The Object class in java contains three final methods that allows threads to communicate about the lock status of a resource. These methods are wait(), notify() and notifyAll(). So today we will look into wait, notify and notifyAll in java program.
wait, notify and notifyAll in Java
The current thread which invokes these methods on any object should have the object monitor else it throws java.lang.IllegalMonitorStateException exception.
wait
Object wait methods has three variance, one which waits indefinitely for any other thread to call notify or notifyAll method on the object to wake up the current thread. Other two variances puts the current thread in wait for specific amount of time before they wake up.
notify
notify method wakes up only one thread waiting on the object and that thread starts execution. So if there are multiple threads waiting for an object, this method will wake up only one of them. The choice of the thread to wake depends on the OS implementation of thread management.
notifyAll
notifyAll method wakes up all the threads waiting on the object, although which one will process first depends on the OS implementation. These methods can be used to implement producer consumer problem where consumer threads are waiting for the objects in Queue and producer threads put object in queue and notify the waiting threads. Let’s see an example where multiple threads work on the same object and we use wait, notify and notifyAll methods.
Message
A java bean class on which threads will work and call wait and notify methods.
package com.journaldev.concurrency;
public class Message {
private String msg;
public Message(String str){
this.msg=str;
}
public String getMsg() {
return msg;
}
public void setMsg(String str) {
this.msg=str;
}
}
Waiter
A class that will wait for other threads to invoke notify methods to complete it’s processing. Notice that Waiter thread is owning monitor on Message object using synchronized block.
package com.journaldev.concurrency;
public class Waiter implements Runnable{
private Message msg;
public Waiter(Message m){
this.msg=m;
}
@Override
public void run() {
String name = Thread.currentThread().getName();
synchronized (msg) {
try{
System.out.println(name+" waiting to get notified at time:"+System.currentTimeMillis());
msg.wait();
}catch(InterruptedException e){
e.printStackTrace();
}
System.out.println(name+" waiter thread got notified at time:"+System.currentTimeMillis());
//process the message now
System.out.println(name+" processed: "+msg.getMsg());
}
}
}
Notifier
A class that will process on Message object and then invoke notify method to wake up threads waiting for Message object. Notice that synchronized block is used to own the monitor of Message object.
package com.journaldev.concurrency;
public class Notifier implements Runnable {
private Message msg;
public Notifier(Message msg) {
this.msg = msg;
}
@Override
public void run() {
String name = Thread.currentThread().getName();
System.out.println(name+" started");
try {
Thread.sleep(1000);
synchronized (msg) {
msg.setMsg(name+" Notifier work done");
msg.notify();
// msg.notifyAll();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
WaitNotifyTest
Test class that will create multiple threads of Waiter and Notifier and start them.
package com.journaldev.concurrency;
public class WaitNotifyTest {
public static void main(String[] args) {
Message msg = new Message("process it");
Waiter waiter = new Waiter(msg);
new Thread(waiter,"waiter").start();
Waiter waiter1 = new Waiter(msg);
new Thread(waiter1, "waiter1").start();
Notifier notifier = new Notifier(msg);
new Thread(notifier, "notifier").start();
System.out.println("All the threads are started");
}
}
When we will invoke the above program, we will see below output but program will not complete because there are two threads waiting on Message object and notify() method has wake up only one of them, the other thread is still waiting to get notified.
waiter waiting to get notified at time:1356318734009
waiter1 waiting to get notified at time:1356318734010
All the threads are started
notifier started
waiter waiter thread got notified at time:1356318735011
waiter processed: notifier Notifier work done
If we comment the notify() call and uncomment the notifyAll() call in Notifier class, below will be the output produced.
waiter waiting to get notified at time:1356318917118
waiter1 waiting to get notified at time:1356318917118
All the threads are started
notifier started
waiter1 waiter thread got notified at time:1356318918120
waiter1 processed: notifier Notifier work done
waiter waiter thread got notified at time:1356318918120
waiter processed: notifier Notifier work done
Since notifyAll() method wake up both the Waiter threads and program completes and terminates after execution. That’s all for wait, notify and notifyAll in java.
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About the author
Java and Python Developer for 20+ years, Open Source Enthusiast, Founder of https://www.askpython.com/, https://www.linuxfordevices.com/, and JournalDev.com (acquired by DigitalOcean). Passionate about writing technical articles and sharing knowledge with others. Love Java, Python, Unix and related technologies. Follow my X @PankajWebDev
Still looking for an answer?
