Java 中的简单超时

Question

谁能指导我如何在java中使用简单的超时？基本上在我的项目中，我正在执行一个语句br.readLine()，这是从调制解调器读取响应。但有时调制解调器没有响应。为此，我想添加一个超时。我正在寻找这样的代码：

try {
    String s= br.readLine();
} catch(TimeoutException e) {
    System.out.println("Time out has occurred");
}

Answer 1

您可以在此处找到您要查找的内容。可能存在一种更优雅的方式来实现这一目标，但一种可能的方法是

选项 1（首选）：

final Duration timeout = Duration.ofSeconds(30);
ExecutorService executor = Executors.newSingleThreadExecutor();
final Future<String> handler = executor.submit(new Callable() {
    @Override
    public String call() throws Exception {
        return requestDataFromModem();
    }
});
try {
    handler.get(timeout.toMillis(), TimeUnit.MILLISECONDS);
} catch (TimeoutException e) {
    handler.cancel(true);
}
executor.shutdownNow();

选项 2：

final Duration timeout = Duration.ofSeconds(30);
ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
final Future<String> handler = executor.submit(new Callable() {
    @Override
    public String call() throws Exception {
        return requestDataFromModem();
    }
});
executor.schedule(new Runnable() {
    @Override
    public void run(){
        handler.cancel(true);
    }      
}, timeout.toMillis(), TimeUnit.MILLISECONDS);
executor.shutdownNow();

这些只是一个草稿，以便您可以获得主要思想。

Answer 2

现在你可以使用

try {
    String s = CompletableFuture.supplyAsync(() -> br.readLine())
                                .get(1, TimeUnit.SECONDS);
} catch (TimeoutException e) {
    System.out.println("Time out has occurred");
} catch (InterruptedException | ExecutionException e) {
    // Handle
}

编辑：波塞冬正确地指出，在超时的情况下，上述方法不会中断正在处理未来任务的底层线程。如果没有中断，底层线程将继续处理未来的任务直至完成，无法知道不再需要结果。通过中断，底层线程至少可以看到（如果它检查）它已被中断，从而允许它优雅地结束处理并退出。

对于 JDK 中执行阻塞 IO 的方法，按照惯例，它们的实现方式是检查调用线程的中断状态（如果为 true，则抛出 InterruptedException）。因此，中断线程可以使其快速退出，即使在潜在的无限等待情况下也是如此，例如从输入源读取。

无需进一步说明，如果我们想在超时时中断底层线程，我们可以调整：

Future<String> future = CompletableFuture.supplyAsync(() -> br.readLine());
try {
    String s = future.get(1, TimeUnit.SECONDS);
} catch (TimeoutException e) {
    System.out.println("Time out has occurred");
    
    future.cancel(true);
} catch (InterruptedException | ExecutionException e) {
    // Handle
}

Answer 3

    @Singleton
    @AccessTimeout(value=120000)
    public class StatusSingletonBean {
      private String status;
    
      @Lock(LockType.WRITE)
      public void setStatus(String new Status) {
        status = newStatus;
      }
      @Lock(LockType.WRITE)
      @AccessTimeout(value=360000)
      public void doTediousOperation {
        //...
      }
    }
    //The following singleton has a default access timeout value of 60 seconds, specified //using the TimeUnit.SECONDS constant:
    @Singleton
    @AccessTimeout(value=60, timeUnit=SECONDS) 
    public class StatusSingletonBean { 
    //... 
    }  
    //The Java EE 6 Tutorial
//https://docs.oracle.com/javaee/6/tutorial/doc/gipvi.html

Answer 4

示例 1 将无法编译。 此版本编译并运行。 它使用 lambda 功能来缩写它。

/*
 * [RollYourOwnTimeouts.java]
 *
 * Summary: How to roll your own timeouts.
 *
 * Copyright: (c) 2016 Roedy Green, Canadian Mind Products, http://mindprod.com
 *
 * Licence: This software may be copied and used freely for any purpose but military.
 *          http://mindprod.com/contact/nonmil.html
 *
 * Requires: JDK 1.8+
 *
 * Created with: JetBrains IntelliJ IDEA IDE http://www.jetbrains.com/idea/
 *
 * Version History:
 *  1.0 2016-06-28 initial version
 */
package com.mindprod.example;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import static java.lang.System.*;
/**
 * How to roll your own timeouts.
 * Based on code at http://stackoverflow.com/questions/19456313/simple-timeout-in-java
 *
 * @author Roedy Green, Canadian Mind Products
 * @version 1.0 2016-06-28 initial version
 * @since 2016-06-28
 */
public class RollYourOwnTimeout
    {
    private static final long MILLIS_TO_WAIT = 10 * 1000L;
    public static void main( final String[] args )
        {
        final ExecutorService executor = Executors.newSingleThreadExecutor();
        // schedule the work
        final Future<String> future = executor.submit( RollYourOwnTimeout::requestDataFromWebsite );
        try
            {
            // where we wait for task to complete
            final String result = future.get( MILLIS_TO_WAIT, TimeUnit.MILLISECONDS );
            out.println( "result: " + result );
            }
        catch ( TimeoutException e )
            {
            err.println( "task timed out" );
            future.cancel( true /* mayInterruptIfRunning */ );
            }
        catch ( InterruptedException e )
            {
            err.println( "task interrupted" );
            }
        catch ( ExecutionException e )
            {
            err.println( "task aborted" );
            }
        executor.shutdownNow();
        }
/**
 * dummy method to read some data from a website
 */
private static String requestDataFromWebsite()
    {
    try
        {
        // force timeout to expire
        Thread.sleep( 14_000L );
        }
    catch ( InterruptedException e )
        {
        }
    return "dummy";
    }
}