What is CompletableFuture.completeExceptionally() in Java?
Overview
completeExceptionally() is an instance method of the CompletableFuture which is used to complete the get() and join() throwing the given exception.
The completeExceptionally method is defined in the CompletableFuture class. The CompletableFuture class is defined in the java.util.concurrent package. To import the CompletableFuture class, we check the following import statement.
import java.util.concurrent.CompletableFuture;
Syntax
public boolean completeExceptionally(Throwable ex)
Parameters
Throwable ex: The exception to throw.
Return value
This method returns true if the method call resulted in the transition of the CompletableFuture to a completed state. Otherwise, it returns false.
Code
Let’s take a look at the code.
import java.util.concurrent.*;public class Main {public static void main(String[] args) {CompletableFuture<Integer> completableFuture = new CompletableFuture<>();RuntimeException runtimeException = new RuntimeException("Runtime Exception");boolean flag = completableFuture.completeExceptionally(runtimeException);if(flag) System.out.println("Future moved to completed stage");if(completableFuture.isCompletedExceptionally()) System.out.println("Future completed exceptionally");Integer result = null;try {result = completableFuture.get();} catch (InterruptedException | ExecutionException e) {System.out.println("Exception while getting the result of the future. " + e.getMessage());}System.out.println("Result - " + result);}}
Explanation
- Line 1: We import the classes in the
concurrentpackage. - Line 5: We create an instance of
CompletableFuturethat is incomplete using the constructor of theCompletableFutureclass. - Line 6: We define the exception to be thrown.
- Line 7: We use the
completeExceptionally()method, the future created in line 5 is moved to the completed stage with the exception defined in line 6. ThecompleteExceptionally()method returns aBooleanthat’s stored in theflagvariable. - Line 8: Depending on the
flagvalue, we print whether the future was moved to the completed stage or not. - Line 9: Using the
isCompletedExceptionally(), we check whether the future was completed exceptionally or not. - Line 10: We retrieve the value of the future using the
get()method. At this point, the exception defined in line 6 is thrown and caught in the catch block. - Line 11: The result will remain
nullas initialized andnullwill be printed to the console.
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