Java CompletionStage

Java CompletionStage

I'd be happy to explain Java CompletionStage in English!

In Java 8, the java.util.concurrent package introduced the concept of a CompletionStage, which is part of the CompletableFuture API. A CompletionStage represents the result of an asynchronous computation that can complete normally or exceptionally.

Think of it like a promise: you make a promise to someone that you'll do something, and when you've completed the task, you let them know that it's done (or not). In this context, the "someone" is the thread scheduler, which manages the execution of tasks in your program. When a CompletionStage completes, it notifies the scheduler that it's time to move on to the next task.

There are two main types of CompletionStages: Supplier and BiFunction. A Supplier represents a computation that produces a value (like Future), whereas a BiFunction represents a computation that takes two inputs and produces a result (like thenApplyToBoth).

Here's the key point: a CompletionStage can be composed with other CompletionStages to create a new one. This is known as "composing" or "chaining". Composing stages allows you to build more complex asynchronous computations by combining smaller ones.

Let me illustrate this with an example:

List data = Arrays.asList("A", "B", "C");
// Create a CompletionStage that fetches the data from a database
CompletableFuture stage1 = CompletableFuture.supplyAsync(() -> {
// Simulate database query
return data.get(0);
});
// Compose stage1 with another CompletionStage to process the result
stage1.thenApply(str -> {
System.out.println("Processing: " + str);
return str.toUpperCase();
}).thenCompose(str -> {
// Create a new CompletionStage that processes the uppercase string
return CompletableFuture.supplyAsync(() -> {
// Simulate additional processing
return str.length() > 3 ? str : null;
});
});

In this example, stage1 represents a computation that fetches data from a database. We then compose it with another stage that processes the result (uppercase conversion). The output of this stage becomes the input for a new stage that simulates additional processing.

When we call stage1.get(), the execution will be as follows:

By composing these CompletionStages, you can create complex asynchronous computations that can handle both normal and exceptional completions.

I hope this explanation helps you understand Java CompletionStage!

ExecutorService Java

I'm happy to help!

ExecutorService is a part of the Java Concurrency API that manages a pool of threads for executing tasks asynchronously. It provides various methods to create, manage, and execute tasks.

What is ExecutorService?

An ExecutorService is an interface in the Java Concurrency API that represents an executor (a thread pool manager) that can execute tasks. It provides methods to submit tasks, shutdown, and terminate the executor service.

Why use ExecutorService?

Using ExecutorService has several benefits:

How to use ExecutorService?

Here's an example of how to create and use an ExecutorService:

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class Main {
public static void main(String[] args) throws InterruptedException, ExecutionException {
// Create a fixed-size thread pool executor service
ExecutorService executor = Executors.newFixedThreadPool(5);
// Submit tasks to the executor service
Future future1 = executor.submit(new Callable() {
@Override
public String call() throws Exception {
return "Task 1 completed";
}
});
Future future2 = executor.submit(new Callable() {
@Override
public String call() throws Exception {
return "Task 2 completed";
}
});
// Get the results of the tasks
String result1 = future1.get();
String result2 = future2.get();
System.out.println("Result 1: " + result1);
System.out.println("Result 2: " + result2);
// Shutdown and terminate the executor service
executor.shutdown();
}
}

In this example, we create a fixed-size thread pool executor service with 5 threads using Executors.newFixedThreadPool(5). We then submit two tasks to the executor service using executor.submit() and get the results of the tasks using future.get(). Finally, we shutdown and terminate the executor service using executor.shutdown().

Conclusion

ExecutorService is a powerful tool in Java that allows you to manage a pool of threads for executing tasks asynchronously. It provides various methods to create, manage, and execute tasks, which can improve the responsiveness and scalability of your application. By understanding how to use ExecutorService effectively, you can write more efficient and concurrent code.

I hope this helps! Let me know if you have any questions or need further clarification.