In computing, a thread pool comprises a set of pre-allocated threads that are adept at executing tasks on demand. Usage of thread pools can drastically minimize resource consumption since the application does not create a new thread each time a thread is required.
Instead, a ready – or runnable – thread (from the thread pool) as it is called, is assigned the task to execute and execution of the new task happens thereafter. At runtime, a task is assigned to one of the threads in the thread pool and then executed.
In this Java programming tutorial, we will discuss how Java thread pools work and how to use them in your applications.
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What are thread pools?
As the name implies, a thread pool comprises a set of threads that are ready to run when necessary. The size and number of threads in the pool depend on how many tasks you want to be able to run simultaneously.
A thread pool can also be used for executing tasks in a controlled manner; for example, by limiting the number of concurrent executions or prioritizing certain tasks over others. In addition, thread pools can use a queue for tasks, which can help ensure that critical tasks are not delayed due to a lack of available threads.
You can learn more about threading in our tutorial: Introduction to Using Threads in Java.
Why Do Developers Use Thread Pools?
In some cases, it is useful to have multiple threads of execution. For example, a developer might want to perform a background task that is independent from the main thread of their application or service. In this case, programmers can use a separate thread for their background task and not have to worry about blocking the main thread from executing.
However, creating new threads has overhead associated with it, which can lead to increased memory usage and slower execution when dealing with large amounts of data. Also, consider that each time you switch between threads (also known as context switching), there is a performance penalty due to context switches, CPU cache flushes, and several other things going on behind the scenes, such as loading different stacks into memory, and so forth.
If context switches are frequent and/or these caches get flushed too frequently, then performance will suffer significantly because there will be more time spent waiting in those caches than actually doing useful work on them. Thread pools allow developers to avoid both of these issues by allowing us control over how many threads are used at any given time while also managing their lifecycle.
Thread pools can improve application performance by allowing tasks to be executed concurrently and by providing a mechanism for controlling the number of threads that are active at any given time. Usage of thread pool can dramatically lower the number of threads required by an application thus lowering resource consumption and improving performance considerably.
What are the advantages of using thread pools
When it comes to writing Java code, using thread pools can offer a number of advantages over creating and managing threads yourself.
Thread pools can help to improve the performance of your applications by reusing threads and avoiding the overhead of creating new threads each time a task is executed. They can also help to ensure that tasks are executed in a timely manner by queuing them up and executing them as soon as a thread becomes available.
Another advantage of using thread pools is that they can make your code more robust by allowing you to gracefully handle situations where there are more tasks to be executed than there are available threads. In such cases, the thread pool will simply queue up the tasks until a thread becomes available to execute them.
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How to Create a Thread Pool in Java
The java.util.concurrent package provides several classes that can be used for this purpose, including the Executors class and the ThreadPoolExecutor class.
Using the Executor class is the simplest way to create a thread pool, but the ThreadPoolExecutor class provides more flexibility and control.
Refer to the code listing given below that shows how you can work with the ExecutorService class to create thread pools in Java:
import java.util.concurrent.*; public class MyThreadPool { public static void main(String[] args) { // Create a fixed-size thread pool with three threads ExecutorService executorService = Executors.newFixedThreadPool(3); // Submit a task to the executor executorService.submit(new Runnable() { public void run() { // Write your custom code here… System.out.println(“Inside run method…”); } }); executorService.shutdown(); // Shut down the executor service } }
Instances of the ThreadPoolExecutor class can be created using one of its static factory methods, such as newFixedThreadPool() or newCachedThreadPool(). Once created, a ThreadPoolExecutor can be used to execute tasks by calling its execute() method. Note that tasks that are submitted to a ThreadPoolExecutor must be instances of the Runnable or Callable interfaces.
Refer to the code listing given below that shows how you can use the ThreadPoolExecutor class to create thread pools in Java:
import java.util.concurrent.Executors; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit; public class MyThreadPool { public static void main(String[] args) { ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) Executors.newFixedThreadPool(2); for (int i = 1; i <= 5; i++) { MyTask task = new MyTask("Task " + i); threadPoolExecutor.execute(task); } threadPoolExecutor.shutdown(); } } public class MyTask implements Runnable { public void run() { System.out.println("Executing the run() method..."); } }
Use cases for thread pools
Thread pools are often used in server applications to improve performance by creating a thread pool (with a max threshold) that can be used to service requests on demand, rather than creating a new thread for each request.
For example, a web server takes advantage of thread pool to serve requests. When a new request arrives, the web server can create a new thread to handle that request. By using a thread pool, the web server can ensure that there are always enough threads available to handle incoming requests.
When Not to Use a Thread Pool
If your application does not handle many threads, you can avoid using a thread pool. Creating and destroying threads takes time, so using a thread pool in this situation would just add to the overhead with no appreciable benefit. Additionally, a thread pool itself consumes resources.
Another situation where a developer would not want to use a thread pool is if your application requires threads that perform unrelated actions. For example, while one thread handles user events, another executes business logic and yet another thread prints data.
Programmers should not use a thread pool if their application is going to be blocked for long periods of time. You should not use thread pools in this case, because if there are too many blocked threads, the tasks will not start at all.
Final Thoughts on Thread Pools
Thread pools are a great way to improve the responsiveness of your Java applications by reusing threads and avoiding the overhead of creating new threads for each task. A thread pool not only pre-allocates resources for one or more threads but it also limits the number of threads that are in use at a given point of time.
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