Thread pool (Executor)

What is a thread pool?

Java5 introduces a new concurrency API called the Executor framework to make programmers’ lives easier. It simplifies the design and development of multithreaded applications. It mainly consists of Executor, the ExecutorService interface, and the ThreadPoolExecutor class, which implements both the Executor and ExecutorService interfaces. The ThreadPoolExecutor class provides an implementation of thread pools. We will learn more about this later in the tutorial.

Why do we need thread pools?

When we create a simple multithreaded application, we create Runnable objects and use Runnable to construct thread objects that we need to create, execute, and manage. We may have a hard time doing that. The Executor framework does this for you. It is responsible for creating, executing, and managing threads, and not only that, it improves application performance.

When you create a new thread for each task, then if the system is highly overloaded, you will get out of memory errors, the system will fail, and even throw oom exceptions. If ThreadPoolExecutor is used, no threads are created for new tasks. Assign tasks to a limited number of threads to execute only Runnable tasks. Once a thread completes a task, it blocks the queue for a Runnable to execute.

How do I create a thread pool?


public interface Executor {
 void execute(Runnable command);
}
 
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There is another interface called ExecutorService, which extends the Executor interface. It can be called an Executor, and it provides methods that control termination and generate methods that track the progress of one or more asynchronous tasks in the future. It has methods like submit, shut down, shut down now, and so on.

ThreadPoolExecutor is the actual implementation of ThreadPool. It extends the implement AbstractThreadPoolExecutor ExecutorService interface. ThreadPoolExecutor can be created from the factory method of the Executor class. It is recommended that you use a method to obtain an instance of ThreadPoolExecutor.

useExecutorsFactory method to create a thread pool:

Provide default static methods

There are four factory methods available for obtaining instances of ThreadPoolExecutor in the Executors class. We’re using Executors’ newFixedThreadPool to get an instance of ThreadPoolExecutor.

ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) Executors.newFixedThreadPool(5);
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methods	instructions
newFixedThreadPool(int nThreads)	This method returns a thread pool executor whose maximum size (say, n threads) is fixed
newCachedThreadPool()	This method returns an infinite thread pool.
newSingleThreadedExecutor()	This method returns a thread executor that is guaranteed to use a single thread.
newScheduledThreadPool(int corePoolSize)	This method returns a fixed-size thread pool that can schedule commands to run after a given delay or to be executed periodically

The customThreadPoolExecutorCreates a thread pool

Provide a default constructor

public ThreadPoolExecutor(int corePoolSize,int maximumPoolSize,long keepAliveTime,
TimeUnit unit,BlockingQueue workQueue ,ThreadFactory threadFactory,RejectedExecutionHandler handler) ;
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parameter	instructions
corePoolSize	Core threads
maximumPoolSize	Maximum number of threads
keepAliveTime	Maximum time for a thread to remain alive
unit	Unit of time
workQueue	Blocking queue
threadFactory	Thread factory
handler	Rejection policies