最初のコールバック呼び出しは非常に高速で、残りは約 50 ミリ秒 (付加的でない) 遅れますが、その理由はわかりません。
public class CfTest {
final static long t0 = System.nanoTime();
public static void main(String[] args) {
CompletableFuture<Integer> cf1 = CompletableFuture.supplyAsync(()->{sleep(2000); return 100;});
CompletableFuture<Long> cf2 = CompletableFuture.supplyAsync(()->{sleep(1000); return 1L;});
CompletableFuture<Long> cfs = cf1.thenCombine(cf2, (a,b)->a+b);
cfs.thenAccept(x->debug("a1. " + x)); // Async process, sync when complete
cfs.thenAccept(x->debug("a2. " + x)); // Async process, sync when complete
cfs.thenAcceptAsync(x->debug("a3. " + x)); // Async process, async after complete too
debug("b. " + cfs.join()); // Wait and process
debug("c. " + cfs.join()); // Wait and process
}
private static void sleep(int i) {
try {
Thread.sleep(i);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
private static void debug(Object res) {
System.out.println(String.format("after %.3fs: %s", (System.nanoTime()-t0) / (float)TimeUnit.SECONDS.toNanos(1), res));
System.out.flush();
}
}
出力：
after 2,067s: a1. 101
after 2,129s: a2. 101
after 2,129s: a3. 101
after 2,129s: b. 101
after 2,129s: c. 101
編集：私をさらに驚かせる他のケース。CompletableFuture は問題ではないと思います。
この行を追加すると:
...
cfs.thenAcceptAsync(x->debug("a3. " + x)); // Async process, async after complete too
System.out.println("Waiting..."); // <-- New Line
debug("b. " + cfs.join()); // Wait and process
debug("c. " + cfs.join()); // Wait and process
私はこの出力を取得しますが、変更はありません:
Waiting...
after 2,066s: a1. 101
after 2,121s: a2. 101
after 2,122s: a3. 101
after 2,122s: b. 101
after 2,122s: c. 101
しかし、代わりに次の行を追加すると:
...
cfs.thenAcceptAsync(x->debug("a3. " + x)); // Async process, async after complete too
debug("Waiting..."); // <-- New Line
debug("b. " + cfs.join()); // Wait and process
debug("c. " + cfs.join()); // Wait and process
遅延がなくなりました!!
after 0,068s: Waiting...
after 2,066s: a1. 101
after 2,067s: a2. 101
after 2,067s: b. 101
after 2,067s: a3. 101
after 2,068s: c. 101