考虑一个简单的程序:
public static void main(String[] args) throws Exception {
// Create an ExecutorService with a fixed number of threads: three
ExecutorService service = Executors.newFixedThreadPool(3);
// Pass a simple Runnable task to the ExecutorService
service.submit(() -> System.out.println("done"));
}
运行程序会产生我们期望的控制台输出:
但这并不是我们通常在 IntelliJ IDEA 中看到的输出:
我们通常会在程序结束时看到。
为什么会这样?
newFixedThreadPool()方法的描述告诉我们,使用ExecutorService创建的线程将继续存在,直到它们被显式停止。这意味着因为我们将任务传递给 ExecutorService ,所以创建了一个线程来执行它,并且即使在任务完成后该线程仍然存在。
在 ExecutorService 处停止
因此,我们需要“关闭”(或停止)ExecutorService。我们可以通过两种方式做到这一点:
-
void shutdown() — 调用此方法后,ExecutorService停止接受新作业。先前提交给ExecutorService 的所有任务将继续运行。
public static void main(String[] args) throws Exception { ExecutorService service = Executors.newFixedThreadPool(3); service.submit(() -> System.out.println("task 1")); service.submit(() -> System.out.println("task 2")); service.shutdown(); // A RejectedExecutionException will occur here service.submit(() -> System.out.println("task 3")); }
-
List<Runnable> shutdownNow() — 此方法尝试停止当前活动的作业。仍在等待轮到他们的任务被丢弃并作为Runnables列表返回。
public static void main(String[] args) throws Exception { ExecutorService service = Executors.newFixedThreadPool(5); List.of(1, 2, 3, 4, 5, 6, 7, 8).forEach(i -> service.submit(() -> System.out.println(i))); List<Runnable> runnables = service.shutdownNow(); runnables.forEach(System.out::println); }
输出:
2
4
3
java.util.concurrent.FutureTask@1e80bfe8[未完成,任务 = java.util.concurrent.Executors$RunnableAdapter@4edde6e5[包装任务 = Test$$Lambda$16/0x0000000800b95040@70177ecd]]
java.util.concurrent .FutureTask@cc34f4d[未完成,任务 = java.util.concurrent.Executors$RunnableAdapter@66a29884[包装任务 = Test$$Lambda$16/0x0000000800b95040@4769b07b]]
java.util.concurrent.FutureTask@6f539caf[未完成,任务= java.util.concurrent.Executors$RunnableAdapter@17a7cec2[Wrapped task = Test$$Lambda$16/0x0000000800b95040@65b3120a]]
5
进程结束,退出代码为 0
输出将因运行而异。输出中有两种行:
-
数字表示 ExecutorService设法处理了相应的任务,显示了我们用来创建任务的列表中的数字。
-
在FutureTask对象上调用toString()方法的结果。这些对象是提交给ExecutorService但未处理的任务。
输出还有另一个有趣的细微差别。在理想情况下,我们首先会看到所有显示的数字,然后是FutureTask对象。但是同步问题使输出中的行混乱。
其他方法
ExecutorService还有几个与停止它相关的方法:
-
boolean awaitTermination(long timeout, TimeUnit unit) — 此方法阻塞调用它的线程。只要发生以下三个事件中的任何一个,块就会结束:
- 调用shutdown()方法后,所有活动作业和所有计划任务都已执行;
- 方法参数确定的超时已经过去;
- 调用awaitTermination()方法的线程被终止。
如果ExecutorService在超时结束前停止,则该方法返回true ,如果超时已经结束,则返回 false 。
public static void main(String[] args) throws Exception { ExecutorService service = Executors.newFixedThreadPool(2); service.submit(() -> System.out.println("task 1")); service.submit(() -> System.out.println("task 2")); service.submit(() -> System.out.println("task 3")); service.shutdown(); System.out.println(service.awaitTermination(1, TimeUnit.MICROSECONDS)); }
-
boolean isShutdown() —如果在ExecutorService上调用了shutdown()或shutdownNow()方法,则返回true。
public static void main(String[] args) throws Exception { ExecutorService service = Executors.newFixedThreadPool(2); service.submit(() -> System.out.println("task 1")); service.submit(() -> System.out.println("task 2")); service.submit(() -> System.out.println("task 3")); System.out.println(service.isShutdown()); service.shutdown(); System.out.println(service.isShutdown()); }
-
boolean isTerminated() —如果已在ExecutorService上调用shutdown()或shutdownNow()方法并且所有任务都已完成,则返回true 。
public static void main(String[] args) throws Exception { ExecutorService service = Executors.newFixedThreadPool(5); List.of(1, 2, 3, 4, 5, 6, 7, 8).forEach(i -> service.submit(() -> System.out.println(i))); service.shutdownNow(); System.out.println(service.isTerminated()); }
使用这些方法的示例代码:
public static void main(String[] args) throws Exception {
ExecutorService service = Executors.newFixedThreadPool(16);
Callable<String> task = () -> {
Thread.sleep(1);
return "Done";
};
// Add 10,000 tasks to the queue
List<Future<String>> futures = IntStream.range(0, 10_000)
.mapToObj(i -> service.submit(task))
.collect(Collectors.toList());
System.out.printf("%d tasks were submitted for execution.%n", futures.size());
// Attempt to shut down
service.shutdown();
// Wait 100 milliseconds to finish the work
if (service.awaitTermination(100, TimeUnit.MILLISECONDS)) {
System.out.println("All tasks completed!");
} else {
// Stop forcibly
List<Runnable> notExecuted = service.shutdownNow();
System.out.printf("%d tasks were not started.%n", notExecuted.size());
}
System.out.printf("Total tasks completed: %d.%n", futures.stream().filter(Future::isDone).count());
}
输出(每次运行都不同):
9170 任务未启动。
完成的任务总数:830 项任务。
进程结束,退出代码为 0
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