如何正确关闭java ExecutorService

Question

我有一个简单的javaExecutorService，它运行一些任务对象（实现callable)。

ExecutorService exec = Executors.newSingleThreadExecutor();
List<CallableTask> tasks = new ArrayList<>();
// ... create some tasks
for (CallableTask task : tasks) {
 Future future = exec.submit(task);
 result = (String) future.get(timeout, TimeUnit.SECONDS);
 // TASKS load some classes and invoke their methods (they may create additional threads)
 // ... catch interruptions and timeouts
}
exec.shutdownNow();

除了尽最大努力尝试停止处理正在执行的任务之外，没有任何保证。例如，典型的实现将通过{@link thread#interrupt}取消，因此任何未能响应中断的任务都可能永远不会终止。

我的问题是，有没有办法确保那些（任务）线程会终止？我想出的最佳解决方案是在程序末尾调用system.exit()，但这显然是愚蠢的。

Answer 1

从ExecutorService的Oracle API文档页推荐的方法：

 void shutdownAndAwaitTermination(ExecutorService pool) {
   pool.shutdown(); // Disable new tasks from being submitted
   try {
     // Wait a while for existing tasks to terminate
     if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
       pool.shutdownNow(); // Cancel currently executing tasks
       // Wait a while for tasks to respond to being cancelled
       if (!pool.awaitTermination(60, TimeUnit.SECONDS))
           System.err.println("Pool did not terminate");
     }
   } catch (InterruptedException ie) {
     // (Re-)Cancel if current thread also interrupted
     pool.shutdownNow();
     // Preserve interrupt status
     Thread.currentThread().interrupt();
   }

如果关闭池需要更多时间，则可以更改

if (!pool.awaitTermination(60, TimeUnit.SECONDS))

到

while (!pool.awaitTermination(60, TimeUnit.SECONDS))

awaitTermination（长超时，时间单位）引发InterruptedException：

阻塞，直到在关机请求、超时或当前线程中断（以先发生者为准）之后所有任务都已执行完毕。