英文:
Not able to cancel list of scheduled tasks using Java Streams
问题
我有一个实现了Runnable接口的类
class AppThread implements Runnable {
String s;
public AppThread(String s) {
this.s=s;
}
@Override
public void run() {
System.out.println(s);
}
}
我创建了一些任务并将它们提交给ScheduledThreadPool，等待10秒后取消了这些计划任务。
ScheduledExecutorService s = Executors.newScheduledThreadPool(1);
List<ScheduledFuture<?>> list = new ArrayList<>();
public static void main( String... args ) throws InterruptedException
{
App app = new App();
app.method("A");
app.method("D");
app.method("C");
app.method("B");
Thread.sleep(10000);
app.cancelTasks();
}
public void method(String s) {
list.add(this.s.scheduleAtFixedRate(new AppThread(s), 1, 2, TimeUnit.SECONDS));
}
我成功地使用list.forEach取消了任务。
Method2
public void cancelTasks() {
System.out.println("Stopping Executors");
this.list.forEach(sf -> sf.cancel(false));
}
但是，当我尝试使用Java流来取消任务时，虽然没有异常，但任务仍然继续执行。
Method2
public void cancelTasks() {
System.out.println("Stopping Executors");
this.list.stream().map(sf -> sf.cancel(false));
}
我仍然在努力理解流的使用，但我不明白为什么第二种方法不起作用。
英文:
I have a class implementing Runnable interface
class AppThread implements Runnable {
String s;
public AppThread(String s) {
this.s=s;
}
@Override
public void run() {
System.out.println(s);
}
}
I am creating few tasks and submitting to a ScheduledThreadPool and after waiting for 10 seconds cancelling the scheduled tasks.
ScheduledExecutorService s = Executors.newScheduledThreadPool(1);
List<ScheduledFuture<?>> list = new ArrayList<>();
public static void main( String... args ) throws InterruptedException
{
App app = new App();
app.method("A");
app.method("D");
app.method("C");
app.method("B");
Thread.sleep(10000);
app.cancelTasks();
}
public void method(String s) {
list.add(this.s.scheduleAtFixedRate(new AppThread(s), 1,2, TimeUnit.SECONDS));
}
I am successfully able to cancel the tasks using list.forEach
Method2
public void cancelTasks() {
System.out.println("Stopping Executors");
this.list.forEach(sf -> sf.cancel(false));
}
But when I do so using Java streams, I get not exception but the tasks still continue to execute.
Method2
public void cancelTasks() {
System.out.println("Stopping Executors");
this.list.stream().map(sf-> sf.cancel(false));
}
I am still trying to get hold of streams, but I do not understand why the second method doesn't work.
答案1
得分: 1
map是一种中间操作，除非列表被终端操作实体化，否则它不会执行任何操作。这样，你可以有效地链接许多中间流操作。这种模式也许更好地被称为惰性评估。
如果你想要对每个流元素执行一个动作，并立即这样做，你需要使用foreach。
因此，你的代码应该变成：
this.list.stream().forEach(sf -> sf.cancel(false));
英文:
map is an intermediate operation, which doesn't do anything at all unless the list is materialized by a terminal operation. This way, you can effectively chain quite a lot of intermediate stream operations. This pattern is maybe better known as lazy-evaluation.
If you want to execute an action on each stream element, and do so immediately, you need to use foreach.
Thus, your code should become
this.list.stream().forEach(sf-> sf.cancel(false));
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