3- Executors

Jul 03, 2026 22:57 · 6:50 · English · Whisper Turbo · 2 Овозли эшиттиришлар

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In Java,

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the concept of a thread pool is represented using the executor service interface

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and its implementations.

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So we have thread pool executor,

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which is a typical thread pool implementation.

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This is the one that we use most of the time.

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We also have scheduled thread pool executor.

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With this,

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we can schedule tasks to run after a delay or periodically.

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For example,

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we can schedule a task to run five hours from now or every two hours.

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We also have fork join pool.

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This is a special type of pool that is designed to recursively split a task

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into smaller tasks and then combine the results of each subtask to

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produce the overall result.

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It's like a divide and conquer algorithm.

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Now let me show you how to create a thread pool.

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We have this executors class that is declared in the java

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.util .concurrent package.

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This class has a bunch of static factory methods for creating

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an executor service.

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So with these methods we can create an instance of these implementations.

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Now, why shouldn't we create a new instance of thread pool executor

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directly?

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We can definitely do that,

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but it's a little bit difficult.

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Let me show you.

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So if you type new thread pool executor,

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take a look.

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This constructor has a bunch of parameters like core pool size,

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maximum pool size,

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keep a lifetime,

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and so on.

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So creating an executor explicitly is a little bit difficult.

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That is why we use the factory methods on the executors class.

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So here we can call new single thread executor.

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This returns an executor with a single thread.

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That's something that we use that often.

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We also have new fixed thread pool.

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This will create a thread pool with a given number of worker threads.

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So that would be an instance of the thread pool executor

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class.

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We also have new schedule thread pool.

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This will return

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instance of the scheduled thread pool executor.

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Okay, so let's call new fixed thread pool

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with two worker threads.

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Now we have stored a result in a variable called executor.

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Now look at the type of this variable.

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That is executor service.

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So we're dealing with an interface here.

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At runtime,

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the type of this object is going to be thread pool executor.

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Let me show you.

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So if you print executor

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.git class

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but get name and run the program we see

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thread pool executor that is a thread pool with a number of worker threats.

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Okay, so we have an executor

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now we can call executor .submit

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to submit a task to this thread pool.

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Now this method is overloaded.

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We can pass a runnable object.

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We can also pass a callable.

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That is a task that returns a result.

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We'll look at that later.

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So for now,

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let's pass a runnable object here.

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I'm going to use a lambda expression.

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So here,

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let's print the name of the current thread.

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So thread .currentThread .getName

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Now technically we don't need the braces here because we have a single print line statement,

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but I'm going to keep them here for clarity because I don't want to put this line over

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here, otherwise it's going to pop out of the screen.

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So that's all we have to do to run a task on a separate

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thread.

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If you run this program,

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you can see our task was executed on this

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thread. Pool one,

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thread one.

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So we didn't have to explicitly create a thread.

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Now if you have a thousand tasks,

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we don't have to worry about creating too many threads and running out of memory.

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We simply submit those tasks to this executor,

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to this thread pool,

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and this pool will assign our tasks to work your threads.

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Let me show you.

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So here we have a pool with two threads.

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Let's add a for loop for i,

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we start from 0 to let's say 10.

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Now in each iteration we submit

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a task to this pool.

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So we have only two threads available,

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but we're submitting 10 tasks.

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Let's see what happens.

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Take a

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look. So,

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some tasks are being executed on thread one,

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other tasks are being executed on thread two.

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See?

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So internally this executor maintains a queue.

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Every task that we submit goes in this queue and waits for an available

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thread.

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Okay?

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Now, let's get rid of this for loop.

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We don't need it anymore.

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So we create an executor and submit a task.

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Now, if I run this program again,

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we get this dialog box saying that our program is still running.

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Why is that?

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Because when we start an executor and submit a task,

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the executor thinks there might be more tasks coming in the future.

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So it's not going to terminate.

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It's going to stay in the memory waiting for new tasks.

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So we have to explicitly shut down an executor to terminate

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our program.

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And to do that we call executor .shutdown.

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We also have shutdown now.

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The difference is that the shutdown method doesn't stop the current tasks.

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So it will wait for the completion of those tasks,

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but it's not going to accept any new tasks.

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In contrast,

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if you call shutdown now,

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this will force the existing tasks to stop.

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Okay, so let's call the shutdown method.

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Now we run the program.

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We get this dialog box because we didn't shut down our executor previously.

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So let's rerun this program.

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All right, our program shut down.

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So if I run it again,

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we don't get that warning.

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Okay.

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Now,

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what if something goes wrong over here?

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Or we might have some code and

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this code throws an exception.

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With that,

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we're not going to be able to shut down an executor properly.

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So as a best practice,

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we should run this inside a try find block.

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So I'm going to add a try block here and the

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try block

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we submit our task and in

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the final block,

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we shut down the executor.

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With this,

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we'll make sure that no matter what,

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we always shut down an executor and release it from the memory.

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So this is the benefit of using the executor framework.

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We don't have to worry about threat manipulation anymore.

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We let Java,

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we let the executor framework take care of all of that.

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But you have to remember,

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even when using the executor framework,

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we still have to worry about the concurrency problems.

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So if two tasks modify an object

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concurrently, we're going to run into issues.

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So the executor framework does not protect us from the concurrency problems

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we talked about in the last section.

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It just simplifies threat manipulation.

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