Java Threads & Concurrency API: Part 2

ExecutorService

Creating Threads with ExecutorService

In Part I of this article, we discussed what threads are in Java, how to create them, the concept of a task, and the differences between using the Callable and Runnable interfaces.

Let's delve a bit deeper into the Concurrency API, where Java introduced the ExecutorService, which assists us in creating and managing threads.

ExecutorService is an interface in Java. To create an instance and utilize this interface, the Concurrency API offers a factory class called "Executors."

Single-Thread Executor

• Create an ExecutorService interface reference variable.

• Initialize it using Executors class. Let us use the simplest one → newSingleThreadExecutor().

• Create a task using Runnable and pass it to a method called on the instance created in the above step.

Output:

Task is smallest unit of work performed by a Thread
Second task
ExecutorService - Main
Third task

You can see that all the tasks using a single thread executor are in order, whereas the one by the Main thread follows no sequence as it’s a separate thread. Hence, results are guaranteed to be executed in the order in which they are added to the executor service (for single thread executor).

But…this guarantee vanishes when the number of threads increases so it’s better not to code relying on this behavior.

Shutting Down a Thread Executor

Executor thread creates a non-daemon thread and hence it is important to call shutdown() method once you are finished using the service.

It doesn't implement AutoCloseable interface so we cannot use it with try-with-resources either.

Note: In the next article we will see how to use submit() instead of execute() and the advantage of doing so.

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