What is CompletableFuture.thenApplyAsync() in Java?
Overview
thenApplyAsync is an instance method that is used to run the passed task in a thread obtained from the default executor/the common pool of ForkJoinPool. The method optionally accepts an Executor. When a custom executor is specified, the passed task is executed in a thread obtained from the passed executor.
The thenApplyAsync method is defined in the CompletableFuture class. The CompletableFuture class is defined in the java.util.concurrent package. To import the CompletableFuture class, check the following import statement.
import java.util.concurrent.CompletableFuture;
Syntax
public <U> CompletableFuture<U> thenApplyAsync(Function<? super T,? extends U> fn)
Parameters
Function<? super T,? extends U> fn: This is the task to execute.
Return value
This method returns a new CompletableFuture object.
Example
Let’s see how the thenApplyAsync() method works.
import java.util.concurrent.*;import java.util.function.Function;public class Main {static void sleep(int millis){try {Thread.sleep(millis);} catch (InterruptedException e) {e.printStackTrace();}}static void executionThread(){System.out.println("Thread execution - " + Thread.currentThread().getName());}public static void main(String[] args) throws ExecutionException, InterruptedException {CompletableFuture<String> completableFuture = CompletableFuture.supplyAsync(() -> {sleep(1000);System.out.println("supplyAsync task");executionThread();return "Hello-Educative";});Function<String, String> taskToExecute = res -> {System.out.println("thenApplyAsync task");sleep(1000);executionThread();return res + "-Edpresso";};CompletableFuture<String> result = completableFuture.thenApplyAsync(taskToExecute);sleep(3000);System.out.println("Result - " + result.get());}}
Explanation
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Lines 1–2: We import the relevant packages and classes.
-
Lines 6–12: We define a function called
sleep(), which makes the current thread sleep for the given amount of time (in milliseconds). -
Lines 14–16: We define a function called
executionThread(), which prints the current thread of execution. -
Lines 19–24: We create a completable future called
completableFutureusing thesupplyAsync()method. We pass a supplier to it that sleeps for one second and then invokes theexecutionThread(). Finally, this method returns a string value. -
Lines 26–31: We define a task
taskToExecuteto complete once thecompletableFutureis completed. The task is a function that sleeps for one second and then invokes theexecutionThread(). Finally, this method returns the future result appended with a string. -
Line 33: We call the
thenApplyAsyncmethod oncompletableFuture. We passtaskToExecuteas an argument to the method. This returns a newCompletableFutureobject calledresult. -
Line 35: The main thread is made to sleep for two seconds.
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Line 36: We print the value of
resultto the console.
