[英]Synchronizing a sequence of asynchronous calls
我正在使用JavaFX的WebView
解析网站。 该站点包含一堆链接-我需要按给定的顺序分别打开每个链接,并从每个链接中检索一个信息。
为了确保WebView
已加载整个网站,我正在侦听WebEngine
changed
事件,并等待newState == Worker.State.SUCCEEDED
。 问题在于此调用是异步的。 当我打电话给webEngine.load(firstAddress);
,代码会立即返回,并且在此页面加载之前,我的代码将调用另一个webEngine.load(secondAddress);
, 等等。
我知道为什么这样做(为什么异步比同步更好),但是我是Java的初学者,所以我不确定什么是解决此问题的最佳方法。 我以某种方式理解多线程和东西,所以我已经尝试了一个信号量( CountDownLatch
类)。 但是代码挂起await
,我不确定自己在做什么错。
有人可以告诉我如何正确地做吗? 也许一些通用模式如何应对这种情况?
我要实现的伪代码:
WebEngine webEngine = new WebEngine();
webEngine.loadPage("http://www.something.com/list-of-cars");
webEngine.waitForThePageToLoad(); // I need an equivalent of this. In the real code, this is done asynchronously as a callback
// ... some HTML parsing or DOM traversing ...
List<String> allCarsOnTheWebsite = webEngine.getDocument()....getChildNodes()...;
// allCarsOnTheWebsite contains URLs to the pages I want to analyze
for (String url : allCarsOnTheWebsite)
{
webEngine.loadPage(url);
webEngine.waitForThePageToLoad(); // same as in line 3
String someDataImInterestedIn = webEngine.getDocument()....getChildNodes()...Value();
System.out.println(url + " : " + someDataImInterestedIn);
}
System.out.println("Done, all cars have been analyzed");
您应该使用在页面加载时被调用的监听器,而不是阻塞直到完成。
就像是:
WebEngine webEngine = new WebEngine();
ChangeListener<State> initialListener = new ChangeListener<State>() {
@Override
public void changed(ObservableValue<? extends State> obs, State oldState, State newState) {
if (newState == State.SUCCEEDED) {
webEngine.getLoadWorker().stateProperty().removeListener(this);
List<String> allCarsOnTheWebsite = webEngine.getDocument()... ;
loadPagesConsecutively(allCarsOnTheWebsite, webEngine);
}
}
};
webEngine.getLoadWorker().addListener(initialListener);
webEngine.loadPage("http://www.something.com/list-of-cars");
// ...
private void loadPagesConsecutively(List<String> pages, WebEngine webEngine) {
LinkedList<String> pageStack = new LinkedList<>(pages);
ChangeListener<State> nextPageListener = new ChangeListener<State>() {
@Override
public void changed(ObservableValue<? extends State> obs, State oldState, State newState) {
if (newState == State.SUCCEEDED ) {
// process current page data
// ...
if (pageStack.isEmpty()) {
webEngine.getLoadWorker().stateProperty().removeListener(this);
} else {
// load next page:
webEngine.load(pageStack.pop());
}
}
}
};
webEngine.getLoadWorker().stateProperty().addListener(nextPageListener);
// load first page (assumes pages is not empty):
webEngine.load(pageStack.pop());
}
如果要同时运行所有任务,但按提交顺序处理它们,请看以下示例:
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import javafx.application.Application;
import javafx.application.Platform;
import javafx.concurrent.Task;
import javafx.scene.Scene;
import javafx.scene.control.ListView;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;
public class ProcessTaskResultsSequentially extends Application {
@Override
public void start(Stage primaryStage) {
ListView<String> results = new ListView<>();
List<Task<Integer>> taskList = new ArrayList<>();
for (int i = 1; i<= 10 ; i++) {
taskList.add(new SimpleTask(i));
}
ExecutorService exec = Executors.newCachedThreadPool(r -> {
Thread t = new Thread(r);
t.setDaemon(true);
return t ;
});
Thread processThread = new Thread(() -> {
for (Task<Integer> task : taskList) {
try {
int result = task.get();
Platform.runLater(() -> {
results.getItems().add("Result: "+result);
});
} catch (Exception e) {
e.printStackTrace();
}
}
});
processThread.setDaemon(true);
processThread.start();
taskList.forEach(exec::submit);
primaryStage.setScene(new Scene(new BorderPane(results), 250, 400));
primaryStage.show();
}
public static class SimpleTask extends Task<Integer> {
private final int index ;
private final static Random rng = new Random();
public SimpleTask(int index) {
this.index = index ;
}
@Override
public Integer call() throws Exception {
System.out.println("Task "+index+" called");
Thread.sleep(rng.nextInt(1000)+1000);
System.out.println("Task "+index+" finished");
return index ;
}
}
public static void main(String[] args) {
launch(args);
}
}
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