8- Race Conditions

Jul 03, 2026 14:45 · 5:53 · English · Whisper Turbo · 2 ஒலிப்பேழைகள்

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Let's say as part of downloading multiple files,

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we want to show the total number of bytes we have downloaded so far.

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So we need to store the total value somewhere and have multiple threads

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incremented as they are downloading files.

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This is going to cause a race condition which means multiple threads

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racing or competing to modify a shared resource.

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Let me show you.

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So I'm going to add a new class in this project.

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We call it download status.

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Here we need a field for storing the total number of bytes we have downloaded.

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So private integer total bytes Now

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let's create a getter for this field.

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So we put the caret on the field name press alt and enter and Create

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a getter.

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We also need a method for incrementing this field.

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So instead of adding a setter I would prefer to add an increment method

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So each thread can call this to increment our field.

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It prevents us from accidentally resetting the total bytes in a thread

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So, public void increment total bytes,

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and here we type total bytes plus plus.

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Now we want all our download threads to report to a single download

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status object.

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So,

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in our demo class,

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let's create a download status object,

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new download status.

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Now we start 10 download threads.

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So,

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let's add a for loop for i equals 0,

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10 times plus plus.

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Here we create a thread,

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new thread.

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Here we pass a new download file task.

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We start the thread.

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Now,

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we should pass this status object to each download file task.

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So,

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let's pass it here.

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Now,

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we let IntelliJ create a constructor for receiving this value.

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So,

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we press alt and enter.

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Create constructor.

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There you go.

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Pretty easy.

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Now,

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we should create a fill to store this status object.

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So, we press alt and enter here one more time.

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and create a field for parameter status.

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There you go.

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So here's our field and we're setting it in the constructor.

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Now let's change the run method a little bit.

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Instead of integer dot max value,

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let's use 10 ,000.

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So we want to simulate a scenario where each file is 10 ,000

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bytes.

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We can also add an underline here.

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This makes our code cleaner and more readable.

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So in each iteration,

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we call status dot increment total bytes.

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Now, we don't need this message anymore,

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so let's simplify our code.

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Now back to our demo class.

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We are starting 10 download threads and sharing a single

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status object across these threads.

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Now,

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once all these threads are complete,

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we should print the total number of bytes we have downloaded.

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So we have to wait for all these threads to finish.

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Now,

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we cannot call thread the join

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because this will make the main thread wait for each download to finish before starting

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another download.

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Because this join method is a blocking method.

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So in the first iteration,

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we create a thread,

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we start it,

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and then we wait for that thread to finish before going to the second iteration to create a

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second thread.

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So we can use the join method here.

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We should start all these threads simultaneously and then join

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with all of them.

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So let's declare a list of threads,

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a list of thread,

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we call it threads and set it to a new array list.

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Now,

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every time we start a thread,

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we add it to our list.

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So threads .add thread.

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Now here we need another for loop to

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iterate over all these threads and join with them.

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So, for thread in threads,

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we simply call thread .join.

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Now once again,

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we should handle the interrupted exception,

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so let's do it real quick.

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So with this for loop,

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we can wait for all these download threads to finish.

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Then we can print the total number of bytes we have downloaded.

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So we call status dot get total bytes.

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So here we have 10 download tasks and

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each task is going to download 10 ,000 bytes.

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So when I run this program,

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we expect to see 100 ,000 bytes,

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but that's not going to happen.

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Let me show you.

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So run.

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So we saw 78 ,000.

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If I run it again,

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we see a different number,

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72 ,000.

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One more time,

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71 ,000.

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This is a race condition in action because multiple threads are

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racing or competing to update the total number of bytes.

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Now let me explain what happens under the hood.

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So back to our download status class,

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look at this increment operator here.

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This operation involves three steps.

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So even though we have only one line of code,

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there will be three steps happening under the hood.

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First,

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the value of this field has to be read from the main memory and stored

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in the CPU.

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Next,

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the CPU is going to increment this value and then the updated value is going

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to be stored in the memory.

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So we have three steps and we call this a non -atomic

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operation because it involves multiple steps.

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In contrast,

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an atomic operation is like an atom.

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We cannot break it down into many steps.

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Now,

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imagine two threads trying to call into this method at the same time.

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Let's say the value of this field is zero.

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Both these threads will read this value concurrently.

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They both increment it and write it to the memory.

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So the result would be one instead of two.

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This is how we lose an update.

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In the next video,

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we'll be looking at various strategies to prevent these problems.

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