5- Joining a Thread

Jul 03, 2026 14:41 · 2:27 · English · Whisper Turbo · 2 اسپیکر

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ليکشن

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Let's say once we download a file we want to kick off another thread to scan the

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downloaded file for viruses.

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Now scanning a file is a time -consuming task It's not going to happen immediately

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like incrementing an integer.

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It may take a few seconds So we should run it in a separate thread But

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we should do that only after we have downloaded a file.

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Here's the thing.

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How can we know if the download thread has finished?

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downloading a file may take a second,

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a minute, or longer.

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We cannot predict it.

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So here we cannot use thread .sleep to make the current thread sleep

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for, let's say,

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five seconds,

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because the download may take longer.

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To solve this problem,

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we should join threads.

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So after we start the download thread,

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we call thread .join,

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and this will make the current thread,

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that is the main thread that is executing this code,

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wait for the completion of the download thread.

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So this call is going to block the current thread until the download thread has

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finished.

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Once this method returns,

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then we can print a message like file is ready to

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be scanned.

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Now,

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just like the sleep method,

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the join method throws an interrupted exception,

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so we should handle it here.

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Let's wrap it with a try -catch block.

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Alright,

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so we start a download thread.

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then we join the current thread with that thread,

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so this will make the current thread wait for the completion of that thread,

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and then we print this message.

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Let's run the program,

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so download started,

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now 1,

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5 seconds later,

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download is complete,

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and we see that the file is ready to be scanned.

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So with the join method,

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we tell the current thread to wait for the completion of another thread,

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and that means,

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while this thread is waiting,

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it cannot do other things.

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For example,

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in the case of a desktop or a mobile app,

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this main thread is responsible for handling UI events like mouse

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clicks and keystrokes.

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So if you make the main thread wait for another thread,

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it cannot respond to these UI events while it's waiting.

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So the UI will freeze and the user will not be able to resize or move

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the application window.

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But don't worry,

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later I will show you a better way to have one task start after another

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task finishes without causing the current thread to wait.

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For now I just want you to understand what joining is and how it works.

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