Why wait(),notify(),notifyAll() are in object class?please reply…Thanks
- Nitya
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. so in that we have to set the time.
- sandeep
i have a doubt 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
- Ruhina
Thanks sir…it was realyyyyyy helpful…god bless u for ur effort
- Jaykishan
Excellent examples… continue more on collections…if you have blogs already please give link here Thanks & Regards, Vignesh Kanna M S/w Eng, Chennai, India.
- Vignesh Kanna M
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.
- Nanda
Hi pankaj and All, i have one requirement in threads. Requirement: i have three threads.so how to make sure that one thread is exeucted after another.ie thread1 followed bye thread two =>is followed by threade three. please help me
- chandu
how to make sure that one thread is executed onther(multiple threads).
- chandu
Thanks a lot for your great job share java to us. In this article, Other two variances puts the current thread in ‘wait’ for specific amount of time before ‘they’ wake up. Maybe it should be ‘it’ wake up? Or puts the current thread in ‘wait sets’? Looking forward to your response！
- byboating
Hi Sir, Could you share or refer some code of how wait , join , notify used in project . even though i have go through the tutorials and understand what you said, however, i have no idea how to put these into my project.
- Paul Yang
Thankyou for the tutorial. I have read that we should always use wait in loop ., to prevent false notification . If I am wrong .Please let me know…
- rajesh
I am confused that waiter and notifier all use “sychronized” on the same “msg” object. 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. But, in fact things don’t go on as my imagination, and 3 threads can reach synchronized code cocurrently… please explain this, thansk a lot~
- lz
Very lucid explanation. I visit this site every time I find any concept difficult to understand. Thanks a lot !!!
- Vidita
package two; public class balance { int currentBalance; public balance(int currentBalance){ this.currentBalance=currentBalance; } public void add(int currentBalance){ this.currentBalance=currentBalance; currentBalance++; } public void deduct(int currentBalance){ this.currentBalance=currentBalance; currentBalance–; } public int showBalance(){ return currentBalance; } } ---------------------------------------------------------------------------------------------------------------------------------------- package two; public class waiter implements Runnable{ private balance bal; int currentBalance= bal.showBalance(); public waiter(balance bal){ this.bal= bal; } @Override public void run() { synchronized (bal){ String name= Thread.currentThread().getName(); System.out.println(name); System.out.println(bal.showBalance()); System.out.println(“in waiter thread waiting for notification”); try { bal.wait(); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } System.out.println(“waiting over now processing”); for (int i=0;i<10;i++){ bal.add(currentBalance); } }} } ----------------------------------------------------------------------------------------------------------------------------------------- package two; public class notifier implements Runnable { private balance bal; int currentBalance= bal.showBalance(); public notifier(balance bal){ this.bal= bal; } @Override public void run() { synchronized (bal){ String name= Thread.currentThread().getName(); System.out.println(name); System.out.println(bal.showBalance()); System.out.println(“in notifier thread”); for (int i=0;i<10;i++){ bal.add(currentBalance); } bal.notifyAll(); } } } --------------------------------------------------------------------------------------------------------------------------------------- package two; public class balcheck { public static void main(String args[]) { balance bal = new balance(10); Thread notifiere= new Thread(new notifier(bal)); notifiere.start(); Thread waiterre= new Thread(new waiter(bal)); waiterre.start(); } } ----------------------------------------------------------------------------------------------------------------------------------------- Hi Pankaj, Can you please help me to debug it, I am getting the following error:: Exception in thread “main” java.lang.NullPointerException at two.notifier.(notifier.java:5) at two.balcheck.main(balcheck.java:6)
- Rahul
Hi This is good example explaining thread communication.You can make it better if you introduce pause in main method after creating waiter thread because when I tried your example main thread was creating notifier thread before waiter thread. Here is new main method .I have put main thread to sleep fo 3 seconds giving time for waiter threads to start. public class WaitNotifyTest { public static void main(String[] args) { Message msg = new Message(“process it”); Waiter waiter = new Waiter(msg); new Thread(waiter, “waiter”).start(); Waiter waiter1 = new Waiter(msg); new Thread(waiter1, “waiter1”).start(); // Allow waiter threads to start try { Thread.sleep(3000); } catch (InterruptedException e) { e.printStackTrace(); } Notifier notifier = new Notifier(msg); new Thread(notifier, “notifier”).start(); System.out.println(“All the threads are started”); } }
- Rishabh Kashyap
how come another waiter thread enter the synchronized block,if already a waiter thread is waiting in that block? please explain I have tested with other examples and all other threads seem to wait to enter the synchronized block when one of the threads is in synchronized block and the new thread enters the synchronized block right after the old one leaves the block
- Sriharsha
‘msg’ field in Waiter and Notifier are different instances. How come notify call on ‘msg’ of Notifier will notify ‘msg’ of Waiter?
