Java Community | Help. Code. Learn.•10mo ago

Concurrent ArrayList read/write

I have a situation where one thread puts data in an ArrayList and another thread reads that data. I am not really concerned whether the thread that reads the data can instantly tell that the data is there, but rather if there is a chance the reader thread might not ever see the data at all. Here is an example:

public class Main {

    
    final static ArrayList<Integer> list= new ArrayList<>();

    public static void main(String[] args) {
        Thread writer = new Thread(Main::write);
        Thread reader = new Thread(Main::read);
        writer.start();
        reader.start();
    }
    public static void write(){
        while (true){
            list.add(1);
        }
    }
    public static void read(){
        int currentIndex=0;
        while (true){
            if(currentIndex>=list.size()){
                try {
                    Thread.sleep(1);
                    continue;
                } catch (InterruptedException e) {
                    return;
                }
            }
            int element = list.get(currentIndex);
            currentIndex++;
            //do stuff with element...
        }
    }
}

public class Main {

    
    final static ArrayList<Integer> list= new ArrayList<>();

    public static void main(String[] args) {
        Thread writer = new Thread(Main::write);
        Thread reader = new Thread(Main::read);
        writer.start();
        reader.start();
    }
    public static void write(){
        while (true){
            list.add(1);
        }
    }
    public static void read(){
        int currentIndex=0;
        while (true){
            if(currentIndex>=list.size()){
                try {
                    Thread.sleep(1);
                    continue;
                } catch (InterruptedException e) {
                    return;
                }
            }
            int element = list.get(currentIndex);
            currentIndex++;
            //do stuff with element...
        }
    }
}

In this scenario if for some reason the writer thread is taking a while to add something to the list, is it possible the reader thread might do something like cache the entire list and pretty much never see when a new element is added so it just keeps sleeping? And if it is possible is there a way to prevent it without synchronizing access to the entire structure, because I really don't need to. I have been running my program like this for quite some time and haven't run into any issues, but this potential problem lingers in the back of my mind.

1 Reply

JavaBot•10mo ago

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Concurrent ArrayList read/write