在this blog中，他为回调地狱提供this（复制/粘贴以下代码）示例。但是，没有提到如何使用Reactive Extensions消除该问题。
所以这里F3取决于F1完成，F4和F5取决于F2完成。
注意：我目前正试图绕过Rx，所以在提出这个问题之前我没有尝试解决这个问题。
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
public class CallbackB {
/**
* Demonstration of nested callbacks which then need to composes their responses together.
* <p>
* Various different approaches for composition can be done but eventually they end up relying upon
* synchronization techniques such as the CountDownLatch used here or converge on callback design
* changes similar to <a href="https://github.com/Netflix/RxJava">Rx</a>.
*/
public static void run() throws Exception {
final ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
/* the following are used to synchronize and compose the asynchronous callbacks */
final CountDownLatch latch = new CountDownLatch(3);
final AtomicReference<String> f3Value = new AtomicReference<String>();
final AtomicReference<Integer> f4Value = new AtomicReference<Integer>();
final AtomicReference<Integer> f5Value = new AtomicReference<Integer>();
try {
// get f3 with dependent result from f1
executor.execute(new CallToRemoteServiceA(new Callback<String>() {
@Override
public void call(String f1) {
executor.execute(new CallToRemoteServiceC(new Callback<String>() {
@Override
public void call(String f3) {
// we have f1 and f3 now need to compose with others
System.out.println("intermediate callback: " + f3 + " => " + ("f4 * f5"));
// set to thread-safe variable accessible by external scope
f3Value.set(f3);
latch.countDown();
}
}, f1));
}
}));
// get f4/f5 after dependency f2 completes
executor.execute(new CallToRemoteServiceB(new Callback<Integer>() {
@Override
public void call(Integer f2) {
executor.execute(new CallToRemoteServiceD(new Callback<Integer>() {
@Override
public void call(Integer f4) {
// we have f2 and f4 now need to compose with others
System.out.println("intermediate callback: f3" + " => " + (f4 + " * f5"));
// set to thread-safe variable accessible by external scope
f4Value.set(f4);
latch.countDown();
}
}, f2));
executor.execute(new CallToRemoteServiceE(new Callback<Integer>() {
@Override
public void call(Integer f5) {
// we have f2 and f5 now need to compose with others
System.out.println("intermediate callback: f3" + " => " + ("f4 * " + f5));
// set to thread-safe variable accessible by external scope
f5Value.set(f5);
latch.countDown();
}
}, f2));
}
}));
/* we must wait for all callbacks to complete */
latch.await();
System.out.println(f3Value.get() + " => " + (f4Value.get() * f5Value.get()));
} finally {
executor.shutdownNow();
}
}
public static void main(String[] args) {
try {
run();
} catch (Exception e) {
e.printStackTrace();
}
}
private static final class CallToRemoteServiceA implements Runnable {
private final Callback<String> callback;
private CallToRemoteServiceA(Callback<String> callback) {
this.callback = callback;
}
@Override
public void run() {
// simulate fetching data from remote service
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
callback.call("responseA");
}
}
private static final class CallToRemoteServiceB implements Runnable {
private final Callback<Integer> callback;
private CallToRemoteServiceB(Callback<Integer> callback) {
this.callback = callback;
}
@Override
public void run() {
// simulate fetching data from remote service
try {
Thread.sleep(40);
} catch (InterruptedException e) {
e.printStackTrace();
}
callback.call(100);
}
}
private static final class CallToRemoteServiceC implements Runnable {
private final Callback<String> callback;
private final String dependencyFromA;
private CallToRemoteServiceC(Callback<String> callback, String dependencyFromA) {
