test/jdk/java/util/concurrent/CompletableFuture/ThenComposeAsyncTest.java

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import org.testng.Assert;
import org.testng.annotations.Test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CountDownLatch;

/**
 * @test
 * @bug 8029164
 * @run testng ThenComposeAsyncTest
 * @run testng/othervm -Djava.util.concurrent.ForkJoinPool.common.parallelism=0 ThenComposeAsyncTest
 * @summary Test that CompletableFuture.thenCompose works correctly if the
 * composing task is complete before composition
 */
@Test
public class ThenComposeAsyncTest {

    public void testThenComposeAsync() throws Exception {
        // Composing CompletableFuture is complete
        CompletableFuture<String> cf1 = CompletableFuture.completedFuture("one");

        // Composing function returns a CompletableFuture executed asynchronously
        CountDownLatch cdl = new CountDownLatch(1);
        CompletableFuture<String> cf2 = cf1.thenCompose(str -> CompletableFuture.supplyAsync(() -> {
            while (true) {
                try {
                    cdl.await();
                    break;
                }
                catch (InterruptedException e) {
                }
            }
            return str + ", two";
        }));

        // Ensure returned CompletableFuture completes after call to thenCompose
        // This guarantees that any premature internal completion will be
        // detected
        cdl.countDown();

        String val = cf2.get();
        Assert.assertNotNull(val);
        Assert.assertEquals(val, "one, two");
    }
}