14- Thread Signalling with wait() and notify()

Jul 03, 2026 22:35 · 3:35 · English · Whisper Turbo · 2 Puheenvuorot

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Luennon hakemisto

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Sometimes we want one thread to wait until some condition becomes true.

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For example,

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over here, the second thread has to wait until the done field becomes

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true.

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Now we're implementing this using a while loop with an empty body.

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The problem with this implementation is that it wastes CPU cycles.

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Because this while loop is continuously running until the done field becomes

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true. It may run millions or even trillions of times.

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So it wastes CPU cycles.

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Now we can solve this problem by using the wait and notify method.

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So every object in Java has this wait and notify method.

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Let me show you.

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So here if you type status dot,

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take a look.

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every object has these methods notify all notify and

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wait. These methods are inherited from the object class,

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which is the parent of all classes in Java.

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So here we call status dot wait.

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And this will make this thread go to sleep until another thread notifies

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this thread that the state of the status object is changed.

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So when we call the wait method,

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this while loop is not going to run a million times.

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In the first iteration,

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this wait method is going to put this thread into sleep until another

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thread wakes it up.

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Now here we need to handle the uninterrupted exception just like before.

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So let's wrap it inside a try catch block.

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So this is the body of our while loop.

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Now the Java virtual machine expects us to call the wait method

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inside a synchronized block.

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So we need to wrap this inside the synchronized block and

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we're going to use the status object as our monitor.

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Now let's move the try catch block over here.

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Good.

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So this is one part of the story.

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Now we need to go to our first thread in our download

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file task class.

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After we call status dot done,

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we need to notify the other thread that the state of this object is

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changed.

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So here we call status dot notify.

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We also have notify all.

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This is useful if you have multiple threads waiting for a change on this

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object.

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With this,

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we can notify them all.

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Now,

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just like the wait method,

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we need to wrap this line inside a synchronized block.

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Otherwise, the Java virtual machine is going to throw a runtime exception.

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So let's add

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a synchronized block.

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We're going to use the status object as the monitor and then

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call the notify all method.

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Now,

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with this implementation,

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the moment the second thread gets started,

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this while loop is going to start and in the first iteration,

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the wait method is going to put this thread into a sleep.

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So the while loop is not going to get executed a million times.

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Now, the moment the first thread finishes its job,

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it's going to notify the second thread.

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So the second thread is going to wake up and then it's going to print the

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total bytes.

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Let's take a look.

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There you go.

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So this is a more efficient implementation than a wide loop with an empty body that

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runs a million times.

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Having said that,

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wait and notify can get pretty tricky in complex applications.

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If you don't use them properly,

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you're going to run into all sorts of weird issues.

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So I would say don't use them in new code.

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These days we have better tools for implementing this kind of communication

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between threads.

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We'll look at them in the next section.

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I just wanted to briefly talk about wait and notify methods in case you

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have seen them before and you were curious.

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what they are and how they work.

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