# Java Threads & Concurrency API: Part 2

## ExecutorService

### Creating Threads with ExecutorService

In [Part I of this article](https://hashnode.com/post/cljsm8wix001b09l019og2hyq), 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.
    

![Using Single Thread Executor](https://cdn.hashnode.com/res/hashnode/image/upload/v1688735920197/2d98ae12-66e2-4889-a96c-182c2a018658.png align="left")

Output:

```java
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*](https://docs.oracle.com/javase/8/docs/api/java/lang/AutoCloseable.html) interface so we **cannot** use it with ***try-with-resources*** either.

![Executor Service Lifecycle](https://miro.medium.com/v2/resize:fit:700/1*BDDcLFoISnscwGnzvosPhQ.jpeg align="left")

**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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