ThreadPoolExecutor

Published: June 20, 2017

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The java.util.concurrent.ThreadPoolExecutor is an implementation of the ExecutorService interface. The ThreadPoolExecutor executes the given task (Callable or Runnable) using one of its internally pooled threads.

The thread pool contained inside the ThreadPoolExecutor can contain a varying amount of threads. The number of threads in the pool is determined by these variables:

corePoolSize
maximumPoolSize
If less than corePoolSize threads are created in the the thread pool when a task is delegated to the thread pool, then a new thread is created, even if idle threads exist in the pool.

If the internal queue of tasks is full, and corePoolSize threads or more are running, but less than maximumPoolSize threads are running, then a new thread is created to execute the task.

Here is a diagram illustrating the ThreadPoolExecutor principles:



However, unless you need to specify all these parameters explicitly for your ThreadPoolExecutor, it is often easier to use one of the factory methods in the java.util.concurrent.Executors class, as shown in the ExecutorService text.

ExecutorService Example
Here is a simple Java ExectorService example:

ExecutorService executorService = Executors.newFixedThreadPool(10);

executorService.execute(new Runnable() {
    public void run() {
        System.out.println("Asynchronous task");
    }
});

executorService.shutdown();
First an ExecutorService is created using the newFixedThreadPool() factory method. This creates a thread pool with 10 threads executing tasks.

Second, an anonymous implementation of the Runnable interface is passed to the execute() method. This causes the Runnable to be executed by one of the threads in the ExecutorService.