency - incl. a video about the ForkJoinPool :-)

[@kimo8154]

thank you verry much. believe me i read/watched every single tutorial you made on your website/youtube channel. you are a legend and thank you again, again and again

[@JakobJenkov]

Thank you very much !! :-)

[@garimadhanania1853]

This is the best ever playlist!! I really enjoyed learning these concepts and its a must watch for those using Java concurrency!! Thanks for your efforts Jakob! 💯💯

[@vijaysekharvn]

thank you for the wonderful cource on threads. i have been looking ofr this kind of information on thread as Thread is scariest topic to face in the interview

[@JakobJenkov]

You are welcome ! ... I am just happy my tutorials are useful to people out there ! :-)

[@matthewsaucedo2471]

Excellent series here. Start to finish exceptional explanations, made it easy to binge!

[@JakobJenkov]

Thank you very much !! :-)

[@alexv3312]

Jakob, this entire series is great. I'll gladly recommend it to anyone as an alternative of companion to the Concurrency in Practice book. Well done!

[@JakobJenkov]

Thank you very much! :-)

[@damercy]

I read your tutorials on your website 2 years ago and learned collections from there. Really grateful for such clear, concise and deep explanations. I'm happy to see the video versions of concurrency etc are being published as well. Thank you for these. I'm now a native android developer who just did his bachelors in CSE last year and thanks to you, I am somewhat confident in these concepts (so much to learn yet!). Thank you Jakob. :) I hope to see more videos on this channel as well.

[@JakobJenkov]

You are welcome! I am happy that my tutorials and videos have helped you ! :-)

[@edihoxhalli4885]

Well done! Very informative video especially regarding spurious wakeups.

[@JakobJenkov]

Thanks Edi! :-)

[@jeandy4495]

Thank you for your videos. I'm studying concurrent systems concepts and your website as well as your videos are a perfect complement.

[@JakobJenkov]

Great! I am happy to hear that! :-)

[@tahiriqbal8543]

such an amazing explaination, great stuff

[@JakobJenkov]

Thank you !!

[@arpitjain4239]

Can you please make a video on Semaphores, Mutexes about differences and similarities. 🙏 Your videos are amazing!!

[@JakobJenkov]

Hmm... I have a long list of video topics, and not so much time, unfortunately ! ... so I am not sure if I can get around to it! ... but who knows! Maybe ! :-)

[@RahulKumar-ed1bt]

Best explanation ever

[@JakobJenkov]

Thanks :-D

[@wujasmarecki]

Perfect! Thank you.

[@JakobJenkov]

You are welcome! :-)

[@adithyabhat4770]

Thanks for the series

[@JakobJenkov]

You are welcome! :-)

[@manoj.tyg01]

Nice explanation!!

[@JakobJenkov]

Thank you! :-)

[@Theonadj]

Awesome plz explain more advance concepts here

[@Hrishikeshdarshan]

Very helpful

[@JakobJenkov]

Thanks :-)

[@leejeam851]

thank you.

[@JakobJenkov]

You are welcome :-)

[@garimadhanania1853]

could you please share some usecases where a thread would need to wait on an object? The only vague thing I can think of is that when a thread is waiting for some data that is probably going to be written to the signalObject by the notifier..kind of like future

[@JakobJenkov]

wait() is used e.g. when a thread is waiting to take some elements from a BlockingQueue that is currently empty. Then the thread waits until another thread puts some elements into the queue and calls notify() .

[@polacofederos]

nice tutorials!!! just wondering, it would be "beneficial" that flag variables be "volatile" ?

[@JakobJenkov]

Only if accessed by more than one thrread.

[@rm6267]

In this case 12:47 it's not necessary to used atomic type in signalRaised because is synchronized all the class with synchronized(this) {}?

[@JakobJenkov]

Inside a synchronized block you do not need to use atomic types, nor volatile variables. However, then all access to these variables (fields) should be within synchronized blocks.

[@garimadhanania1853]

question: if say more than 1 thread called wait and then 1 called notify(), how is it decided which one to notify?

[@JakobJenkov]

There is no guarantee about which waitinf thread gets notified with a notify() call. This can result in "Thread Starvation" . The solution is to implement "fairness" among the waiting threads.

[@arpitjain4239]

What is different between a Condition Variable and Any Other Object when used for signalling?

[@JakobJenkov]

What do you mean by "condition variable" ?

[@arpitjain4239]

@@JakobJenkov I mean't using Condition Interface in Locks.

[@AMITTIWARI-re9sg]

Great video, But I have one issue with below code - Where when my Female thread called first my male thread is keep on waiting. I don't want a signal to be missed nor I want my thread to keep on waiting. import java.util.ArrayList; import java.util.List; /* Objective is to use two threads for stundent list and each thread will responsible for doing the needful based on Gender In below example I have just used to print the name of male and fémale student with the help of different thread. Another important aspect is both thread are communicating with standard wait and notify mechanism and the lock is on same signal object */ public class MultiThread { static List studentList = new ArrayList(); private boolean isThreadWaiting = false; private boolean signalRaised = false; public static void main(String[] args) throws InterruptedException { Student s1= new Student("Amit", "male"); Student s2= new Student("Amita", "female"); Student s3= new Student("Chandan", "male"); Student s4= new Student("Amrita", "female"); Student s5= new Student("Ajay", "male"); Student s6= new Student("Chandana", "female"); studentList.add(s1); studentList.add(s2); studentList.add(s3); studentList.add(s4); studentList.add(s5); studentList.add(s6); Object signalObject = new Object(); MultiThread multiThread = new MultiThread(); Runnable runnable=() -> { synchronized (signalObject) { System.out.println("Calling " + Thread.currentThread()); try { if (multiThread.signalRaised) { multiThread.signalRaised=false; return; } System.out.println(Thread.currentThread().getName() + " going to wait"); multiThread.isThreadWaiting=true; signalObject.wait(); multiThread.isThreadWaiting=false; System.out.println(Thread.currentThread().getName() + " wait ended"); } catch (InterruptedException e) { e.printStackTrace(); } studentList.stream().sorted().filter(student -> student.getGender().equals("male")).forEach(student -> System.out.println(student)); } }; Runnable runnable2=() -> { synchronized (signalObject) { System.out.println("Calling "+Thread.currentThread()); studentList.stream().sorted().filter(student -> student.getGender().equals("female")).forEach(student -> System.out.println(student)); if(!multiThread.isThreadWaiting) { multiThread.signalRaised=true; } signalObject.notify(); System.out.println(Thread.currentThread().getName()+" After notify existing synchronized block"); } }; startThreads(runnable, runnable2); } private static void startThreads(Runnable runnable, Runnable runnable2) { Thread t1= new Thread(runnable, "Male Thread 1"); Thread t2= new Thread(runnable2, "Female Thread 1"); t2.start(); t1.start(); } }

[@ivib570]

Hello Jakob, you upload this to github??? thank you :)

[@JakobJenkov]

I just did ! :-) ... it's now part of my Java Examples repository! github.com/jjenkov/java-examples

[@bruceliebewilma]

Why so complex so? If one solution works, then I think pp would interest in the working solution...

[@JakobJenkov]

I teach principles and techniques - bit final answers 😉