this.callback = callback;
this.dependencyFromA = dependencyFromA;
}
@Override
public void run() {
// simulate fetching data from remote service
try {
Thread.sleep(60);
} catch (InterruptedException e) {
e.printStackTrace();
}
callback.call("responseB_" + dependencyFromA);
}
}
private static final class CallToRemoteServiceD implements Runnable {
private final Callback<Integer> callback;
private final Integer dependencyFromB;
private CallToRemoteServiceD(Callback<Integer> callback, Integer dependencyFromB) {
this.callback = callback;
this.dependencyFromB = dependencyFromB;
}
@Override
public void run() {
// simulate fetching data from remote service
try {
Thread.sleep(140);
} catch (InterruptedException e) {
e.printStackTrace();
}
callback.call(40 + dependencyFromB);
}
}
private static final class CallToRemoteServiceE implements Runnable {
private final Callback<Integer> callback;
private final Integer dependencyFromB;
private CallToRemoteServiceE(Callback<Integer> callback, Integer dependencyFromB) {
this.callback = callback;
this.dependencyFromB = dependencyFromB;
}
@Override
public void run() {
// simulate fetching data from remote service
try {
Thread.sleep(55);
} catch (InterruptedException e) {
e.printStackTrace();
}
callback.call(5000 + dependencyFromB);
}
}
private static interface Callback<T> {
public void call(T value);
}
}
答案 0 :(得分：32)
我是关于回调和Java期货的引用博客文章的原作者。以下是使用flatMap，zip和merge异步进行服务组合的示例。
它获取一个User对象，然后同时获取Social和PersonalizedCatalog数据，然后对于来自PersonalizedCatalog的每个视频同时获取书签，评级和元数据，将它们拉到一起，并将所有响应合并为渐进流输出作为服务器 - 发生的事件。
return getUser(userId).flatMap(user -> {
Observable<Map<String, Object>> catalog = getPersonalizedCatalog(user)
.flatMap(catalogList -> catalogList.videos().<Map<String, Object>> flatMap(
video -> {
Observable<Bookmark> bookmark = getBookmark(video);
Observable<Rating> rating = getRatings(video);
Observable<VideoMetadata> metadata = getVideoMetadata(video);
return Observable.zip(bookmark, rating, metadata, (b, r, m) -> combineVideoData(video, b, r, m));
}));
Observable<Map<String, Object>> social = getSocial(user).map(s -> {
return s.getDataAsMap();
});
return Observable.merge(catalog, social);
}).flatMap(data -> {
String json = SimpleJson.mapToJson(data);
return response.writeStringAndFlush("data: " + json + "\n");
});
由于我无法在此处提供所有信息，您还可以在https://speakerdeck.com/benjchristensen/reactive-streams-with-rx-at-javaone-2014?slide=32以演示文稿形式（带有视频链接）找到解释。
答案 1 :(得分：5)
根据你的代码。假设使用Observable完成远程调用。
Observable<Integer> callRemoveServiceA() { /* async call */ }
/* .... */
Observable<Integer> callRemoveServiceE(Integer f2) { /* async call */ }
你想要什么：
serviceA，然后使用serviceB serviceA
serviceC然后致电serviceD和serviceE，结果为serviceC serviceE和serviceD，构建新值serviceB 使用RxJava，您可以使用以下代码实现此目的：
Observable<Integer> f3 = callRemoveServiceA() // call serviceA
// call serviceB with the result of serviceA
.flatMap((f1) -> callRemoveServiceB(f1));
Observable<Integer> f4Andf5 = callRemoveServiceC() // call serviceC
// call serviceD and serviceE then build a new value
.flatMap((f2) -> callRemoveServiceD(f2).zipWith(callRemoveServiceE(f2), (f4, f5) -> f4 * f5));
// compute the string to display from f3, and the f4, f5 pair
f3.zipWith(f4Andf5, (childF3, childF4Andf5) -> childF3 + " => " + childF4Andf5)
// display the value
.subscribe(System.out::println);
此处的重要部分是使用flapMap和zip（或zipWith）
flapMap会将值转换为另一个Observable。这里的Observable将是您的新异步调用。 （http://reactivex.io/documentation/operators/flatmap.html）zip将从两个不同的Observable组成一个新值。因此，您可以使用两个（或更多）Obsevable（http://reactivex.io/documentation/operators/zip.html）您可以在此处获取有关flapMap的更多信息：When do you use map vs flatMap in RxJava?