11- Locks

Jul 03, 2026 15:09 · 3:17 · English · Whisper Turbo · 2 Altaveus
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S… Speaker 1 (11- Locks)
Another strategy for preventing race conditions and visibility problems
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is to prevent multiple threads from accessing an object at the same time.
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This is called synchronization.
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So we have to synchronize or coordinate multiple threads trying to
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access an object.
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We do that using locks.
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So we put a lock on a block of our code and the Java virtual machine will
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ensure that only one thread at a time can execute that block of code,
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which is called the critical section.
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In reality,
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our code is going to get executed sequentially,
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so we lose concurrency.
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As a metaphor,
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think of a hotel room.
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Only one guest at a time can be in that room.
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When a guest goes in,
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they lock the door so others cannot enter.
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They have to wait until that guest checks out.
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Using locks in code is pretty much the same.
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We have the same concept in databases as well.
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I talked about this in my Ultimate SQL course.
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So, we have the same code as before.
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We are sharing a single status object across many
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download tasks.
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If we run this program,
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we have a race condition because every time we see
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a different value.
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S… Speaker 2 (11- Locks)
So,
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multiple threads are racing to update the total bytes field.
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S… Speaker 2 (11- Locks)
Now,
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let's see how we can use a log to solve this problem.
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S… Speaker 2 (11- Locks)
So,
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back to our download status class,
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we want to ensure that only one thread at a time can increment this field.
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So here we declare a private field of type lock.
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This is an interface declared in the java .util .concurrent
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package.
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So lock,
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we call it lock,
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and set it to a new reentrant lock.
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This is one of the implementations of the lock interface.
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Now,
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in our increment method,
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before we increment this field,
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we call lock .lock.
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So we call the lock method to lock this lock object.
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I know it sounds weird.
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Then we increment the total bytes fill and finally we
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call lock that unlock.
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So when a thread enters this method,
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it will lock this lock object.
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S… Speaker 1 (11- Locks)
It's like a guest going to a hotel room and locking the door.
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Other guests or other threads have to wait for this guest to
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come out.
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S… Speaker 1 (11- Locks)
This is how locking works.
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Now as a best practice,
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we should execute this line in a finally block because if an exception is
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thrown, we don't want to keep this lock locked forever.
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Otherwise,
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no other threats can execute this code.
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And this may cause a deadlock and our application will crash.
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So let's add a try block here.
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We execute the main logic inside the try block.
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S… Speaker 1 (11- Locks)
If this causes an exception,
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but the finally block will ensure
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that we always unlock this lock.
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Now in this particular case,
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we don't really need the try finally block because incrementing this field is not
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going to throw an exception.
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But in your applications,
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you might have more complex logic and somewhere along this logic,
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you might get an exception.
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So make sure to unlock this lock inside of finally block.
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S… Speaker 2 (11- Locks)
Now,
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let's run our program and see the result.
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So we get 100 ,000.
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Let's run it one more time.
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S… Speaker 2 (11- Locks)
There you go.
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So, this is how we can use locks to prevent concurrency problems.

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