13- The volatile Keyword

Jul 03, 2026 22:33 · 6:33 · English · Whisper Turbo · 2 اسپيڪر

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In Java,

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we have another tool for writing thread -safe code,

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but without the overhead of synchronization.

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It's the volatile keyword.

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It solves the visibility problem,

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but not the race condition.

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So it doesn't prevent two threads simultaneously modifying some

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data.

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Instead, it ensures that if one thread changes some data,

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other threads can see the changes.

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Let me show you a real example.

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So here in the download status class,

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let's add a new field.

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Private Boolean is done.

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With this flag,

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we can tell if you have downloaded all the files or not.

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Now let's create a getter and a setter for this field.

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So Alt and Enter,

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create getter and setter.

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Now,

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I prefer to rename the setter to

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done and remove this parameter.

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So here we explicitly set this field to true.

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We're essentially sending the done message to this object and let this

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object take care of setting its status.

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Now, in our demo class,

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we want to create two threads.

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One thread simulates downloading a file,

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and the other checks to see if the download is finished.

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When the download is finished,

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we want to report something to the user.

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So,

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first let's create a download status object,

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download status.

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Now we create our first thread,

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so thread one,

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we set this to a new thread with a new instance of our download

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file task,

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and pass our status object.

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Now,

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Back to our download file task.

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I want to run this loop for 1

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million times because I don't want this to finish so quickly.

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Now,

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after this loop,

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we're going to call status .don.

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We're sending the don message to our status object.

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Now,

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back to our demo class.

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We create our second thread,

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so var thread2.

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We create a new thread.

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Now this time,

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I want to pass a lambda expression to represent a runnable

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object.

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We could also create a separate class that implements the runnable interface,

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but I want to change things up for a little bit here.

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So,

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you pass a lambda expression for the runnable interface.

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That's basically a function with no parameters that

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returns void.

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Now,

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in the body of this lambda,

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we should continuously ask this status object if it's done or not.

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So, we had a while loop

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We ask the status object.

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Are you done?

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As long as it's not done,

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we're going to wait.

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So here we have a while loop with an empty body.

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We're just waiting.

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The moment our download is finished,

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you're going to print status dot get total bytes.

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Now,

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let's start both these threads.

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So thread one dot start and thread two dot

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start.

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Now run our program.

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Thread 1 started and finished,

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but thread 2 is still going.

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In fact, it's going to go forever.

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Why is this happening?

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Well, let me stop the program first.

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Back to our download status class.

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Earlier we used the synchronous keyword in two places

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in this class.

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So how come the synchronous keyword didn't help us here?

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Well,

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the synchronous keyword is not a silver bullet.

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In this particular case,

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we want to make sure that only one thread at a time can update this field.

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But here we're dealing

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with another field.

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These are two separate fields.

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Now,

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if you want to solve the problem in this demo,

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we should mark these two methods as synchronized.

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I told you that this is a bad practice.

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You should prefer to use the synchronized block as opposed to declaring an entire method

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synchronized,

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but let's not worry about the problem with this approach and see if the synchronized

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keyword solves the problem.

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Now,

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run the program.

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So, thread 1 started,

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it finished,

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thread 2 got notified and printed the total number of bytes.

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So the problem is fixed.

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Now,

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back to our demo class.

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Even though the synchronous keyword solved this problem,

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it can cause a lot of overhead,

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because in this while loop,

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we're constantly calling the isDone method.

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While we're calling this method,

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no other threads can do anything else with this status object.

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They cannot call any other methods in this object.

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So all these method calls will run in sequence.

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That will show you a better way.

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So,

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back to our download status,

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let's remove the synchronous keyword.

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Back to the

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original problem.

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When we run this program,

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the second thread waits indefinitely because

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it doesn't see the change to the is done field.

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This is the visibility problem.

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So one thread changes some data,

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the other thread cannot see the change.

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But why does this happen?

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Well,

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the Java virtual machine makes some optimizations under the hood to make our code

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run faster.

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One of these optimizations is caching values.

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So let's say we have an integer field with the value 1.

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This value is stored in the main memory or RAM.

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Now,

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we have two threads running by two different CPU cores.

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Each CPU has a cache,

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which is a small amount of memory available locally in that CPU.

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Reading the data from the cache is faster because the data is closer to

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the CPU.

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So it doesn't have to travel far.

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It doesn't have to travel between the CPU and the main memory.

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Okay.

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Now,

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this is what happens.

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Two threads read the value of a field and store it locally.

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Now,

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the first thread changes its value,

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but this change is only local to this thread.

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So the second thread doesn't see the change.

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Even if it writes the change back in the memory,

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the second thread doesn't see the change because it already has the value

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of this field in its cache.

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This is the visibility problem.

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To solve this problem without the overhead of synchronization,

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we can declare this field as volatile.

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So we type volatile.

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volatile means unstable.

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So we're telling the Java virtual machine,

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hey, this field is unstable.

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It may change.

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So don't rely on the value stored in the cache.

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Always read it from the main memory.

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And that means if another thread updates the value of this field,

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the change will be immediately written to the main memory.

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Now let's run a program and make sure the problem is solved.

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So,

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run.

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There you go.

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So the volatile keyword guarantees that the changes to a field

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is visible across threads.

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