ThreadPoolExecutor如何让主线程等待其他线程完成 [英] How to make the main thread wait for the other threads to complete in ThreadPoolExecutor

问题描述

我正在使用 ThreadPoolExecutor 在我的 Java 应用程序中实现线程.

I am using the ThreadPoolExecutor to implement threading in my Java Application.

我有一个 XML，我需要解析它并将它的每个节点添加到线程以执行完成.我的实现是这样的:

I have a XML which I need to parse and add each node of it to a thread to execute the completion. My implementation is like this:

parse_tp 是创建的线程池对象 &ParseQuotesXML 是具有 run 方法的类.

parse_tp is a threadpool object created & ParseQuotesXML is the class with the run method.

        try {     
           List children = root.getChildren();               
        Iterator iter = children.iterator();

        //Parsing the XML     
        while(iter.hasNext()) {       
           Element child = (Element) iter.next();           
           ParseQuotesXML quote = new ParseQuotesXML(child, this);         
           parse_tp.execute(quote);         
        }
    System.out.println("Print it after all the threads have completed");
        catch(Exception ex) {  
        ex.printStackTrace();      
        }
        finally {  
    System.out.println("Print it in the end.");
if(!parse_tp.isShutdown()) {
                if(parse_tp.getActiveCount() == 0 && parse_tp.getQueue().size() == 0 ) {
                    parse_tp.shutdown();                    
                } else {
                    try {
                        parse_tp.awaitTermination(30, TimeUnit.SECONDS);
                    } catch (InterruptedException ex) {
                        log.info("Exception while terminating the threadpool "+ex.getMessage());
                        ex.printStackTrace();
                    }
                }
            }
          parse_tp.shutdown();  
        }

问题是，这两个打印输出语句是在其他线程退出之前打印出来的.我想让主线程等待所有其他线程完成.在普通的线程实现中，我可以使用 join() 函数来实现，但在 ThreadPool Executor 中无法实现相同的目标.还想问一下finally里写的代码块关闭线程池是否正确?

The problem is, the two print out statements are printed before the other threads exit. I want to make the main thread wait for all other threads to complete. In normal Thread implementation I can do it using join() function but not getting a way to achieve the same in ThreadPool Executor. Also would like to ask if the code written in finally block to close the threadpool proper ?

谢谢，阿米特

推荐答案

为了回答您的第二个问题，我认为您在尝试清理线程池方面做得很合理.

To answer your second question, I think you are doing a reasonable job trying to clean up your thread pool.

关于你的第一个问题，我认为你想使用的方法是 提交 而不是execute.与其试图用文字解释这一切，不如用我编写的单元测试中的一个编辑片段来完成许多任务，让每个任务完成总工作的一个片段，然后在起点相遇以添加结果:

With respect to your first question, I think the method that you want to use is submit rather than execute. Rather than try to explain it all in text, here's an edited fragment from a unit test that I wrote that makes many tasks, has each of them do a fragment of the total work and then meets back at the starting point to add the results:

final AtomicInteger messagesReceived = new AtomicInteger(0);

// ThreadedListenerAdapter is the class that I'm testing 
// It's not germane to the question other than as a target for a thread pool.
final ThreadedListenerAdapter<Integer> adapter = 
    new ThreadedListenerAdapter<Integer>(listener);
int taskCount = 10;

List<FutureTask<Integer>> taskList = new ArrayList<FutureTask<Integer>>();

for (int whichTask = 0; whichTask < taskCount; whichTask++) {
    FutureTask<Integer> futureTask = 
        new FutureTask<Integer>(new Callable<Integer>() {
        @Override
        public Integer call() throws Exception {
            // Does useful work that affects messagesSent
            return messagesSent;
        }
    });
    taskList.add(futureTask);
}

for (FutureTask<Integer> task : taskList) {
    LocalExecutorService.getExecutorService().submit(task);
}

for (FutureTask<Integer> task : taskList) {
    int result = 0;
    try {
        result = task.get();
    } catch (InterruptedException ex) {
        Thread.currentThread().interrupt();
    } catch (ExecutionException ex) {
        throw new RuntimeException("ExecutionException in task " + task, ex);
    }
    assertEquals(maxMessages, result);
}

int messagesSent = taskCount * maxMessages;
assertEquals(messagesSent, messagesReceived.intValue());

我认为这个片段与您想要做的类似.关键组件是 提交 和 get 方法.

I think this fragment is similar to what you're trying to do. The key components were the submit and get methods.

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