Java 中 Future 的 get 方法超时会怎样？ |
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Java 中 Future 的 get 方法超时会怎样？
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一、背景 很多 Java 工程师在准备面试时，会刷很多八股文，线程和线程池这一块通常会准备线程的状态、线程的创建方式，Executors 里面的一些工厂方法和为什么不推荐使用这些工厂方法，ThreadPoolExecutor 构造方法的一些参数和执行过程等。 工作中，很多人会使用线程池的 submit 方法 获取 Future 类型的返回值，然后使用 java.util.concurrent.Future#get(long, java.util.concurrent.TimeUnit) 实现“最多等多久”的效果。 但很多人对此的理解只停留在表面上，稍微问深一点点可能就懵逼了。  在这里插入图片描述比如，java.util.concurrent.Future#get(long, java.util.concurrent.TimeUnit) 超时之后，当前线程会怎样？线程池里执行对应任务的线程会有怎样的表现？ 如果你对这个问题没有很大的把握，说明你掌握的还不够扎实。 最常见的理解就是，“超时以后，当前线程继续执行，线程池里的对应线程中断”，真的是这样吗？ 二、模拟2.1 常见写法下面给出一个简单的模拟案例： 代码语言：javascript复制package basic.thread; import java.util.concurrent.*; public class FutureDemo { public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException { ExecutorService executorService = Executors.newFixedThreadPool(2); Future future = executorService.submit(() -> { try { demo(); } catch (InterruptedException e) { throw new RuntimeException(e); } }); String threadName = Thread.currentThread().getName(); System.out.println(threadName + "获取的结果 -- start"); Object result = future.get(100, TimeUnit.MILLISECONDS); System.out.println(threadName + "获取的结果 -- end :" + result); } private static String demo() throws InterruptedException { String threadName = Thread.currentThread().getName(); System.out.println(threadName + ",执行 demo -- start"); TimeUnit.SECONDS.sleep(1); System.out.println(threadName + ",执行 demo -- end"); return "test"; } }输出结果： 代码语言：javascript复制main获取的结果 -- start pool-1-thread-1,执行 demo -- start Exception in thread "main" java.util.concurrent.TimeoutException at java.util.concurrent.FutureTask.get(FutureTask.java:205) at basic.thread.FutureDemo.main(FutureDemo.java:20) pool-1-thread-1,执行 demo -- end我们可以发现：当前线程会因为收到 TimeoutException 而被中断，线程池里对应的线程“却”继续执行完毕。 2.2 尝试取消我们尝试对未完成的线程进行取消，发现 Future#cancel 有个 boolean 类型的参数。 代码语言：javascript复制 /** * Attempts to cancel execution of this task. This attempt will * fail if the task has already completed, has already been cancelled, * or could not be cancelled for some other reason. If successful, * and this task has not started when {@code cancel} is called, * this task should never run. If the task has already started, * then the {@code mayInterruptIfRunning} parameter determines * whether the thread executing this task should be interrupted in * an attempt to stop the task. * * After this method returns, subsequent calls to {@link #isDone} will * always return {@code true}. Subsequent calls to {@link #isCancelled} * will always return {@code true} if this method returned {@code true}. * * @param mayInterruptIfRunning {@code true} if the thread executing this * task should be interrupted; otherwise, in-progress tasks are allowed * to complete * @return {@code false} if the task could not be cancelled, * typically because it has already completed normally; * {@code true} otherwise */ boolean cancel(boolean mayInterruptIfRunning);看源码注释我们可以知道： 当设置为 true 时，正在执行的任务将被中断（interrupted）； 当设置为 false 时，如果任务正在执行中，那么仍然允许任务执行完成。 2.2.1 cancel(false)此时，为了不让主线程因为超时异常被中断，我们 try-catch 包起来。 代码语言：javascript复制package basic.thread; import org.junit.platform.commons.util.ExceptionUtils; import java.util.concurrent.*; public class FutureDemo { public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException { ExecutorService executorService = Executors.newFixedThreadPool(2); Future future = executorService.submit(() -> { try { demo(); } catch (InterruptedException e) { throw new RuntimeException(e); } }); String threadName = Thread.currentThread().getName(); System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果 -- start"); try { Object result = future.get(100, TimeUnit.MILLISECONDS); System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果 -- end :" + result); } catch (Exception e) { System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果异常:" + ExceptionUtils.readStackTrace(e)); } future.cancel(false); System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果 -- cancel"); } private static String demo() throws InterruptedException { String threadName = Thread.currentThread().getName(); System.out.println(System.currentTimeMillis() + "," + threadName + ",执行 demo -- start"); TimeUnit.SECONDS.sleep(1); System.out.println(System.currentTimeMillis() + "," + threadName + ",执行 demo -- end"); return "test"; } }结果： 代码语言：javascript复制1653751759233,main获取的结果 -- start 1653751759233,pool-1-thread-1,执行 demo -- start 1653751759343,main获取的结果异常:java.util.concurrent.TimeoutException at java.util.concurrent.FutureTask.get(FutureTask.java:205) at basic.thread.FutureDemo.main(FutureDemo.java:23) 1653751759351,main获取的结果 -- cancel 1653751760263,pool-1-thread-1,执行 demo -- end我们发现，线程池里的对应线程在 cancel(false) 时，如果已经正在执行，则会继续执行完成。 2.2.2 cancel(true)代码语言：javascript复制package basic.thread; import org.junit.platform.commons.util.ExceptionUtils; import java.util.concurrent.