CompletableFuture in Java simplifies asynchronous programming by enabling non-blocking code with greater flexibility than the traditional Future interface. 1. It allows manual completion of tasks using complete(), 2. supports async execution via runAsync() or supplyAsync(), 3. enables chaining operations with thenApply (transforms result), thenAccept (consumes result), and thenRun (runs after completion), 4. combines futures using thenCompose (chaining futures) and thenCombine (merging results), 5. handles errors via exceptionally (fallback values) and handle (custom error/result handling), and 6. offers async versions of methods for thread control, enhancing parallelism and readability in asynchronous workflows.
CompletableFuture is a class in Java introduced in Java 8 as part of the java.util.concurrent package. It's designed to make asynchronous programming easier by allowing you to write non-blocking code that handles futures (results of asynchronous computations) more cleanly and flexibly than the older Future interface.
The main idea behind CompletableFuture is that it gives you more control over how tasks are chained and combined, including handling exceptions, combining multiple futures, and specifying which thread should run the next step.
Basic Use: Creating and Completing Futures
You can create a CompletableFuture in several ways:
-
Manually complete it using
complete() -
Run an async task using methods like
runAsync()orsupplyAsync()
For example:
CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
// Simulate a long-running task
try {
Thread.sleep(1000);
} catch (InterruptedException e) { }
return "Hello";
});
future.thenAccept(result -> System.out.println("Result: " result));This sets up a background task that returns "Hello" after one second and then prints it when done.
If you want to manually complete a future (say, for testing or custom logic), you can do:
CompletableFuture<Integer> manualFuture = new CompletableFuture<>(); manualFuture.complete(42); manualFuture.thenAccept(System.out::println); // prints 42
Chaining Operations with thenApply, thenAccept, and thenRun
One of the big advantages of CompletableFuture is the ability to chain operations without blocking threads.
Here’s how each method works:
thenApplytransforms the resultthenAcceptconsumes the result but doesn’t return anythingthenRunruns some code after completion, ignoring the result
Example:
CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> "Apple")
.thenApply(s -> s.length()) // turns "Apple" into 5
.thenApply(len -> len * 2); // 5 becomes 10
future.thenAccept(System.out::println); // prints 10These operations are typically run on the same thread that completed the previous future, unless you specify a different executor.
Combining Futures with thenCompose and thenCombine
When working with multiple asynchronous steps, two key methods are:
thenCompose— use this when you have a future that returns another futurethenCombine— use this when you want to combine results from two independent futures
For example:
CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> 10);
CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> 20);
// Combine both results
future1.thenCombine(future2, (a, b) -> a b)
.thenAccept(System.out::println); // prints 30And if you're returning a future inside a future:
CompletableFuture<Integer> composed = CompletableFuture.supplyAsync(() -> 5)
.thenCompose(val -> CompletableFuture.supplyAsync(() -> val * 2));
composed.thenAccept(System.out::println); // prints 10Handling Errors with exceptionally and handle
Things don’t always go smoothly. You can handle exceptions using:
exceptionally— provides a fallback value if an error occurshandle— lets you inspect both result and exception and choose what to return
Example:
CompletableFuture<Integer> faultyFuture = CompletableFuture.supplyAsync(() -> {
if (true) throw new RuntimeException("Oops!");
return 100;
});
faultyFuture
.exceptionally(ex -> {
System.out.println("Error occurred: " ex.getMessage());
return 0; // fallback value
})
.thenAccept(System.out::println); // prints 0Using handle gives you more flexibility:
CompletableFuture<Integer> handled = faultyFuture.handle((result, ex) -> {
if (ex != null) {
System.out.println("Handling error: " ex.getMessage());
return 1; // default
}
return result;
});Async Versions for Better Control Over Threads
Most methods come with an async version (e.g., thenApplyAsync, thenAcceptAsync) that allow you to specify which thread pool to use for the next operation.
This is useful when you want to avoid blocking the original thread or want to parallelize work.
Example:
CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 10)
.thenApplyAsync(val -> val * 2, Executors.newFixedThreadPool(2));This ensures the multiplication happens in a separate thread pool.
So, basically, CompletableFuture helps you write clean, readable, and efficient asynchronous code in Java. It builds on top of the old Future interface by giving you tools to chain, combine, and manage asynchronous tasks easily — without getting stuck in callback hell.
That's the core of it. Not too complicated once you get used to the patterns.
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