layout, title, folder, permalink, categories, tags
| layout | title | folder | permalink | categories | tags | |
|---|---|---|---|---|---|---|
| pattern | Async Method Invocation | async-method-invocation | /patterns/async-method-invocation/ | Concurrency |
|
含义
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。
解释
真实世界案例
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。
简而言之
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。
维基百科的解释
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。
编程示例
在这个例子中，我们正在发射太空火箭和部署月球车。
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。
public interface AsyncResult<T> {
boolean isCompleted();
T getValue() throws ExecutionException;
void await() throws InterruptedException;
}
public interface AsyncCallback<T> {
void onComplete(T value, Optional<Exception> ex);
}
public interface AsyncExecutor {
<T> AsyncResult<T> startProcess(Callable<T> task);
<T> AsyncResult<T> startProcess(Callable<T> task, AsyncCallback<T> callback);
<T> T endProcess(AsyncResult<T> asyncResult) throws ExecutionException, InterruptedException;
}
ThreadAsyncExecutor 是 AsyncExecutor 的一个实现。接下来将着重说明它的一些关键部分。
public class ThreadAsyncExecutor implements AsyncExecutor {
@Override
public <T> AsyncResult<T> startProcess(Callable<T> task) {
return startProcess(task, null);
}
@Override
public <T> AsyncResult<T> startProcess(Callable<T> task, AsyncCallback<T> callback) {
var result = new CompletableResult<>(callback);
new Thread(
() -> {
try {
result.setValue(task.call());
} catch (Exception ex) {
result.setException(ex);
}
},
"executor-" + idx.incrementAndGet())
.start();
return result;
}
@Override
public <T> T endProcess(AsyncResult<T> asyncResult)
throws ExecutionException, InterruptedException {
if (!asyncResult.isCompleted()) {
asyncResult.await();
}
return asyncResult.getValue();
}
}
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。
public static void main(String[] args) throws Exception {
// construct a new executor that will run async tasks
var executor = new ThreadAsyncExecutor();
// start few async tasks with varying processing times, two last with callback handlers
final var asyncResult1 = executor.startProcess(lazyval(10, 500));
final var asyncResult2 = executor.startProcess(lazyval("test", 300));
final var asyncResult3 = executor.startProcess(lazyval(50L, 700));
final var asyncResult4 = executor.startProcess(lazyval(20, 400), callback("Deploying lunar rover"));
final var asyncResult5 =
executor.startProcess(lazyval("callback", 600), callback("Deploying lunar rover"));
// emulate processing in the current thread while async tasks are running in their own threads
Thread.sleep(350); // Oh boy, we are working hard here
log("Mission command is sipping coffee");
// wait for completion of the tasks
final var result1 = executor.endProcess(asyncResult1);
final var result2 = executor.endProcess(asyncResult2);
final var result3 = executor.endProcess(asyncResult3);
asyncResult4.await();
asyncResult5.await();
// log the results of the tasks, callbacks log immediately when complete
log("Space rocket <" + result1 + "> launch complete");
log("Space rocket <" + result2 + "> launch complete");
log("Space rocket <" + result3 + "> launch complete");
}
以下是控制台输出。
21:47:08.227 [executor-2] INFO com.iluwatar.async.method.invocation.App - Space rocket <test> launched successfully
21:47:08.269 [main] INFO com.iluwatar.async.method.invocation.App - Mission command is sipping coffee
21:47:08.318 [executor-4] INFO com.iluwatar.async.method.invocation.App - Space rocket <20> launched successfully
21:47:08.335 [executor-4] INFO com.iluwatar.async.method.invocation.App - Deploying lunar rover <20>
21:47:08.414 [executor-1] INFO com.iluwatar.async.method.invocation.App - Space rocket <10> launched successfully
21:47:08.519 [executor-5] INFO com.iluwatar.async.method.invocation.App - Space rocket <callback> launched successfully
21:47:08.519 [executor-5] INFO com.iluwatar.async.method.invocation.App - Deploying lunar rover <callback>
21:47:08.616 [executor-3] INFO com.iluwatar.async.method.invocation.App - Space rocket <50> launched successfully
21:47:08.617 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <10> launch complete
21:47:08.617 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <test> launch complete
21:47:08.618 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <50> launch complete
类图
适用场景
在以下场景可以使用异步调用模式
- 你有多有可以并行执行的独立任务
- 你需要提高一组串行任务的性能
- 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