- Niraj
There is one edge case scenario though in above example. If notifier calls notify(), it will wake up single waiter thread, either waiter or waiter2. We must call notifyAll() in such cases or the other thread will keep waiting.
- Anirudh Kanth
Nice. Never marked so well the difference between notify() & notifyAll()
- Aditya Rewari
Which thread will be invoked after notify and notifyAll is this details stored anywhere if yes then where?
- Bhavesh Vani
This will not work all the time. If the notifier thread get the lock first, its notify call will be ignored and other threads will always be waiting.
- Java Dev
Hi Pankaj, Can you also please explain the usage of Condition methods in place of wait(), notify() and notifyAll()?
- Mahdavi
Hey, still on the Notify exaple (notiftyAll commented out). How can I check something like “if one of them is stuck, release it”? I tried the following: if (t1.isAlive() ^ t2.isAlive) // one of them is alive and the other finished msg.notify(); But that doesn’t really do anything. (What I’m asking is, how to terminate the program “safely” because we know for sure some thread is stuck. Also, is this a case of starvation?)
- Yoad
Hi Sir, My question is very simple Why “wait” method is there in object class bcz of we are mostly using in wait method in thread class and is there any reason apart from thread u can also use "wait"method in other place then please write the few code without using the thread concept
- pratap singh
Example public class Data { int msg; boolean valueSet=false; public synchronized void get() { try {Thread.sleep(3000);}catch(Exception e) {} while(!valueSet) { try {wait();}catch(Exception e) {} } System.out.println(“Consume :”+msg); valueSet=false; notify(); } public synchronized void set(int msg) { try {Thread.sleep(3000);}catch(Exception e) {} while(valueSet) { try {wait();}catch(Exception e) {} } this.msg=msg; System.out.println(“Produce :”+msg); valueSet=true; notify(); } } -------------------------------------------------------------------------------- public class Producer implements Runnable{ Data d; Producer(Data d){ this.d=d; new Thread(this,“Producer”).start(); } public void run() { int i=1; while(true) { d.set(i++); } } } ------------------------------------------------------------------------ public class Consumer implements Runnable { Data d; boolean ValueSet=false; Consumer(Data d){ this.d=d; new Thread(this,“Consumer”).start(); } public void run() { while(true) { d.get(); } } } ------------------------------------------------------------- public class InterThread { public static void main(String[] args) { // TODO Auto-generated method stub Data d=new Data(); new Producer(d); new Consumer(d); } }
- sagar
Hello Sir, Your content and the way of explaining the things is awesome. I like everything of this tutorial except that why there are this much ads in this website? Like it’s very annoying. Also it slow down the speed.
- Sanjana Chourishi
Journal Dev has been always the best portfolio for me to learn and brush up concepts with hands on example in a simpler and easy way.Thank You So so much.
- Sachin Pradhan
Hi, Does any one know why if you include a println statement in the run method of the waiter class before the synchronized block like in the notifier class the thread keeps blocked?, for example: public void run() { String name = Thread.currentThread().getName(); System.out.println(“Test”); //------------------------------------> This one keeps the thread blocked. synchronized (msg) { try{ System.out.println(name+" waiting to get notified at time:"+System.currentTimeMillis()); msg.wait(); }catch(InterruptedException e){ e.printStackTrace(); } Thanks!!!
- Armando
Hi - it might be late query. I have tried test class as below and it is not notifying even single waiter thread. If i copy samea above client test code ,it is working. Please help somebody why it is not able to notify new Thread(new Waiter(new Message(“Process It”)),“WaiterThread1”).start(); new Thread(new Waiter(new Message(“Process It”)),“WaiterThread2”).start(); new Thread(new Notifier(new Message(“Process It”)),“NotifierThread”).start(); System.out.println(“ALL THREADS ARE STARTED”);
- Lakshman
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