Java - CompletableFuture API 改進

Hello there, future Java developers! I'm thrilled to embark on this exciting journey with you as we explore the wonderful world of Java's CompletableFuture API improvements. Don't worry if you're new to programming; we'll start from the basics and work our way up. By the end of this tutorial, you'll be amazed at how much you've learned!

What is CompletableFuture?

Before we dive into the improvements, let's understand what CompletableFuture is. Imagine you're cooking a complex meal. You don't want to wait for the pasta to boil before you start chopping vegetables, right? CompletableFuture is like having multiple chefs in your kitchen, each working on different tasks simultaneously. It's Java's way of handling asynchronous programming, allowing your code to do multiple things at once without getting all tangled up.

Basic CompletableFuture Example

Let's start with a simple example:

CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return "Hello, Future!";
});

System.out.println(future.get());

In this code, we're creating a CompletableFuture that will return a String. The supplyAsync method runs our code in a separate thread. We're simulating some work by making the thread sleep for a second. After that, it returns our greeting. The get() method waits for the future to complete and gives us the result.

Support for Delays and Timeouts

One of the exciting improvements in the CompletableFuture API is better support for delays and timeouts. This is like setting a timer in your kitchen - you don't want your sauce to simmer forever!

Delay Example

CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> "Hello")
.thenApply(s -> s + ", World!")
.completeOnTimeout("Timeout occurred", 2, TimeUnit.SECONDS)
.orTimeout(3, TimeUnit.SECONDS);

System.out.println(future.get());

In this example, we're creating a future that says "Hello", then modifies it to say "Hello, World!". We're using two new methods:

1. completeOnTimeout: This will complete the future with a default value if it doesn't complete within 2 seconds.
2. orTimeout: This will throw an exception if the future doesn't complete within 3 seconds.

This way, we ensure our code doesn't hang indefinitely if something goes wrong.

Improved Support for Subclassing

The CompletableFuture class is now easier to extend, allowing you to create your own specialized versions. This is like being able to create your own custom kitchen appliance that does exactly what you need!

Subclassing Example

public class MyFuture<T> extends CompletableFuture<T> {
@Override
public <U> MyFuture<U> thenApply(Function<? super T, ? extends U> fn) {
return (MyFuture<U>) super.thenApply(fn);
}

// Other overridden methods...
}

MyFuture<String> myFuture = new MyFuture<>();
myFuture.complete("Hello, Custom Future!");
System.out.println(myFuture.get());

In this example, we're creating our own MyFuture class that extends CompletableFuture. We're overriding the thenApply method to return our custom future type. This allows us to chain operations while keeping our custom type.

New Factory Methods

Java has introduced new factory methods to make creating CompletableFutures even easier. It's like having pre-set recipes in your cookbook!

Factory Methods Table

Method	Description
failedFuture(Throwable ex)	Creates a CompletableFuture that is already completed exceptionally
completedStage(T value)	Creates a new CompletionStage that is already completed with the given value
failedStage(Throwable ex)	Creates a new CompletionStage that is already completed exceptionally

Factory Method Example

CompletableFuture<String> successFuture = CompletableFuture.completedFuture("Success!");
CompletableFuture<String> failedFuture = CompletableFuture.failedFuture(new Exception("Oops!"));

try {
System.out.println(successFuture.get());
System.out.println(failedFuture.get());
} catch (Exception e) {
System.out.println("An error occurred: " + e.getMessage());
}

In this example, we're using the completedFuture method to create a future that's already successful, and the failedFuture method to create one that's already failed. This can be useful for testing or when you need to integrate synchronous and asynchronous code.

Conclusion

Wow! We've covered a lot of ground today. From understanding the basics of CompletableFuture to exploring its new improvements, you've taken your first steps into the world of asynchronous programming in Java. Remember, like learning to cook, mastering these concepts takes practice. Don't be afraid to experiment and make mistakes - that's how we learn!

In my years of teaching, I've found that the students who excel are those who aren't afraid to get their hands dirty with code. So, I encourage you to take these examples, modify them, break them, and see what happens. That's the best way to truly understand how they work.

As we wrap up, I'm reminded of a student who once told me that learning CompletableFuture was like learning to juggle - at first, it seems impossible to keep all the balls in the air, but with practice, it becomes second nature. So keep practicing, and before you know it, you'll be juggling complex asynchronous operations like a pro!

Happy coding, future Java masters!

Credits: Image by storyset