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docs: translated the async method invocation pattern into Chinese (iluwatar#1741)
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‎zh/async-method-invocation/README.md

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---
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layout: pattern
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title: Async Method Invocation
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folder: async-method-invocation
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permalink: /patterns/async-method-invocation/
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categories: Concurrency
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tags:
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- Reactive
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---
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## 作用
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异步方法是一种调用线程在等待任务结果时候不会被阻塞的模式。该模式提供了对多个任务的并行处理，并通过回调或等待，在所有任务完成后在提供结果读取。
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## 解释
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真实世界案例
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> 发射太空火箭是一项令人兴奋的事业。在任务指挥部下达发射命令后， 经过一些未确定的时间，火箭要么成功发射，要么重演挑战者悲剧。
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简而言之
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> 异步方法调用开始任务处理并，在任务结果准备好之前立即返回。任务处理的结果会在稍后再返回给调用者。
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维基百科的解释
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> 在多线程计算机编程中，异步方法调用（AMI），也被称为异步方法调用或异步模式。这是一种设计模式，在这种模式下，调用点在等待被调用代码完成时不会被阻塞。相反，当返回点到达时，调用线程会得到通知。轮询结果是一种不受欢迎的选择。
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**编程示例**
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在这个例子中，我们正在发射太空火箭和部署月球车。
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该应用演示了异步方法调用模式。该模式的关键部分是 `AsyncResult`，它是一个异步计算值的中间容器，`AsyncCallback` 可以在任务完成时提供执行行动作，`AsyncExecutor` 负责管理异步任务的执行。
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```java
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public interface AsyncResult<T> {
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boolean isCompleted();
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T getValue() throws ExecutionException;
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void await() throws InterruptedException;
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}
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```
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```java
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public interface AsyncCallback<T> {
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void onComplete(T value, Optional<Exception> ex);
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}
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```
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```java
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public interface AsyncExecutor {
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<T> AsyncResult<T> startProcess(Callable<T> task);
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<T> AsyncResult<T> startProcess(Callable<T> task, AsyncCallback<T> callback);
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<T> T endProcess(AsyncResult<T> asyncResult) throws ExecutionException, InterruptedException;
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}
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```
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`ThreadAsyncExecutor` 是 `AsyncExecutor` 的一个实现。接下来将着重说明它的一些关键部分。
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```java
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public class ThreadAsyncExecutor implements AsyncExecutor {
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@Override
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public <T> AsyncResult<T> startProcess(Callable<T> task) {
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return startProcess(task, null);
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}
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@Override
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public <T> AsyncResult<T> startProcess(Callable<T> task, AsyncCallback<T> callback) {
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var result = new CompletableResult<>(callback);
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new Thread(
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() -> {
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try {
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result.setValue(task.call());
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} catch (Exception ex) {
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result.setException(ex);
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}
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},
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"executor-" + idx.incrementAndGet())
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.start();
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return result;
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}
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@Override
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public <T> T endProcess(AsyncResult<T> asyncResult)
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throws ExecutionException, InterruptedException {
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if (!asyncResult.isCompleted()) {
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asyncResult.await();
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}
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return asyncResult.getValue();
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}
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}
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```
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然后我们准备发射一些火箭，看看所有东西是如何一起运作的。
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```java
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public static void main(String[] args) throws Exception {
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// construct a new executor that will run async tasks
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var executor = new ThreadAsyncExecutor();
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// start few async tasks with varying processing times, two last with callback handlers
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final var asyncResult1 = executor.startProcess(lazyval(10, 500));
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final var asyncResult2 = executor.startProcess(lazyval("test", 300));
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final var asyncResult3 = executor.startProcess(lazyval(50L, 700));
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final var asyncResult4 = executor.startProcess(lazyval(20, 400), callback("Deploying lunar rover"));
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final var asyncResult5 =
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executor.startProcess(lazyval("callback", 600), callback("Deploying lunar rover"));
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// emulate processing in the current thread while async tasks are running in their own threads
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Thread.sleep(350); // Oh boy, we are working hard here
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log("Mission command is sipping coffee");
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// wait for completion of the tasks
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final var result1 = executor.endProcess(asyncResult1);
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final var result2 = executor.endProcess(asyncResult2);
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final var result3 = executor.endProcess(asyncResult3);
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asyncResult4.await();
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asyncResult5.await();
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// log the results of the tasks, callbacks log immediately when complete
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log("Space rocket <" + result1 + "> launch complete");
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log("Space rocket <" + result2 + "> launch complete");
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log("Space rocket <" + result3 + "> launch complete");
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}
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```
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以下是控制台输出。
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```java
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21:47:08.227 [executor-2] INFO com.iluwatar.async.method.invocation.App - Space rocket <test> launched successfully
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21:47:08.269 [main] INFO com.iluwatar.async.method.invocation.App - Mission command is sipping coffee
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21:47:08.318 [executor-4] INFO com.iluwatar.async.method.invocation.App - Space rocket <20> launched successfully
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21:47:08.335 [executor-4] INFO com.iluwatar.async.method.invocation.App - Deploying lunar rover <20>
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21:47:08.414 [executor-1] INFO com.iluwatar.async.method.invocation.App - Space rocket <10> launched successfully
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21:47:08.519 [executor-5] INFO com.iluwatar.async.method.invocation.App - Space rocket <callback> launched successfully
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21:47:08.519 [executor-5] INFO com.iluwatar.async.method.invocation.App - Deploying lunar rover <callback>
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21:47:08.616 [executor-3] INFO com.iluwatar.async.method.invocation.App - Space rocket <50> launched successfully
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21:47:08.617 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <10> launch complete
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21:47:08.617 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <test> launch complete
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21:47:08.618 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <50> launch complete
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```
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## 类图
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![alt text](../../async-method-invocation/etc/async-method-invocation.png "Async Method Invocation")
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## 适用性
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在以下场景可以使用异步调用模式
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* 你有多有可以并行执行的独立任务
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* 你需要提高一组串行任务的性能
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* 你的处理能力有限、或者有长期运行的任务，调用者不应该等待任务所有任务运行结束
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## 现实示例
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* [FutureTask](http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/FutureTask.html)
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* [CompletableFuture](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletableFuture.html)
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* [ExecutorService](http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ExecutorService.html)
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* [Task-based Asynchronous Pattern](https://msdn.microsoft.com/en-us/library/hh873175.aspx)
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