我如何遍历可能由另一个线程添加到的列表

Question

I have a list that I iterate over and perform some actions on. One of the actions that can be performed can result in work being passed off to another thread, which may add more elements to my list while I'm still iterating over it in the first thread.

Is there a way for me to iterate over the list in such a way that the iterator includes the additions from the other thread?

Here's some pseudo Java

Class1{

   List multiThreadList = getMyList(); //points to list in Class2
   for(Element element:multiThreadList)
      //perform some actions, these actions may results in another thread being called
      //which will cause addToMyList() to be called while I'm still iterating over it
      //I want to keep iterating if an element gets added on the other thread
}
Class2{
   List originalList = new ArrayList();

   public getMyList()
     return originalList;

   void addToMyList(Element element)
     originalList.add(element);
}

Answer 1

Are you sure List is the kind of collection you need?

I would use BlockingQueue, and remove elements from BlockingQueue in one Thread, and add in another. This way you would not need any additional concurrency control.

BlockingQueue<String> bounded   = new LinkedBlockingQueue<String>();

bounded.put("Value");

String value = bounded.take();

Your pseudo code becomes

Class1{

   BlockingQueue queue = getMyList();
   Object element = queue.poll(0, TimeUnit.SECONDS);
   while(element != null) {
      //perform some actions, these actions may results in another thread being called
      //which will cause addToMyList() to be called while I'm still iterating over it
      //I want to keep iterating if an element gets added on the other thread  
      element = queue.poll(0, TimeUnit.SECONDS);
   }
}
Class2{
   BlockingQueue originalList = new LinkedBlockingQueue();

   public BlockingQueue getMyList()
     return originalList;

   void addToMyList(Element element)
     originalList.put(element);
}

But one thing, you need to understand, that this task in current form would give you inconsistent results. Since you don't control another Thread, your iterator might finish, before new element was added, and you will miss it, while iterating, depending on the state of the system you can miss from zero to all new elements. So you either need to join all created Threads, before you finish iterating, or change the approach.

Answer 2

You would have to use some type of concurrency control mechanism, such as a Mutex lock, a semaphore, or a monitor. These create a 'critical section' of your code which would allow only one thread to access it at a time.

Unfortunately, there is no solution to the problem without somehow locking the code and making a portion of it serial.