*; public class FutureDemo { public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException { ExecutorService executorService = Executors.newFixedThreadPool(2); Future future = executorService.submit(() -> { try { demo(); } catch (InterruptedException e) { System.out.println(System.currentTimeMillis() + "," + Thread.currentThread().getName() + ", Interrupted:" + ExceptionUtils.readStackTrace(e)); throw new RuntimeException(e); } }); String threadName = Thread.currentThread().getName(); System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果 -- start"); try { Object result = future.get(100, TimeUnit.MILLISECONDS); System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果 -- end :" + result); } catch (Exception e) { System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果异常:" + ExceptionUtils.readStackTrace(e)); } future.cancel(true); System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果 -- cancel"); } private static String demo() throws InterruptedException { String threadName = Thread.currentThread().getName(); System.out.println(System.currentTimeMillis() + "," + threadName + ",执行 demo -- start"); TimeUnit.SECONDS.sleep(1); System.out.println(System.currentTimeMillis() + "," + threadName + ",执行 demo -- end"); return "test"; } }执行结果： 代码语言：javascript复制1653752011246,main获取的结果 -- start 1653752011246,pool-1-thread-1,执行 demo -- start 1653752011347,main获取的结果异常:java.util.concurrent.TimeoutException at java.util.concurrent.FutureTask.get(FutureTask.java:205) at basic.thread.FutureDemo.main(FutureDemo.java:24) 1653752011363,pool-1-thread-1, Interrupted:java.lang.InterruptedException: sleep interrupted at java.lang.Thread.sleep(Native Method) at java.lang.Thread.sleep(Thread.java:340) at java.util.concurrent.TimeUnit.sleep(TimeUnit.java:386) at basic.thread.FutureDemo.demo(FutureDemo.java:36) at basic.thread.FutureDemo.lambda$main$0(FutureDemo.java:14) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) 1653752011366,main获取的结果 -- cancel可以看出，此时，如果目标线程未执行完，那么会收到 InterruptedException ，被中断。 当然，如果此时不希望目标线程被中断，可以使用 try-catch 包住，再执行其他逻辑。 代码语言：javascript复制package basic.thread; import org.junit.platform.commons.util.ExceptionUtils; import java.util.concurrent.*; public class FutureDemo { public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException { ExecutorService executorService = Executors.newFixedThreadPool(2); Future future = executorService.submit(() -> { demo(); }); String threadName = Thread.currentThread().getName(); System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果 -- start"); try { Object result = future.get(100, TimeUnit.MILLISECONDS); System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果 -- end :" + result); } catch (Exception e) { System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果异常:" + ExceptionUtils.readStackTrace(e)); } future.cancel(true); System.out.println(System.currentTimeMillis() + "," + threadName + "获取的结果 -- cancel"); } private static String demo() { String threadName = Thread.currentThread().getName(); System.out.println(System.currentTimeMillis() + "," + threadName + ",执行 demo -- start"); try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { System.out.println(System.currentTimeMillis() + "," + threadName + ",执行 demo 被中断，自动降级"); } System.out.println(System.currentTimeMillis() + "," + threadName + ",执行 demo -- end"); return "test"; } }执行结果： 代码语言：javascript复制1653752219803,main获取的结果 -- start 1653752219803,pool-1-thread-1,执行 demo -- start 1653752219908,main获取的结果异常:java.util.concurrent.TimeoutException at java.util.concurrent.FutureTask.get(FutureTask.java:205) at basic.thread.FutureDemo.main(FutureDemo.java:19) 1653752219913,main获取的结果 -- cancel 1653752219914,pool-1-thread-1,执行 demo 被中断，自动降级 1653752219914,pool-1-thread-1,执行 demo -- end三、回归源码我们直接看 java.util.concurrent.Future#get(long, java.util.concurrent.TimeUnit) 的源码注释，就可以清楚地知道各种情况的表现： 代码语言：javascript复制 /** * Waits if necessary for at most the given time for the computation * to complete, and then retrieves its result, if available. * * @param timeout the maximum time to wait * @param unit the time unit of the timeout argument * @return the computed result * @throws CancellationException if the computation was cancelled * @throws ExecutionException if the computation threw an * exception * @throws InterruptedException if the current thread was interrupted * while waiting * @throws TimeoutException if the wait timed out */ V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException;我们还可以选取几个常见的实现类，查看下实现的基本思路： java.util.concurrent.FutureTask#get(long, java.util.concurrent.TimeUnit) 代码语言：javascript复制 public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException { if (unit == null) throw new NullPointerException(); int s = state; if (s |
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