11- Locks

Jul 03, 2026 15:09 · 3:17 · English · Whisper Turbo · 2 Speakers

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Lecture Index

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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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Now,

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let's see how we can use a log to solve this problem.

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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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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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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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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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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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There you go.

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So, this is how we can use locks to prevent concurrency problems.

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