Difference between notify and notifyAll in java

In this post, we are going to see difference between notify and notifyall in java.

notify():

When you call notify method on the object, it wakes one of thread waiting for that object. So if multiple threads are waiting for an object, it will wake of one of them. Now you must be wondering which one it will wake up. It actually depends on OS implementation.

notifyAll() :

notifyAll will wake up all threads waiting on that object unlike notify which wakes up only one of them.Which one will wake up first depends on thread priority and OS implementation.
Lets understand it with the help of example:
1. Create a class named File.java:
It is java bean class on which thread will act and call wait and notify method.

2. Create a class named FileReader.java
This thread will wait until other thread call notify method, then after it will complete its processing. It will first take a lock on file object and will be called from synchronized block .So in this example, it will wait for FileWriter to complete the file.

3. Create a class named FileWriter.java
This class will notify thread(in case of notify) which is waiting on file object. It will not give away lock as soon as notify is called, it first complete its synchronized block. So in this example, FileWriter will complete the file and notify it to FileReaders.

4. Create a class NotifyAndNotifyAllMain,java.
This is our main class which will create object of above classes and run it.

In case of notify():
When you run above program, you will get following outputs:

So here,two FileReader threads(reader 1 and reader 2) are waiting for file to be completed,so they called file.wait(). Once FileWriter completes it’s file, it called file.notify() and reader 2 thread gets up and completes its processing.
In case of notifyAll() :
Lets change FileWriter class to call file.notifyAll().

When you run above program, you will get following output:

In case of notifyAll(), it notifies all threads waiting on that object.

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